Most routine tests used to diagnose CKD do not identify abnormal findings until there is advanced disease (stage 2 or higher). Currently, the most common way to diagnose CKD is by first detecting evidence of changes in renal function (e.g., azotemia, proteinuria) that arise as a result of renal lesions (Lees, 2004). Looking for subtle changes (e.g., gradually increasing serum creatinine over time, progressive decline in urine concentrating ability or presence of mild proteinuria) is helpful for identifying CKD at earlier stages. (Lees, 2004) Symmetric dimethylated arginine (SDMA) has recently become available in the veterinary marketplace as a reliable, early surrogate marker of GFR and, like creatinine, its reciprocal has a linear relationship with GFR (Braff et al, 2014). It has been shown to be an earlier marker of kidney dysfunction than creatinine in both dogs and cats (Hall et al, 2014, Nabity et al, 2015, Yerramilli et al, 2014, Yerramilli et al, 2016). Importantly SDMA does not appear to be affected by extra-renal factors, importantly lean body mass, making it a more sensitive marker for kidney disease than serum creatinine in patients with muscle loss (Hall et al, 2014, Hall et al, 2015, Relford et al, 2016). Both IRIS and ISFM guidelines for diagnosis and management of CKD recommend incorporating SDMA for early detection of kidney disease in dogs and cats (Sparkes et al, 2016, IRIS Staging of CKD). Persistent increases in SDMA greater than 14 mg/dL suggest reduced kidney function and the possibility of IRIS CKD stage 1 in patients with serum creatinine level less than the IRIS cutoff of 1.4 mg/ dL for dogs and 1.6 mg/dL for cats. SDMA can help to identify dogs and cats in IRIS stage 1 and early IRIS stage 2 in which clinical signs are absent or mild and serum creatinine level is not above the reference interval. Most animals with early kidney disease have an SDMA level between 15 and 20 mg/dL. Because SDMA level increases as kidney function decreases, SDMA levels greater than 20 mg/dL are typically seen in more advanced disease along with an increased serum creatinine level. A complete urinalysis should be performed to evaluate for inappropriate specific gravity, proteinuria, and other evidence of kidney disease. Making an early diagnosis of CKD generally requires finding 1 or more of the following: serum creatinine level increasing within the reference range, persistently increased SDMA, abnormal kidney imaging or persistent renal proteinuria (weeks to months). Earlier diagnosis of CKD allows for investigation for underlying causes and earlier therapeutic intervention, which could slow or halt disease progression.

Increased serum concentrations of urea nitrogen or creatinine may result from prerenal, renal or postrenal disorders. Prerenal azotemia may be caused by catabolic states (e.g., treatment with corticosteroids), consumption of a high-protein food, gastrointestinal hemorrhage, dehydration, hypovolemia or decreased cardiac output. Renal structure remains normal and the kidneys are capable of normal function if the prerenal insult is corrected before permanent damage occurs. Renal azotemia is caused by kidney disease and generally occurs when 75% of nephrons are nonfunctional. Renal azotemia should be further classified as either acute or chronic because of differences in treatment and prognosis. Postrenal azotemia is caused by disorders that impair elimination of urine from the body (e.g., urinary tract obstruction or rupture). Sites most often affected are the urethra and urinary bladder, and less often, ureters and kidneys. For upper urinary tract disease to cause postrenal azotemia, bilateral renal or ureteral disease must be present (unless the patient has concomitant kidney disease). As with prerenal disorders, renal function in patients with postrenal azotemia is normal initially; development of irreversible renal injury depends on severity, duration and nature of the disorder impairing urine outflow.

One of the most useful tests for distinguishing between pre- renal and renal azotemia is analysis of urine obtained before any treatment, especially fluid therapy. Patients with azotemia and evidence of adequate urine concentration (i.e., specific gravity >1.030 in dogs and >1.040 in cats) usually have prerenal disorders. There are two exceptions to this rule:

1. Some cats with CKD may have renal azotemia and still retain urine concentrating ability (specific gravity >1.040); it may be many months before they finally develop concurrent azotemia and inadequate concentrating ability.
2. Some dogs with glomerular disease may develop azotemia initially and then lose concentrating ability; this “glomerulotubular imbalance” should be suspected in dogs that have significant proteinuria, azotemia and urine specific gravity values greater than 1.030.

When azotemia is initially identified, it’s important to determine if the patient has received any treatment that may interfere with urine concentrating ability such as intravenous fluids, diuretics or corticosteroids. Also, disorders that may cause prerenal azotemia but concomitantly decrease urine specific gravity must be excluded; examples include hypoadrenocorticism, diabetic ketoacidosis, hypercalcemia, hepatic disease and pyometra. Hypoadrenocorticism may be easily misdiagnosed as acute kidney failure because of similar clinical and laboratory findings. Dogs and cats with renal azotemia usually have either isosthenuria (urine specific gravity of 1.008 to 1.013) or minimally concentrated urine (specific gravity <1.025). However, as previously noted, some patients with CKD may retain the ability to produce concentrated urine.

Like serum creatinine, results from SDMA analysis must always be interpreted in light of patient USG and physical examination findings in order to rule out volume-responsive and postrenal causes of azotemia. For example, a dehydrated dog or cat with a SDMA concentration of 16 mcg/dL and a USG of > 1.040 likely has prerenal azotemia. Longitudinal assessment of serum SDMA concentrations is preferred over one-time assessments and persistent elevations in SDMA in hydrated animals are indicative of kidney damage (Grauer, 2016).

