Barbiturates, such as thiopental, are frequently used for sedation and anesthesia during laryngeal examination, but they can be potent depressants of the central nervous system. Dose-dependent respiratory depression inhibits evaluation of laryngeal paralysis, and dogs have been mistakenly diagnosed with this disease because of idiosyncratic reactions to various sedatives and anesthetics during laryngoscopy. On the other hand, laryngeal paralysis is a common airway disease in large- and giant-breed dogs, usually in middle to old age, and selection of an appropriate surgical or anesthetic protocol could be the key to correct diagnosis of the condition.
One study assessed the effects of various anesthetic agents on laryngeal motion during laryngoscopy in healthy dogs. The ideal drug or combination of drugs would provide relaxation of jaw muscles while permitting normal laryngeal movement and range of motion during laryngoscopy without waking the patient. Each of 6 healthy, large-breed dogs with no history of respiratory dysfunction were randomly assigned to different injectable anesthetic protocols once weekly for 6 weeks, and then they all were anesthetized with isoflurane in the 7th week. Video laryngoscopy was performed and recorded immediately after induction for each protocol until dogs could no longer be restrained. Laryngeal motion, which is defined as change in normalized glottal gap area and measured in pixels, was measured at induction and at recovery. Arytenoid motion before recovery was significantly greater with thiopental when compared with propofol, ketamine + diazepam, acepromazine + thiopental, and acepromazine + propofol. No significant difference in arytenoid motion was seen immediately after induction or before recovery when acepromazine - butorphanol + isoflurane and thiopental were compared. Thus, intravenous thiopental was the best choice when given to effect, followed by acepromazine plus isoflurane by mask if further restraint was required. The authors advise against using acepromazine plus thiopental, acepromazine plus propofol, or ketamine plus diazepam, because no detectable respiratory motion was seen in some dogs given these drugs.
In another study on transnasal laryngoscopy, which has been used successfully in awake or sedated horses and cattle, the purpose was to determine if the technique could be used successfully on sedated dogs. After intramuscular and topical anesthesia was administered, a video endoscope was passed through the left nasal passage (as standardized based on equine studies) for evaluation of 4 dogs with clinical signs of laryngeal paralysis and 3 healthy dogs. Laryngeal opening, structure, and function were observed in every dog during spontaneous ventilation, and manipulation of the epiglottis was not required. Transnasal laryngoscopy was successful in each dog, and all were comfortable and did not resist examination. The dogs later underwent traditional laryngoscopy to confirm diagnosis. Laryngeal paralysis was diagnosed in the 4 dogs with clinical signs and was confirmed with traditional laryngoscopy in 3. Of the healthy dogs, 2 required mechanical stimulation of the laryngeal mucosa for full evaluation with the transnasal technique. Ultrasonography-a noninvasive alternative to direct visualization-has also been used but is difficult to perform and interpret. Transnasal laryngoscopy eliminates both the difficulty of performing and interpreting ultrasonographic findings as well as the potentially adverse effects of deep sedation and general anesthesia with traditional laryngoscopy.
COMMENTARY: Laryngeal paralysis is a relatively common airway disease seen by small animal practitioners. It can be a diagnostic challenge to evaluate arytenoid movement under light anesthesia, particularly if appropriate anesthetic agents are not used. The anesthesia article is valuable in helping the clinician select an agent to maximize the potential for correct diagnosis of the disease.
COMMENTARY: The ability to diagnose-correctly-laryngeal paralysis can depend on the sedation/anesthetic protocol used for the evaluation. Laryngeal paralysis can be falsely diagnosed if the depth of sedation and/or anesthesia abolishes laryngeal motion in a healthy dog during laryngoscopy. For oral examination, my preference has been to use the lightest dose of thiopental that will permit laryngeal examination without struggle. We have used doxapram hydrochloride clinically as recommended by Jackson and colleagues and have found it useful in stimulating laryngeal motion in healthy dogs with minimal or no movement on initial examination. Subjectively, this has resulted in a more healthy respiratory pattern than when the laryngeal mucosa is stimulated directly. We have not had an opportunity to use the transnasal technique, but it appears that it might be a useful alternative to oral examination if endoscopic equipment is available.
Effects of various anesthetic agents on laryngeal motion during laryngoscopy in normal dogs. Jackson AM, Tobias K, Long C, et al. VET SURG 33:102-106, 2004.
Transnasal laryngoscopy for the diagnosis of laryngeal paralysis in dogs. Radlinsky MG, Mason DE, Hodgson D. JAAHA 40:2211-2215, 2004.