Diagnosis and treatment of glaucoma

Glaucoma is a group of diseases that cause loss of retinal ganglion cells and optic nerve demyelination, resulting in rapid loss of vision. A common clinical sign is an elevation in intraocular pressure (IOP). In addition to loss of vision, significant discomfort can also be present if the IOP is elevated.

Clinical signs of acute glaucoma include diffuse corneal edema, mydriasis, conjunctival and sclera hyperemia and loss of vision. Three things to consider in glaucoma: Is it glaucoma? Measure the IOP. Normal IOP is between 10 and 20 mm Hg. A persistent difference between the two eyes is significant.
Is it primary or secondary? Primary glaucoma is most commonly seen in cocker spaniels, basset hounds, beagles, poodle and the arctic breeds, but many other breeds are also predisposed. It is a hereditary malformation of the iridocorneal angle that progressively deteriorates during life. It is bilateral, and predisposes the other eye to glaucoma in the future. Both eyes need to be treated. The prognosis for long term vision is poor. Secondary glaucoma is secondary to another disease in the eye. Common examples are chronic uveitis, retinal disease, intraocular neoplasia, luxated lens. Treatment and prognosis depends on the etiology.
Is it acute or chronic? If it is acute, potential for return of vision exists. In acute glaucoma, animals need to be treated VERY aggressive to give them the best possible chance for return of vision. Chronic glaucoma has destroyed all potential for vision in the eye. The globe is typically enlarged, corneal striae can be present, the lens can be (sub) luxated, the optic nerve is demyelinated (small, round and dark) and cupped and the fundus is diffusely hyperreflective. These eyes need to be managed surgically.

Glaucoma medications 

Glaucoma medications reduce the intraocular pressure by reducing the production of aqueous humor, increasing the outflow of aqueous humor, or a combination or both. They are either administered systemically, or topically.

Systemic therapy

Osmotic agents reduce intraocular pressure by increasing plasma osmolality which results in diffusion of water from the intraocular fluids back into the plasma. An intact blood aqueous barrier is needed to lower the intraocular pressure and water needs to be withheld for 4 hours after administration of the osmotic agent. Intravenous mannitol is the most commonly used osmotic agent in treatment of acute glaucoma. Mannitol may be used in diabetics, but should be used with caution in patients with cardiac disease or renal insufficiency. The usual dose is 1-2 gram/kg given slowly over 20-30 minutes. Oral glycerin has the advantage of not requiring intravenous access, but the hypotensive effect can be unpredictable and vomiting may occur. It should be avoided in diabetic patients.

Carbonic anhydrase inhibitors (CAI)

Carbonic anhydrase inhibitors decrease the production of aqueous humor. Common side-effects include metabolic acidosis, hypokalemia and gastrointestinal problems such as anorexia, vomiting and diarrhea. Respiratory side-effects include increased respiratory effort and panting. Weakness and ataxia are uncommon side-effects. The most commonly used oral CAI is methazolamide at a dose of 2 mg/kg BID to TID. Although dose ranges of up to 10 mg/kg BID to TID have been reported, side-effects are common at higher doses

Topical glaucoma medications 

Carbonic anhydrase inhibitors: 2% dorzolamide. This is a commonly used medication in the management of glaucoma in dogs and cats. It is usually used BID to TID and is usually well tolerated. Potential sideeffects include burning/stinging upon application to the eye. 1% brinzolamide. Brinzolamide may be better tolerated than dorzolamide, but appears less effective in cats.

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