Glaucoma: Taking the Pressure Off

The pathophysiology of glaucomatous optic neuropathy, a leading cause of blindness in dogs, is not fully known. Increased intraocular pressure (IOP) is believed to be a major risk factor. Calcium-channel blockers have multiple ocular effects and have been reported to increase or decrease IOP. To determine the effects of the calcium-channel blocker flunarizine on IOP of dogs, a study of 20 clinically normal dogs was performed. Each dog was treated topically with 1 drop of 0.5% flunarizine in 1 eye twice daily for 5 days. The contralateral eye of each dog served as the control. Through use of a rebound tonometer, baseline IOP was measured before the first treatment day and diurnal IOP was measured during the 5-day treatment period. Pupillary diameter and mean arterial pressure (MAP) were also measured. Serum flunarizine concentrations were measured on the final treatment day. The study found a significant bilateral decrease in IOP after 2 days of treatment, which persisted for the remainder of the study. No significant differences in IOP were noted between treated and untreated eyes, suggesting systemic as well as local effects of the drug. This was supported by detection of flunarizine in all of the serum samples, although other possible reasons for the bilateral decrease in IOP were possible. No significant treatment effect was noted on pupillary diameter or MAP. It was concluded that flunarizine may prove to be an effective treatment for dogs with glaucoma.

COMMENTARY: Glaucoma is one of the most common causes of blindness and ocular pain in dogs. Surgical management is not readily available and is too expensive for many clients. Therefore, any new medical treatment would be worthy of consideration. A calcium-channel blocker that could not only provide neuroprotection to the retinal ganglion but also lower IOP would be beneficial. The authors report a "significant decrease in IOP after 2 days of topical flunarizine in clinically normal dogs." However, it is questionable whether this decrease is clinically significant and what the effect would be in glaucomatous eyes. Also, would the drug have an additive effect with the other drugs currently used to treat glaucoma? Future studies might give us additional means of dealing with this severe disease.

Effects of the topically applied calcium-channel blocker flunarizine on intraocular pressure in clinically normal dogs. Greller AL, Hoffman AR, Liu C, et al. AM J VET RES 69:273-278, 2008.