Diagnostic Ophthalmology

History and clinical signs

A 6-month-old female Cavalier King Charles spaniel was examined at the ophthalmology service at the Western College of Veterinary Medicine for evaluation of ocular abnormalities in the right eye. The menace responses, palpebral, occulocephalic, and direct and consensual pupillary light reflexes were present bilaterally. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were 20 mm/min bilaterally. The intraocular pressures were estimated with a rebound tonometer (Tonovet; Tiolat, Helsinki, Finland) and were 12 and 11 mmHg in the right and left eyes, respectively. Fluorescein staining (Fluorets; Bausch & Lomb Canada, Markham, Ontario) was negative bilaterally. On direct examination 2 linear, pigmented structures were extending from the iris to the lens, associated with a circular lens opacity. Following application of 0.5% tropicamide (Mydriacyl; Alcon Canada) biomicroscopic examination (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) revealed the linear pigmented structures to be uveal tissue extending from the collarette region of the iris to the anterior lens capsule. The lens opacity involved the axial anterior cortex of the lens. Indirect ophthalmoscopic (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) examinations were completed bilaterally and no further abnormalities were detected. A photograph of the right eye at presentation is provided for your assessment (Figure 1).

Figure 1.

Photograph of the right eye of a 6-month-old Cavalier King Charles spaniel.

What are your clinical diagnosis, differential diagnoses, therapeutic plan, and prognosis?

Discussion

The clinical diagnosis was iris-to-lens persistent pupillary membranes (PPMs) with incipient cortical cataract. The pupillary membrane is an embryonic mesodermal structure consisting of vascular and connective tissue that forms a solid sheet continuous with the iris at the level of the collarette (1,2). The iris anatomy consists of a central pupillary zone and a peripheral ciliary zone, and the collarette is the demarcation between these zones (3). Regression of the pupillary membrane normally occurs between the 6th week of canine fetal development and 6 wk after birth (4,5). However, the process can take longer in some dogs (6). Incomplete regression of the pupillary membrane results in remnant strands of uveal tissue termed persistent pupillary membranes (PPMs) (2). Persistent pupillary membrane strands originate at the level of the collarette and may bridge from the iris collarette in one location to the collarette in another location (iris-to-iris PPM); or they may bridge from the iris collarette posteriorly to the lens (iris-to-lens PPM) or anteriorly to the cornea (iris-to-cornea PPM). The differential diagnosis for an iris-to-lens PPM is a posterior synechiae. Synechia can be distinguished from PPM, however, as they originate from the iris pupillary margin rather than the collarette.

Persistent pupillary membranes are the most common form of anterior segment dysgenesis, a term used to describe the congenital malformation of the anterior segment of the eye. The most common form of PPMs are single iris-to-iris strands and are usually incidental and of little significance. More significant forms of PPMs include those that bridge from iris-to-lens and iris-to-cornea as these forms cause opacities of the lens and cornea, respectively. Contact of the uveal tissue with the lens results in lens capsule and anterior cortical cataract as well as capsular pigmentation (2). Severe forms of the condition can result in blindness due to multiple strands of uveal tissue attaching to the lens or disrupting the corneal endothelium resulting in extensive opacification of the lens or cornea, respectively. Persistent pupillary membranes may be unilateral or bilateral and different forms may occur in each eye.

Persistent pupillary membranes may occur in any breed of dog. Although the mode of inheritance is not known, they are suspected to be inherited in the basenji, Pembroke Welsh corgi, and mastiff (2,4,7). In these breeds it is not recommended to breed any dog with an iris-to-lens or iris-to-cornea PPM. In general, breeding of any dog with a severe form of the condition is not recommended by the Genetics Committee of the American College of Veterinary Ophthalmologists.

Treatment is rarely necessary for most forms of PPMs and prognosis for the minor forms is excellent. For eyes with severe manifestations of generalized corneal or lens opacity, the therapeutic options are limited and prognosis for vision is poor. Phacoemulsification may be completed in eyes with extensive cataract associated with iris-to-lens PPMs. Pre-operative electroretinography and ocular ultrasound are standard prior to any cataract surgery and should be completed to evaluate for other ocular anomalies and adequate retinal function before surgery in cases with iris-to-lens PPMs. In this case, the PPMs were causing only minor lens opacity and were not associated with clinically detectable disruption of vision, thus cataract surgery was not considered to be of therapeutic benefit. It was recommended that the dog return for re-evaluation to monitor for progression of the cataract on a 6-month to yearly basis.