45 Secondary canine glaucoma
Most dogs with secondary glaucoma will present with a painful eye and colour change – redness or grey/blueing of the cornea. The owner might or might not be aware of visual impairment in the affected eye. Since the condition is normally unilateral the reduction in vision in the affected eye is not always apparent to owners. The dog can be any breed, including crossbreeds, and is usually middle aged or older. The owner might have noticed the dog rubbing and the eye looking red or simply ‘different’ for days or weeks prior to seeking veterinary advice. The dog might also be slightly ‘depressed’ and quiet. Occasionally the presentation will be very acute, with no previous ocular changes, although this is less common with secondary than with primary glaucoma.
Many dogs will have a history of previous ocular disease, although this is not always the case, and the dog might not have received veterinary treatment despite the owner being aware that there was something ‘not quite right’ with the eye for a while before it became red and painful. The most frequent previous ocular problem will be anterior uveitis – traumatic, immune-mediated or infectious – which responded to treatment initially but on cessation of anti-uveitis medication the eye deteriorated again a few days or weeks later. Sometimes owners will have restarted the previously prescribed medication but this did not improve the eye and so they returned for another examination. The cases with an acute history might be associated with lens luxation, so this should be considered in all terrier breeds (see Chapter 33 on uveitis and Chapter 42 on lens luxation).
On general clinical examination the dog might be slightly depressed with a mild pyrexia. It might be head shy and resent examination of the affected eye. A thorough general examination should be undertaken since the ocular problems might be associated with systemic disease (see Chapter 33). The presentation is normally unilateral for secondary glaucoma, but if a bilateral uveitis has been present, then both eyes can develop secondary glaucoma more or less simultaneously.
On ocular examination the affected eye might be noticeably enlarged. This is more common in long-standing cases or in younger dogs and puppies where the globe is able to stretch more easily (see Figure 37.5). The eye is likely to be red, with conjunctival hyperaemia and episcleral congestion; some peripheral corneal vascularization (ciliary flush) is also likely to be present. The eye is probably blind with no menace response, and the pupil is normally dilated and unresponsive, although if a previous uveitis has been present the pupil might be miotic, especially if extensive posterior synechiae are present. Alternatively, the pupil might be misshapen (dyscoria) (Figure 45.1). Thus the actual size of the pupil is not always useful in assisting with the diagnosis of secondary glaucoma.
The cornea will not be normal. Some corneal oedema is likely, giving a steamy appearance, such that detailed intraocular examination might be difficult. Corneal vascularization, pigmentation and ulceration are all possible (often associated with exposure keratitis). In addition, corneal striae (Haab’s striae; see Figure 44.4) might be present. These appear as a double white line on the cornea, almost like narrow train tracks, and are due to breaks in Descemet’s membrane which form as the globe stretches. If there is any ongoing uveitis, this might manifest as aqueous flare, hypopyon or hyphaema.
Careful examination of the iris is important as neoplasia with a uveal mass is a common cause of secondary glaucoma in older dogs. If a mass is suspected, it is important to look tangentially through the pupil to see if any mass is present in the vitreous extending from the ciliary body – what is visible on the anterior surface of the iris is often just the tip of the iceberg, with a large ciliary body component also present. If fundus examination is possible this is likely to reveal optic disc cupping and possibly some retinal degeneration.
Measurement of intraocular pressure is essential in any suspected cases. Referral should be offered to the client if this is not possible in the practice. Both the absolute reading and the comparison between the two eyes are considered.