Postrenal azotemia should be suspected in patients with stranguria, dysuria, pollakiuria, abdominal pain, ascites, firm/painful urinary bladder, subcutaneous swelling or discoloration of the perineum or a history of recent abdominal trauma. Palpable urethroliths or masses in the urethra, urinary bladder or prostate gland also suggest a postrenal cause of azotemia. Complete absence of urine production (i.e., anuria) most often is caused by lower urinary tract obstruction, although it may occur in some cases of acute kidney disease (e.g., ethylene glycol toxicosis). An attempt should be made to pass a urinary catheter if there is any question regarding patency of the lower urinary tract. However, the ability to pass a urinary catheter does not definitively exclude urethral obstruction. Urine specific gravity often is not helpful for distinguishing between renal and postrenal azotemia because urinary tract obstruction may cause renal tubular dysfunction and interfere with urine concentrating ability. Abdominal ultrasonography is helpful for detecting masses and accumulation of fluid when urinary tract obstruction or rupture is suspected. Abdominal fluid analysis in patients with uroabdomen reveals a modified transudate or exudate characterized cytologically by neutrophils, macrophages and mesothelial cells; bacteria may be seen if there is urinary tract infection. If uroabdomen is suspected, a sample of abdominal fluid should be submitted for measurement of creatinine and potassium concentrations so these values can be compared to concomitant serum concentrations. Measurement of urea nitrogen concentration in abdominal fluid often equals that of serum or blood and is therefore not helpful in patients with uroabdomen. Contrast urethrocystography is indicated when rupture or obstruction of the urethra or urinary bladder is likely; whereas, excretory urography is indicated when rupture of the upper urinary tract is suspected. If available, urethrocystoscopy may also be used to confirm rupture of the bladder or urethra.

Response to treatment may help localize azotemia. In general, pre- and postrenal azotemia resolve rapidly within one to three days after the underlying cause is corrected. In contrast, renal azotemia usually decreases more slowly, persists after appropriate treatment or recurs soon after discontinuation of treatment. Note that severe or prolonged pre- or postrenal azotemia may cause renal injury, which eventually leads to permanent kidney disease. It is also possible for renal azotemia to exist concomitantly with either pre- or postrenal disorders; this possibility should be suspected in patients that do not respond to treatment as expected.

Historically, kidney disease has been broadly defined into two categories: chronic kidney disease (CKD) and acute kidney injury (AKI).

Each category has distinctive features and has been defined by unique categorization schemes such as the IRIS CKD staging system for CKD and the IRIS AKI grading system for AKI (IRIS Guidelines). Conventionally, CKD has been perceived as slow in onset, progressive over time and irreversible, whereas AKI is thought to develop rapidly and maintain the potential for repair and return of kidney function. Recently, it has been suggested that these categories of kidney disease are not distinct entities but rather are closely associated and interconnected with common risk factors and disease modifiers (Cowgill et al, 2016, Yerramilli et al, 2016). CKD is a known risk factor for the development of AKI, and AKI is recognized increasingly as a potential mediator for progressive CKD and end-stage kidney disease (Belayev et al, 2014, Chawla et al, 2014, Heung et al, 2014). Persistent or repetitive injury over time may cause AKI to progress to CKD. This new understanding highlights the need for a panel of appropriate biomarkers that reflect functional as well as structural damage and recovery, predict potential risk and are prognostic (Hokamp et al, 2016). If successfully developed sensitive and specific biomarkers of active kidney injury are likely to lead new diagnostic and therapeutic approaches. Detection of active kidney injury (whether classified as CKD or AKI) may require a new designation based on whether a patient’s condition resolves, fails to develop progressive clinical manifestations over time, or progresses to overt clinical disease (Cowgill et al, 2016). Given the common pathophysiologic roots, the distinctions between CKD and AKI might better be viewed as a singular process rather than two distinct diseases.

A combination of diagnostics that assess kidney function (SDMA) and ongoing pathology in patients (active injury markers) will provide practitioners a more complete toolkit to better manage patients and achieve better outcomes. Until these new markers are available, after renal azotemia is confirmed, additional evaluation can be used to distinguish between AKI and CKD (Vaden, 2000). Careful review of history, physical examination findings and laboratory evaluation results usually distinguishes between AKI and CKD (Table 37-7). A careful medical history may reveal causes of AKI (e.g., ingestion of a nephrotoxin such as ethylene glycol). Patients with AKI generally are healthy before sudden onset of lethargy, depression and vomiting, whereas clinical signs in CKD such as inappetence, weight loss and polyuria/polydipsia occur more gradually. However, CKD can be present long before clinical signs are apparent. Patients with acute exacerbation of CKD are common and may present a diagnostic challenge. However, careful questioning of owners in these cases usually establishes a more chronic history. If it is still not possible to distinguish between acute and chronic disease, renal biopsy may be helpful, particularly if results will alter treatment or provide prognostic information that would help owners decide on a course of action.

