D-208 Veterinary Medical Center, Department of Small Animal Clinical Sciences, Michigan State University, East Lansing, MI, USA
Glaucoma can be one of the more challenging conditions to treat successfully. In the early stages of glaucoma it may not be obvious that the patient has glaucoma and a delay in diagnosis can lead to irreversible vision loss. Another challenge can be identifying the underlying cause of the glaucoma. Diagnosis of glaucoma can be based partly on clinical signs but accurate diagnosis and monitoring of treatment requires tonometry to measure intraocular pressure (IOP).
Clinical signs of glaucoma are dependent on several factors, including the cause of the glaucoma, the degree of IOP rise and the duration of the glaucoma. There are also species differences, for example cats can tolerate a raised IOP with less obvious signs of ocular pain compared to dogs. The owner's presenting complaint may include one or more of the following: ocular pain or discomfort; loss of vision (not always obvious with unilateral glaucoma); altered appearance including a red eye, a cloudy eye, a bulging eye. A full ophthalmic examination should be performed.
Vision assessment. A number of methods of vision assessment can be performed including obstacle course testing, menace response, visual tracking and visual placing responses.
Globe. Enlargement of the globe occurs when a raised IOP is sustained. To differentiate globe enlargement (buphthalmos) from anterior displacement of the globe (exophthalmos) the position of the eyes should be observed from directly above. The buphthalmic globe is enlarged 'in situ' and not anteriorly displaced. Globe enlargement may develop to such an extent that the lids are not able to close completely over the cornea. Comparing limbal diameter between the two eyes can help in judging a disparity in globe size (assuming one eye is normal).
Red eye. Episcleral congestion is a feature of raised IOP as well as other intraocular disease such as uveitis. Conjunctival involvement is also common ranging from vascular congestion to oedema, dependent on the degree of IOP elevation and duration. Corneal vascularisation may also develop with sustained high IOPs. This can appear as a brush border of deep vessels within the cornea.
Cloudy eye. Raised IOP can result in corneal oedema as the raised pressure overcomes the ability of the corneal endothelial pump mechanism to clear fluid from the corneal stroma. With chronicity additional corneal changes can develop including those resulting from corneal exposure. Intraocular opacity may also be present and can result from aqueous flare or cell and cataract formation. Anterior lens luxation is a possible cause of glaucoma and may result in increased corneal oedema. Intraocular tumour formation can result in glaucoma and an opaque and painful eye. If the intraocular structures cannot be directly examined because of opacity then ocular ultrasonography is recommended.
Intraocular structures. If examination of intraocular structures is possible this may reveal changes such as subluxation of the lens and degenerative changes involving retina and optic nervehead (atrophy and cupping).
Measuring IOP. IOP can be measured using a tonometer. The Schiotz tonometer is the cheapest of the tonometers commonly used in small animal veterinary medicine. Used correctly it can give a reasonable estimate of IOP. However, it can be difficult to use accurately, particularly for those not practised with it. Many more veterinary practices now own either a Tonopen or TonoVet. The Tonopen is an applanation tonometer that is easy to use and reasonably accurate. It has a transducer in its tip that measures the force required to flatten the cornea that the tip is pressed against. This is proportional to the IOP. The TonoVet is a rebound tonometer that estimates IOP by bouncing a tiny probe off the corneal surface. The restraint of the animal for IOP measurement is important and can result in inaccurate measurements. Ideally the patient should be relaxed and not tightly restrained. Tight restraint around the neck can artificially raise the IOP; this is most commonly a problem with animals that struggle. Care should be taken to only gently retract the lids because if not performed carefully pressure from the lids on the globe can result in an artificially high IOP measurement. With 'educated' fingers digital tonometry can distinguish between a 'hard' and 'soft' globe but is far too inaccurate for diagnosing all glaucomas and monitoring IOP.
Management for glaucoma depends on several factors including the duration of the condition, whether sight is present or not and the aetiology of the glaucoma. Referral to a veterinary ophthalmologist for assessment and treatment should be considered and is highly recommended. Emergency reduction in IOP to try and limit intraocular damage from a high IOP can be achieved by topical prostaglandin analogues in dogs (not effective in cats). However, a pupil block (e.g., from anterior lens luxation, or posterior synechiation) may be a contraindication to this class of drugs because they typically cause an intense miosis. Intravenous mannitol is another option for emergency reduction of IOP so that intraocular damage is limited. Maintenance of IOP control using IOP-lowering drugs may be an option in some cases. Usually a combination of drugs is used (e.g., topical carbonic anhydrase inhibitors and beta-blockers and prostaglandin analogues). Specialised surgical intervention may be required, such as lens luxation surgery (for primary lens luxations), aqueous drainage surgery, or cryodestructive (laser) therapy. Enucleation may be indicated for permanently blind eyes with intractable glaucoma or for glaucoma secondary to intraocular tumours and serious intraocular infections.
Glaucoma remains a challenge to treat effectively, but early diagnosis and consultation with, or referral to, a veterinary ophthalmologist will help increase the chances of saving vision and the eye.