SUDDEN ACQUIRED RETINAL DEGENERATION IN A DOG

History

A seven year old, male neutered Lhasa Apso presented to the clinic with sudden onset vision loss. The owner reported a recent (two month) history of marked polyphagia, with mild polydipsia, polyuria and some weight gain.

Examination

A full clinical examination was undertaken along with an ophthalmic examination as per Appendix 1. Clinical examination was unremarkable apart from being mildly overweight (11.2 kg) and having a slightly pendulous abdomen. All relevant findings are recorded below.

OU: - Mild serous discharge

- Hair at caruncles

- Mild conjunctivitis

- Menace response absent

- Resting pupil mid dilation

- PLR, direct and indirect - slow

- Equivocal dazzle response

- Fundic examination unremarkable

STT L = 21 mm/minute, R = 19 mm/minute

IOP L = 22mmHg, R = 22mmHg

Diagnosis Acute prechiasmal blindness: - Sudden acquired retinal degeneration syndrome (SARDS) - Bilateral optic neuritis (retrobulbar) - Optic chiasma neoplasia / granuloma / compression - Central nervous system disease

Further tests

Electroretinogram (ERG) - performed on my behalf by a colleague

Routine haematology and biochemistry

Urinalysis

Results

ERG - flatline OU

Blood results:

- moderately raised white cell count with lymphocytosis

[19.75 x 109/ litre (6-7)]

- raised alanine aminotransferase

[215 U/litre (10-118)]

Urinalysis - unremarkable

Treatment

Fucidic acid (Fucithalmic, Leo) BID, OU Amoxycillin with clavulanic acid (Synulox, Pfizer) 12.5 mg/kg BID PO

Follow up

Repeat blood tests ten days later showed the biochemistry results to be similar however the white cell count had returned to within normal range.

Ophthalmic re-examination one month later showed evidence of retinal degeneration: tapetal hyperreflectivity and visualisation of streaking choroidal vessels under the thinned retina.

Discussion

SARDS was first described in the early 1980s. SARDS is a sudden onset disease causing irreversible blindness of unknown aetiology (1). It is typically seen in middle aged dogs with females being over represented along with Brittany Spaniels, Dachshunds and Miniature Schnauzers (2). It is usually accompanied by systemic clinical signs such as polyuria, polydipsia, polyphagia, weight gain and hepatomegaly. Many of these dogs have signs (including blood results) suggestive of Cushing's disease however they test negative for adrenocortical dysfunction and these clinical signs often resolve after a few months (3).

The main differential diagnoses are bilateral optic neuritis (affecting the retrobulbar section of the optic nerve or the optic chiasm, as the optic discs appear normal), neoplasia/ other space occupying lesion affecting the optic chiasm, or CNS disease.

The ERG is an electrical summation generated from the outer half of the retina (4). In SARDS there is a complete lack of photoreceptor activity, extinguishing the ERG reading. A "flat line" ERG trace thus distinguishes SARDS from the other differential diagnoses and is therefore pathognomic for this condition (5).

Although described as sudden onset blindness (or so it may appear to the owner) it can actually occur over a few days to weeks. The immediate histological change is destruction of the photoreceptor outer segments, and the visible signs of retinal atrophy follow more slowly (6). The PLR is not immediately eradicated in these dogs and this is true of many functionally blind dogs with retinal atrophy due to other causes. This may be because some nerve fibres contain a light sensitive pigment of their own at low levels (melanopsin), which may cause this persistence of the PLR.

There have been some suggestions that this is an immune mediated condition, especially as there is a similar immune mediated syndrome in people - cancer associated retinopathy (CAR). In humans the most common tumour associated with this syndrome is small cell lung carcinoma. When a relationship was sought between neoplasia and SARDS none could be found (3). In addition no increase in the amount of antiretinal autoantibodies, in comparison to normal dogs, was seen in dogs affected by SARDS (7).

Histologically there is apoptosis (a gene directed, active process of cellular suicide with no associated inflammation) within the outer layer of the retina (8), suggesting that this is one of the main mechanisms for photoreceptor death in SARDS. Apoptosis is otherwise a very rare event in adult tissue but has been shown to be the end stage event in several inherited photoreceptor degenerative diseases. Steroid hormones can cause apoptosis in some cell types and because these dogs exhibit signs of Cushing's disease, without having the disease, it may be that the retinal degeneration is toxic in origin due to an aberrant corticosteroid.

All of these theories, put forward as possible causes for SARDS, are as yet unproven. As we do not know the cause we cannot offer any treatment to these animals, or predict when this syndrome will occur. The retinal degeneration is total and irreversible although the systemic signs do not seem to precede any ongoing systemic disease so the prognosis for the health of the dog is good.

References

1. Cullen CL, Grahn BH (2002) Diagnostic ophthalmology. Acute prechiasmal blindness due to sudden acquired retinal degeneration syndrome. Canadian Veterinary Journal 43 (9), 729 -730.

2. Millichamp NJ (1990) Retinal degeneration in the dog and cat. Veterinary Clinics of North America: Small Animal Practice. 20 (3), 799 - 835.

3. Gilmour MA, Cardenas MR, Blaik MA, Bahr RJ, McGinnis JF (2006) Evaluation of a comparative pathogenesis between cancer- associated retinopathy in humans and sudden acquired retinal degeneration syndrome in dogs via diagnostic imaging and western blot analysis. American Journal of Veterinary Research 67 (5), 877 - 881.

4. Martin CL (2001) Evaluation of patients with decreased vision of blindness. Clinical Techniques in Small Animal Practice 16 (1), 62 - 70.

5. Curtis R (1988) Retinal diseases in the dog and cat: an overview and update. Journal of Small Animal Practice 29, 397 - 415.

6. O'Toole D, Roberts S, Nunamaker C (1992) Sudden acquired retinal degeneration ('silent retina syndrome') in two dogs. Veterinary Record 130, 157 - 161.

7. Keller RL, Kania SA, Hendrix DVH, Ward DA, Abrams K (2006) Veterinary Ophthalmology 9 (3), 195 -200.

8. Miller DE, Galbreath EJ, Kehren JC, Steinberg H, Dubielzig RR (1998) Photoreceptor cell death by apoptosis in dogs with sudden acquired retinal degeneration syndrome. American Journal of Veterinary Research 59 (2) 149 - 152.