While progression, or sustained decline of kidney function over time is a hallmark of CKD in most animals, its pathogenesis remains unknown. There is accumulating evidence from models of AKI that suggest a variety of intrinsic repair processes are activated after injury to the kidneys. Recovery from these insults is thought to represent adaptive repair, whereas progression results from maladaptive repair processes (Basile et al, 2016). Triggers for sustained or active injury include unresolved primary disease (eg, glomerulonephritis), comorbid conditions (eg, systemic hypertension, heart disease, and regional ischemia), chronic medications (eg, angiotensin-converting enzyme inhibitors, diuretics, antibiotics, and nonsteroidal anti-inflammatory drugs), chronic inflammation, chronic immune stimulation, recurrent infection, and proteinuria, among others (Table 37.8). There is compelling evidence that proteinuria per se propagates a maladaptive cycle of tubular injury, epithelial degeneration, and scarring in the adjacent interstitium (Figure 37.2) (Zoja et al, 2015). Unrelated to the nature of the insult, active and ongoing stress, metabolic dysregulation, and loss of morphologic and functional integrity of the tubular epithelium leading to interstitial inflammation and fibrosis are a common theme for progression of CKD. Progression of CKD to end-stage kidney disease may occur in a stepwise fashion from intermittent active insults or sustained injury from ongoing stresses / altered metabolism which perpetuate further loss of kidney structure and function (Cowgill et al, 2016). The growing interest and research directed at discovering biomarkers that can predict early onset of AKI, active injury and repair may help elucidate both the etiology and mechanisms of progression of kidney disease in veterinary patients. Until these tools are available understanding the mechanisms currently associated with progression of disease may help guide selection of treatment for patients with CKD. See sections below for more detailed information about specific mechanisms and how they may contribute to progression of CKD in dogs and cats.

In normal kidneys, single-nephron GFR and single-nephron plasma flow are submaximal under basal conditions. Reduction of nephron mass leads to hypertrophy of the residual nephrons with increases in filtration and perfusion of surviving nephrons to maintain total GFR (Polzin et al, 2005). Although these compensatory increases in single-nephron GFR and renal plasma flow initially help maintain homeostasis, eventually they contribute to progressive kidney damage. Single-nephron GFR increases are accompanied by glomerular hyperfiltration and intraglomerular hemodynamic changes, which increase flux of plasma proteins through the glomerular mesangium. These proteins stimulate mesangial cell proliferation and matrix production and eventually lead to glomerulosclerosis (Figure 37-3). Glomerular capillary hypertension is the critical intraglomerular hemodynamic factor responsible for promoting glomerular injury, perhaps through increasing proteinuria. Decreased dietary protein intake prevents these hemodynamic changes and preserves normal glomerular structure in rats (Brenner et al, 1982). The impact of dietary protein intake on glomerular hemodynamics and structure in dogs and cats is less certain.

As kidney disease develops, the afferent renal arterioles dilate, directly exposing glomeruli to systemic blood pressure; this causes glomerular hypertension, which distends the capillaries. The resultant mesangial stretch stimulates accumulation of collagen and progressive loss of glomerular function (Figure 37-4) (Riser et al, 1992). Continued strain on mesangial cells is a stimulus for cytokine release and extracellular matrix production (Polzin et al, 2005). Mesangial cells are stretched because of their relationship to capillaries and their attachment to the glomerular basement membrane. When mesangial cells in culture are stretched and relaxed repeatedly, stretch-induced release of transforming growth factor-β mediates production of collagen (Cortes et al, 1994). Intraglomerular hypertension also may lead to decreased glomerular permselectivity with resultant proteinuria (Polzin et al, 2005). Proteinuria, in turn, may mediate progressive injury of glomeruli and the renal tubulointerstitium (Lees et al, 2005; Polzin et al, 2005). Proteinuria has been associated with more rapid progression of CKD in dogs ( Jacob et al, 2005) and cats (Syme et al, 2006).  

Evidence linking renal proteinuria to progression of CKD is beginning to accumulate in dogs and cats. Numerous studies have found an association between the magnitude of proteinuria and risk for progression of kidney disease (Jacob et al, 2005, Syme et al, 2006, Kuwahara et al, 2006 Jepson et al, 2007, King et al 2007, Jepson et al, 2009, Chakrabarti et al, 2012, McLeland et al, 2015, Hokamp et al, 2016). In dogs with naturally occurring CKD, the relative risk of a uremic crisis or mortality was 3 times greater in dogs with UPC >1.0 compared with dogs with UPC <1.0 (Jacob et al, 2005). Further, the risk of an adverse outcome was 1.5 times greater for every 1 unit increase in UPC and the decline in renal function was greater in dogs with higher UPC. Interestingly, 19% of apparently healthy senior dogs were found to have persistent renal proteinuria (Marynissen et al, 2017). Serum screening evaluations ruled out hypertension, hypothyroidism and overt azotemia as underlying conditions, however markers of early kidney disease (SDMA) were not evaluated. In cats the relative risk of renal related death was three and four times greater for cats with moderate and marked proteinuria (UPC 0.2–0.4 and UPC > 0.4, respectively) compared those with mild proteinuria (UPC ≤ 0.2) (King et al 2007, Syme et al, 2006). A study designed to evaluate the presence and severity of histopathologic changes in the kidneys of cats with IRIS Stage 1-4 CKD documented proteinuria was associated with increased severity of tubular degeneration, inflammation, fibrosis, tubular epithelial single-cell necrosis, and decreased normal parenchyma (McLeland et al, 2014). In a prospective study of 213 cats with CDK followed for 1 year 47% (101) had progressive disease as defined by a Serum creatinine increase of ≥ 25%. High plasma phosphate concentration and high urine protein-to-creatinine ratio (UPC) predicted progression in all cats (Chakrabarti et al, 2012). Persistent proteinuria also has extra-renal consequences including sodium retention, edema, ascites, hypercholesterolemia hypertension, hypercoagulability, muscle wasting, and weight loss (Grauer et al, 2000).⁸⁹ Treatments that attenuate proteinuria in dogs and cats with CKD have been associated with slowed progression of CKD, improved survival, or both (Jepson et al, 2007, King et al, 2007, Grauer et al, 2000, Mizutani et al, 2006). However, it is possible for urinary proteinuria levels to reduce while mortality remains the same or increases in both cats and humans (King et al., 2006; Onuigbo, 2009).

