44 Primary canine glaucoma
Dogs with primary glaucoma will be presented with a painful, cloudy eye. Increased lacrimation is normally present as well. Owners might be aware of visual loss in the affected eye, but frequently this is not something which they have noticed. The patient is likely to be depressed and may be inappetant. In addition, the owner might report seeing their pet make attempts at rubbing or hiding their face. The presenting signs normally develop very quickly. Affected animals are usually pedigree dogs, and aged 5–8 years, although some breeds develop primary glaucoma at a younger age – discussed in more detail later.
In most cases there is no previous history of ocular disease or relevant systemic disease. Indeed, the patient is usually completely normal one day, then suddenly develops a very sore, cloudy eye – perhaps noticed when the owner gets up or when they come in from work. Occasionally there is a history of some intermittent slight clouding and redness on a few occasions which settled before the owner became particularly worried, but this is the exception rather than the rule.
General clinical examination is unremarkable, although a mild, pain-induced pyrexia is sometimes present. The dog might resent examination of the face and the affected eye can be difficult to examine due to the severe blepharospasm. However, with patience, gentle handling, and the use of topical anaesthesia if necessary, a rewarding ophthalmic examination should be possible.
The affected eye is likely to be blind, or with severely reduced vision, demonstrated by a reduced or absent menace response. Corneal oedema will manifest as a blue–grey corneal discolouration. In addition to the clouding, the eye will also be red – episcleral congestion is usually marked and peripheral corneal vascularization develops which manifests as a red fringe around the limbus (ciliary flush) (Figure 44.1). The pupil can be assessed using a bright light source and is usually mid sized or dilated with very limited constriction when illuminated. A dazzle reflex remains if the pressure has not got too high. The indirect or consensual pupillary light reflex should be assessed and this can help with the prognosis regarding any return of vision – if there is no constriction of the fellow pupil there is less chance of a return of any useful eyesight in the affected eye.
Detailed intraocular examination is hampered by the corneal oedema but the use of indirect ophthalmoscopy will be more informative than direct ophthalmoscopy since it provides better imaging through semi-opaque media. Thus, on fundus examination, cupping of the optic disc might be appreciated – seen as a small, dark disc with the blood vessels seeming to dip down into it from the retina (Figure 44.2). Peripapillary retinal degeneration is sometimes seen, along with more widespread hyper-reflectivity and blood vessel attenuation in advanced cases.
Measurement of intraocular pressure is essential to reach a definitive diagnosis, to provide prognostic advice, to draw up a treatment plan and to monitor the effectiveness of the treatment. Without tonometry it is very difficult to assess the patient properly. If a tonometer is not available, then referral for pressure measurement should be considered. The technique for tonometry, using a Schiotz tonometer, Tonopen or TonoVet, is detailed in Chapter 1. Remember that there are several factors which can affect the intraocular pressure readings (Table 44.1).
|Breed||Higher in terriers and fluctuates more in these breeds as well|
|Age||Decreases with age|
|Degree of restraint required||Pressure around the neck will result in higher readings|
|Position of head||Higher with the head back and elevated|
|Instrument used||Slight variations between types of tonometer|
|Amount of pressure used to measure IOP||Pressure on the globe (including the forced lid opening required to access the cornea) will increase readings|
Measurement of the intraocular pressure will give the diagnosis of glaucoma but does not tell us the cause for the raised pressure. Primary glaucoma develops in the absence of other intraocular disease and is an inherited condition in several breeds of dog. Goniodysgenesis, or malformation of the drainage angle, is the most frequent cause of acute primary glaucoma in dogs, and is a bilateral condition (although usually presented as a unilateral problem). Thus it is necessary to evaluate the contralateral eye. Gonioscopy is the examination of the drainage angle and referral for this procedure should be considered in all dogs which present with acute glaucoma. It is rarely possible to perform gonioscopy on the affected eye (due to the corneal oedema, peripheral corneal vascularization and pain for example) but examination of the fellow eye will reveal evidence of goniodysgenesis and thus the second eye is also at risk from the development of glaucoma.
Other tests which can be performed as part of the case work-up include ultrasonography. This will allow detailed visualization of intraocular structures despite the opaque ocular media and can help to differentiate primary from secondary glaucoma. For example, an iridal mass or lens luxation could be detected, which would suggest that the raised intraocular pressure developed secondary to other intraocular disease. Ultrasonography can also allow measurement of the globe diameter to determine if it has been stretched and enlarged (hydro-phthalmos) as a result of the increased pressure.