Proteinuria may mediate progressive renal injury through several mechanisms (Polzin et al, 2005; Elliott and Syme, 2006). Impaired glomerular permselectivity allows passage of proteins that are not normally filtered including albumin, transferrin and complement (Polzin et al, 2005). Proteinuria may result in direct mesangial cell toxicity, fibrosis of glomeruli and subsequent glomerulosclerosis. Progression of CKD in experimental models more closely relates to the degree of tubulointerstitial disease than to the severity of glomerular lesions. Proteinuria may injure tubular cells through overloading tubular reabsorptive mechanisms or by receptor-mediated mechanisms (Polzin et al, 2005). Proximal tubular cells reabsorb abnormally filtered proteins such as albumin through endocytosis and lysosomal degradation. Excessive albuminuria can overload this resorptive capacity, causing lysosomal swelling and rupture, leading to lysosomal enzyme-mediated injury of tubular cells. Excessive albuminuria also increases oxidative stress, which appears to be an important mechanism of progressive renal injury (see Renal Oxidative Stress).

Abnormally filtered transferrin, a plasma protein that transports iron, increases absorption of iron by proximal tubular cells. Increased intracellular iron concentration of tubular cells produces reactive oxygen species (ROS) leading to oxidative injury. Complement binds to the luminal membrane of tubular cells and activates the membrane attack complex, culminating in cellular injury and lysis. These mechanisms contribute to loss of tubular cells and ultimately loss of nephrons. Cellular activation of inflammatory genes also stimulates secretion of inflammatory mediators into the interstitium, which promotes interstitial fibrosis. These mechanisms of glomerular and tubular injury explain why even modest levels of proteinuria are associated with more rapid progression of CKD in dogs (Jacob et al, 2005) and cats (Syme et al, 2006).    

In dogs and cats, hypertension usually occurs secondary to other diseases including kidney disease, obesity, hyperadrenocorticism, hyperthyroidism, pheochromocytoma and diabetes mellitus (Kobayashi et al, 1990; Rocchini et al, 1987; Brown et al, 2007). However, CKD appears to be the disease most commonly associated with systemic hypertension. (Brown et al, 2007) When considering hypertension in CKD, it is important to note that CKD may cause hypertension and hypertension can promote progression of CKD. Systemic hypertension can also damage a number of other end organs, including the eyes, central nervous system and cardiovascular system (Morgan, 1986). The IRIS scheme for staging CKD in dogs and cats identifies substages based on magnitude of systemic blood pressure and risk of end-organ damage (Table 37-1b).

Impaired autoregulation occurs in dogs with ischemic acute kidney failure and reduced renal mass. In normal dogs, the renal autoregulatory mechanism limits the effect of systemic blood pressure changes on renal blood flow and GFR. This protection is achieved by adjusting preglomerular resistance so that renal hemodynamics remain stable between mean systemic arterial blood pressures of 70 to 150 mm Hg. Dogs with severe reductions in functional mass have impaired renal autoregulation with elevations in renal arterial pressure. Impaired autoregulation may lead to renal injury during systemic hypertensive episodes and contribute to a progressive decline in kidney function (Brown et al, 1995; Polzin et al, 2005). Hypertension has been associated with increased risk of uremic crisis and death in dogs with CKD (Jacob et al, 2003). An association between the magnitude of proteinuria and survival has been shown in cats both with CKD and systemic hypertension (Syme et al, 2006, Jepson et al, 2007). However, since elevated SBP may induce proteinuria it is unlikely that they are independent contributors to the progression of CKD in cats. The relationship between systemic hypertension and progressive kidney disease, although suspected, remains unconfirmed in cats.  

Hyperphosphatemia and renal secondary hyperparathyroidism (RHPT) have been incriminated as causes of progressive renal injury (Felsenfeld and Llach, 1993; Lumlertgul et al, 1986, Chakrabarti et al, 2012). Disturbances of mineral metabolism and the multiple clinical syndromes they lead to are collectively known as CKD-mineral and bone disorder (CKD-MBD) (Foster, 2016). Consequences of CKD-MBD that  are known to occur in dogs and cats include RHPT, progression of CKD, increased mortality rate, renal osteodystrophy, cardiac arrhythmias, extraskeletal calcification, hypocalcemia, hypercalcemia, hypomagnesemia and hypermagnesemia. The development of RHPT is influenced by complex interactions of ionized calcium, phosphorus, vitamin D metabolites, parathyroid hormone (PTH), and fibroblast growth factor-23 (FGF-23). Renal secondary hyperparathyroidism appears to be an inevitable consequence of CKD (Nagode and Chew, 1992; Nagode et al, 1996; Barber and Elliott, 1998; Barber et al, 1999) (Figure 37-5). The inciting event in the pathogenesis of RHPT is phosphate retention (Figure 37-6). Destruction of nephrons decreases phosphorus filtration with a subsequent increase in phosphorus retention, increased serum FGF-23, decrease in calcitriol synthesis, hypocalcemia increased serum PTH (Foster, 2016). In a normal kidney and in early CKD, one effect of PTH is to decrease phosphate resorption in the proximal tubules so that more phosphate is excreted, and serum phosphorus concentration is maintained within the normal range. However, as CKD progresses, and more nephrons become nonfunctional, a greater concentration of PTH is required to maintain serum phosphorus concentration and eventually hyperphosphatemia develops. The primary consequence of hyperphosphatemia is development and progression of hyperparathyroidism. Although hyperparathyroidism helps maintain serum phosphorus concentrations initially, it has other effects that may be harmful. PTH stimulates resorption and release of minerals (e.g., phosphate) from bone, which increases the amount of phosphate that remaining nephrons must excrete. Increased PTH concentration correlates with histologic evidence of renal tissue inflammation and mineralization; therefore, hyperparathyroidism may damage the kidneys (Finco et al, 1992, 1992a; Ross et al, 1982; Brown et al, 1991).

The recently identified fibroblast growth factor (FGF)-23 is a hormone produced mainly by osteoblasts and osteocytes which promotes renal phosphorous excretion. FGF-23 is secreted in response to hyperphosphatemia, early in the course of CKD. It down regulates 1α-hydroxylase activity, thus further decreasing calcitriol levels and worsening renal secondary hyperparathyroisim (Nabsehima, 2008, Parker et al, 2015, Parker et al, 2017). Elevations in serum FGF-23 or PTH concentration likely suggest the presence of phosphorus retention, even in normophosphatemic patients. Both cats and dogs with CKD have been shown to have elevations in FGF-23 and PTH preceding hyperphosphatemia. Studies suggest this process begins early in CKD and as kidney disease progresses, the aberrations in serum phosphorus, calcium, FGF-23, PTH, and calcitriol concentrations typically increase in magnitude (Harjes, et al, 2017, Cortadella et al, 2010, Geddes et al, 2013, Finch et al, 2013, Geddes et al, 2015). One study documented 36% of IRIS stage 1 dogs had renal secondary hyperparathyroidism, whereas only 18% had hyperphosphatemia (Cortadella et al, 2010). In this same study the overall prevalence of RHPT in dogs with CKD was 76%. That increased to ≥ 96% in dogs with stages 3 and 4. Similar results were demonstrated in cats, whereby serum PTH and FGF-23 concentrations increased with advancing severity of CKD despite most cats being normophosphatemic (Geddes et al, 2013). In a population of 62 client owned apparently healthy geriatric cats monitored for 12 months, baseline FGF-23 concentrations were found to be significantly elevated in cats that ultimately developed azotemia compared to cats that did not (Finch et al, 2013). In a more recent study serum FGF-23 concentration was found to be an independent predictor of CKD progression in cats, and occurred prior to increases in PTH (Geddes et al, 2015). Commercial assays are currently available for canine and feline PTH measurement; it is hoped that FGF-23 assays will become readily available in the future. Despite the improved understanding of CDK-MBD pathophysiology additional research into cardiovascular complications, bone heath, and overall mortality are needed to refine our treatment goals and to improve clinical outcomes (Foster, 2016).

The kidney has a very high rate of oxygen consumption, the majority of which is expended reabsorbing sodium. With kidney damage, surviving nephrons increase sodium resorption and correspondingly increase oxygen consumption. The renal medulla concentrates urine by means of the countercurrent system of blood vessels and tubules that actively absorb sodium. The major determinant of medullary oxygen demand is the rate of active absorption in the medullary thick ascending loop, which is a relatively hypoxic environment. Hypoxia of the renal medulla can predispose to acute and chronic renal injury because the kidneys are extremely susceptible to hypoxic injury (O’Connor, 2006; Eckardt et al, 2005; Brezia and Rosen, 1995). In cats high urinary vascular endothelial growth factor (VEGF), a marker of hypoxia in tissues, has been associated with shorter survival and progression of azotemia (Chakrabarti et al, 2012). A more recent study found urinary VEGF was significantly lower in cats with CKD compared to healthy controls which may suggest an inadequate response of the kidney to the hypoxic environment (Habenicht et al, 2013). In CKD, increased fibrosis in the kidneys may result from intra-renal hypoxia due to increased oxygen consumption by surviving nephrons. Acute kidney injury often is associated with altered intra-renal microcirculation and oxygenation (Rosenberger et al, 2006). Hypoxia deprives tissues of energy and induces various regulatory mechanisms. The transcription factor, hypoxia-inducible factor, is involved in cellular regulation of development of new blood vessels, blood vessel tone, glucose metabolism and cell death. Kidney disease activates hypoxia- inducible factor, which presumably is renoprotective during oxygen deprivation (Eckardt et al, 2005). Hypoxia induces profibrogenic changes in proximal tubular epithelial cells and interstitial fibrosis (Norman and Fine, 2006). Hypoxia causes release of cytokines such as TGF-β and platelet derived growth factor, which stimulate intra-renal production of collagen. Furthermore, anemia may contribute to progression of CKD because anemia reduces oxygen delivery within the kidney, further promoting hypoxia and progressive renal damage (Rossert and Froissart, 2006).

A variety of mechanisms regulate medullary oxygen homeostasis; these include medullary vasodilators (e.g., nitric oxide, prostaglandin E₂, adenosine, dopamine and urodilatin) and vasoconstrictors (e.g., endothelin, angiotensin II and vasopressin). Tubuloglomerular feedback controls glomerular filtration and, indirectly, medullary oxygen demand. Reduced resorption of sodium activates signals that constrict the glomerulus, reducing glomerular filtration and subsequent delivery and resorption of sodium from the tubule. A related reaction is shifting of the corticomedullary blood flow to the medulla when renal blood flow is reduced. Because the work of concentrating urine predisposes a patient to medullary hypoxic injury, reducing the need for concentration of urine may prevent medullary injury. Reducing transport activity protects medullary tubules from hypoxic injury. Dehydration, volume depletion and renal hypoperfusion stimulate urine concentration; avoiding these conditions reduces the work of urine concentration and stimulates intra-renal protective mechanisms, such as prostaglandin and dopamine production.

Renal oxidative stress (ROS) occurs when there is an imbalance between the production of ROS (eg, superoxide, hydroxyl radical, and hydrogen peroxide) and availability of antioxidant defense mechanisms (eg, superoxide dismutase, catalase, glutathione peroxidase, and glutathione) (Brown, 2008). ROS are highly reactive molecules, which can cause damage to DNA, lipid, protein, and carbohydrate, resulting ultimately in structural and functional cellular damage that leads to apoptosis and necrosis, stimulating inflammation and fibrosis. This imbalance is a situation that is referred to as oxidative stress. (Small et al, 2012). Antioxidant defense mechanisms are designed to minimize damage by ROS including superoxide dismutase, catalase, nitric oxide synthase, glutathione peroxidase, vitamins E and C and carotenoids (Brown, 2008). Erythrocytes and albumin may also play important roles in minimizing oxidative injury to tissues (Agarwal, 2003; Brown, 2008; Rossert and Froissart, 2006). Erythrocytes represent a major antioxidant component of blood through enzymes such as superoxide dismutase, catalase and glutathione peroxidase. Also, erythrocyte glutathione reductase can regenerate reduced glutathione from its oxidized form (Rossert and Froissart, 2006). Oxidative damage has been incriminated as a cause of progressive renal injury in several types of kidney disease (Diamond et al, 1986; Agarwal, 2003; Vasavada and Agarwal, 2005; Brown, 2008, Hojs et al, 2016) (Figure 37-7). 

In rats with remnant kidneys, increased oxygen  consumption associated with increased dietary protein is accompanied by increased urinary clearance of oxidative products (Nath et al, 1994). In the remnant kidney model long term supplementation with omega -3 fatty acids has been shown to attenuate tubulointerstitial injury by mitigating oxidative stress, inflammation and fibrosis (An et al, 2009).

The role of ROS in progressive renal injury has also been evaluated in studies using vitamin E and selenium-deficient diets (Nath et al, 1994). Vitamin E is a major scavenger of ROS in lipid bilayers and selenium is required for glutathione peroxidase activity. Glutathione peroxidase is the enzyme that degrades hydrogen peroxide. Deficiency of vitamin E or selenium favors hydrogen peroxide accumulation and its associated oxidative effects. Increased renal oxidative stress has been linked to proteinuria as a potential mediator of tubulointerstitial damage and to progression of CKD (Brown, 2008; Agarwal, 2003; Agarwal et al, 2004; Vasavada and Agarwal, 2005). Overloading tubular mechanisms for resorption of filtered albumin by proximal tubular cells can stimulate production of proinflammatory and profibrotic cytokines by activation of the redox-sensitive gene nuclear factor-ⱪB thereby contributing to tubulointerstitial damage (Agarwal, 2003; Rossert and Froissart, 2006). Two studies have attempted to evaluate oxidative stress in cats with CKD. Results from one study indicated that cats with CKD had significantly higher GSH:GSSG ratios and significantly reduced antioxidant capacity (Keegan et al, 2010). There was no significant difference in superoxide dismutase activity between groups, whereas neutrophil burst was significantly higher in the CKD cats. Together, these results suggest antioxidant mechanisms are activated in cats with CKD.

In the other study selenium concentrations, an integral component of glutathione peroxidase, were investigated. This study identified significantly higher plasma glutathione peroxidase activity in IRIS stage 4 cats but no significant difference in other markers either among IRIS stage or between CKD and control cats (Krofic Zel et al, 2014). These results indicate that at stage 4 CKD cats may still be able to induce antioxidant mechanism and that selenium deficiency does not seem to be a factor. 

Several investigators have recognized an association between CKD and hypokalemia in cats (Lulich et al, 1992; Dow and Fettman, 1992; DiBartola, 1994). Hypokalemia is a common finding in cats with stage 2 and 3 CKD with approximately 20% to 30% of cats affected (DiBartola et al, 1987 , Elliott et al, 1998). Hypokalemia is less common in stage 4 CKD cats because of markedly decreased glomerular filtration (Polzin, 2013).135 Hypokalemia is typically less common in dogs because of angiotensin converting enzyme inhibitor therapy for proteinuria. Hypokalemia in cats with CKD may result from decreased renal potassium reabsorption, insufficient intake, renal tubular acidosis or hyperaldosteronism (Reynolds et al, 2013). Potassium depletion may induce metabolic acidosis, kidney dysfunction (hypokalemic nephropathy), muscle weakness and cardiac arrhythmia.

In cats with CKD and hypokalemia, renal function may improve after potassium supplementation and restoration of normokalemia, suggesting that hypokalemia may be associated with a reversible, functional decline in GFR. Renal function was adversely affected in normal cats when an acidified, low-potassium food was fed (Dow et al, 1990). In this study, potassium depletion and acidosis appeared to have additive effects on impairing renal function. Limited evidence suggests; however, that hypokalemia is a cause of, and contributing factor to, CKD in cats rather than simply a consequence of the disease. In an uncontrolled study, renal lesions and dysfunction developed in three of nine cats fed a potassium-restricted, acidifying food for several months (DiBartola et al, 1993). In another study, four of seven cats with induced CKD fed a food containing 0.3% dry matter (DM) potassium developed hypokalemia, but four cats with normal renal function fed the same food did not develop hypokalemia (Adams et al, 1993). Muscle potassium content is decreased in normokalemic cats with spontaneous CKD, indicating that a total body deficit of potassium may develop well before the onset of hypokalemia (Theisen et al, 1997). The latter findings support the concept that reduced renal function precedes the development of hypokalemia.

In dogs hyperkalemia has been well documented with one retrospective study finding that 47% of dogs with CKD had at least one documented episode of hyperkalemia (Segev, 2010). Hyperkalemia may occur in dogs with CKD due to several mechanisms including as a complication of angiotensin-converting enzyme inhibitor use or in cases in which potassium load overwhelms the kidneys’ excretory ability. An important conclusion from this study was the importance of evaluating serum potassium in dogs with CKD and in those with hyperkalemia, identifying the potassium content of the diet being fed. In some cases, a home prepared diet controlled in potassium may be necessary and play an important role in avoiding deleterious effects of hyperkalemia. 

Metabolic acidosis appears to be a common complication of CKD in dogs and cats (DiBartola et al, 1987; Lulich et al, 1992; Jacob et al, 2002). In one report, six of 38 dogs with CKD had metabolic acidosis of sufficient severity to warrant treatment (Jacob et al, 2002).  A cross-sectional study involving 59 cats with CKD showed that more than half of patients with severe CKD had acidemia and low plasma bicarbonate concentrations (Elliott et al, 2003, 2003a). These data also suggested that biochemical evidence of severe metabolic acidosis does not generally occur in cats until late in the course of CKD (Elliott et al, 2003). Patients with CKD tend to develop metabolic acidosis because of impaired ability of the failing kidneys to excrete the daily net acid load. The kidney eliminates hydrogen ions by three major mechanisms: reclaiming filtered bicarbonate, buffering secreted hydrogen ions with filtered phosphate and sulfate (titratable acidity) and renal ammoniagenesis. Of these three mechanisms, renal ammoniagenesis can be markedly upregulated to increase net acid secretion by the kidneys. As functional renal mass decreases in CKD, ammonia production per surviving nephron is increased several-fold, although total ammonia production is still reduced. Because ammonia is nonpolar, it diffuses into the tubular lumen and the surrounding interstitium.

Ammonia activates the alternate complement cascade, which may lead to renal injury by several mechanisms including: release of cytokines, prostanoids and ROS, cell lysis and stimulation of collagen synthesis. In studies involving rats, supplementation with bicarbonate reduced concentrations of complement components (i.e., C₃ and C_5b-9) (Nath et al, 1985). Bicarbonate administration also reduced cortical levels of ammonia, decreased proteinuria, reduced structural damage and improved tubular function. The interaction of ammonia and complement in the etiopathogenesis of tubulointerstitial disease has also been demonstrated in studies of hypokalemic nephropathy in rats (Nath et al, 1985). Studies in rats, however, failed to demonstrate a role for acidemia and increased renal ammoniagenesis as a cause of renal injury and progression of kidney disease (Throssell et al, 1995). In an investigation of cats with induced CKD, those fed an acidifying food for six months did not develop progressive glomerular dysfunction or renal tubulointerstitial injury vs. cats fed a non-acidifying food (James, 2001). Therefore the relative importance of renal ammoniagenesis in progressive renal injury in dogs and cats with CKD is unknown.  

Cholesterol, triglycerides and possibly some classes of lipoproteins are cytotoxic to endothelial cells and stimulate glomerular mesangial cell proliferation and production of excess mesangial matrix. Abnormalities of lipid metabolism in dogs with kidney disease generally include increased serum concentrations of total cholesterol, low-density lipoproteins and triglycerides (Brown et al, 1991). Cats with experimentally induced renal dysfunction demonstrate hypercholesterolemia compared with normal cats. Despite occurrence of lipid abnormalities  in dogs and cats with CKD, there is little evidence to show they play a role in causing progression of disease.

End-stage kidney disease is characterized by glomerulosclerosis, tubulointerstitial fibrosis and tubular atrophy (Wolf, 2006; Polzin et al, 2005). Tubulointerstitial changes are a consistent feature in CKD, irrespective of the cause or initial structure involved (Eddy, 1994). The extent of tubulointerstitial injury correlates with the decline in renal function, whereas the severity of glomerular injury does not correlate well with progression of CKD models. It appears that GFR is influenced to a greater degree by interstitial fibrosis than by glomerulosclerosis (Nath, 1992).

Although chronic, progressive tubulointerstitial disease plays a critical role in progression of renal lesions, the basic mechanisms that generate the tubulointerstitial damage remain unclear. There appears to be a clinically silent acute phase that is characterized by inflammation and tubular cell injury. Possible mediators of tubular injury include antibodies, ROS, obstruction, complement and lysosomal enzymes (Eddy, 1994). Damaged tubular cells can regenerate or die. Factors responsible for recruitment of mononuclear cells to the interstitium are important because of evidence that monocytes and macro- phages play a key role in interstitial fibrosis (Eddy et al, 1991). Recruitment is probably mediated by the release of fibrosis-promoting cytokines, such as TGF-β (Wolf, 2006). TGF-β directly stimulates transcription of many extracellular matrix genes in renal cells including mesangial, endothelial and tubular cells. TGF-β also appears to trigger increased matrix production by perivascular and interstitial fibroblasts. Dietary protein restriction inhibits secretion of TGF-β and glomerular scarring in rats with glomerulonephritis (Fukui et al, 1993). Furthermore, the renin-angiotensin-aldosterone system is linked to activation of the TGF-β pathway, which in turn promotes interstitial fibrosis (Figure 37-7) (Wolf, 2006).

Tubulointerstitial injury can impair renal function by a number of mechanisms:

1. vascular effects
2. glomerular injury
3. interstitial and tubuloepithelial processes
4. nephron obstruction
5. deposition of crystals (Nath, 1992).

Post-glomerular blood flow is decreased when the cortical interstitium expands due to fibrosis and mononuclear infiltration. Decreased blood supply also results from release of vasoactive cytokines, growth factors and ROS produced by the interstitial infiltrate and damaged tubules. Decreased post-glomerular blood flow decreases tubular blood flow and changes glomerular size and pressure. Decreased tubular blood flow may impair tubular function and glomerular size and pressure changes may lead to glomerular injury (Nath, 1992).

As discussed above, abnormal glomerular function can incite tubulointerstitial injury (Diamond and Anderson, 1990). Loss of glomerular permselectivity and resultant proteinuria are accompanied by tubulointerstitial damage. Increased trafficking of protein in the proximal tubules may cause cellular damage. Filtered protein is endocytosed in the proximal tubules and subsequently degraded by lysosomal action. Excessive release of lysosomal enzymes may be one of the pathways for tubular damage. Tubular damage may also be induced by plasma proteins that have escaped into the urine. Incriminated plasma proteins include albumin, lipoproteins, complement components and transferrin. Studies in cats with spontaneously occurring CKD demonstrated that progression of CKD is most closely linked to severity of proteinuria, which may be explained by tubular damage during tubular resorption of leaked proteins (Syme et al, 2006; Jepson et al, 2007). Progression of spontaneously occurring CKD in dogs is also related to severity of proteinuria (Jacob et al, 2005).

Aging is a programmed biological process regulated by many genes. These changes result in impairment to normal adaptive responses and homeostatic mechanism, increasing organs susceptibility to both internal or external stressors. Numerous mechanisms may contribute to age-related organ dysfunction, including mitochondrial injury, telomere shortening, oxidative stress, profibrogenic and pro-inflammatory mediators, and an imbalance between cell repair and proliferation versus apoptosis and cell death (Jepson, 2016). However, many of these mechanisms occur not only during aging but also as part of an organ’s response to injury. Because of the commonality of response to aging and injury, a portion the increased prevalence of kidney disease in geriatric pets may be attributed to the cumulative effects of multiple intermittent or sustained low grade insults over time (Cowgill et al, 2016, Yerramilli et al, 2016). Evidence that supports a link between aging and CKD in dogs and cats include: increasing prevalence of CKD in older pets (O’Neill et al, 2014, Lefebvre, 2011, White et al, 2006, Marino et al, 2014, Babyak et al, 2017, Pelander  et al, 2015), increased prevalence of sclerotic glomeruli in kidneys from geriatric cat (McLeland et al, 2015), shortened telomere length in aged cats with CKD (Quimby et al, 2013), alterations in antioxidant defenses in dogs and cats with CKD (Keegan et al, 2010, Krofic Zel et al, 2014, Yu et al, 2006, Lippi et al, 2017), and increased prevalence of comorbid conditions, such as hyperthyroidism, systemic hypertension, dental disease, and inflammatory bowel disease (Syme et al, 2002, Brown et al, 2007, Taylor et al, 2017, Greene et al, 2014, Finch, et al, 2016, Glickman et al, 2011, O’Neill et al, 2013, Pavlica et al, 2008). 

Renal changes associated with aging are manifested by significant structural and functional alterations. Functional changes include decreased GFR, decreased renal blood flow, decreased urine concentrating ability and decreased ability to maintain sodium, water, endocrine and acid-base homeostasis. Senescent cells have altered secretion of products such as TGF-b, epithelial growth factor, insulin-like growth factor, and VEGF. The net effect of these changes is reduced capacity of the kidney to respond to repair and withstand normal stressors, also reducing its ability to recover from periods of ischemic injury and promotion of inflammation and fibrosis (Sturmlechner et al, 2017).141 Structural changes include alterations in renal weight, volume and histologic appearance. Fibroconnective tissue replaces functionally active parenchyma in aging kidneys. In a study of dogs with spontaneous glomerulonephritis, the incidence of interstitial nephritis increased with increasing age. Interstitial nephritis was present in 10% of dogs less than one year of age, in 60% of dogs between one and five years of age and in 85% of dogs more than five years of age (Muller- Peddinghaus and Trautwein, 1977). In another study, 59% of the dogs older than four years had evidence of interstitial nephritis (Shirota et al, 1979). Glomerular lesions were noted in 43 to 78% of these dogs. Based on these reports, interstitial nephritis and glomerulosclerosis apparently are common and occur with increased frequency in aging dogs. 

It is possible that CKD occurs as a consequence of life-preserving adaptive mechanisms that accompany the aging process. In cats, there is evidence that tubulointerstitial inflammation begins early in life and that these histopathological changes increase significantly with age (Lawler et al., 2006). A study of postmortem data collected from 1979 to 2001 revealed that of 676 cats living in a research colony, cats that died from kidney disease most often had renal histologic changes (i.e., progressive tubular deletion and peritubular interstitial fibrosis); however, their mean lifespan was longer than cats that died from other causes (Lawler et al, 2006). In addition, among cats that died from causes other than kidney disease, those with renal histologic changes had a longer mean lifespan compared with cats that had no changes in their kidneys. A recent study with a large population of cats with CKD and cats > 9 years old with non-renal diseases from first opinion practices identified specific renal lesions in only 16% of cats (Chakrabarti et al., 2013). The majority with CKD were found to have non-specific renal lesions and the predominant morphological diagnosis in these cases was chronic tubulointerstitial inflammation and fibrosis. It has been hypothesized that these renal changes may represent an intrinsic mechanism that is protective until the point of failure. While aging may be a component of the decline in renal function seen in older cats, it is also likely that other individual and environmental factors contribute to an individual’s overall risk of developing CKD. Regardless of the initiating cause, CKD often is characterized by irreversible loss of renal functional mass. After a critical amount of kidney damage occurs, CKD tends to be a progressive condition that often terminates with uremia-associated death.