Bilateral sudden onset blindness in a Labrador

Vet Ophthalmology

Dr. Andra Enache -Res.ECVO, Royal Veterinary College, UK.

 

Signalment: Izzy, 7 years old female neutered Labrador

History:

  • Referred for assessment of sudden onset blindness
  • Developed acute blindness two months prior to referral
  • She started bumping into things all of a sudden
  • Increased thirst
  • Izzy’s pupils have been very dilated
  • The owners think that she may have some peripheral vision.
  • She has been more clingy, however no abnormal behaviour.
  • Up to date with vaccinations, worming and flea treatment.
  • No travel history

Examination

  • Physical examination:
    • bright, alert and responsive, BCS 8/9 , cardiovascularly stable with a HR of 90 bpm, RR of 24 bpm, with no audible murmur or arrhythmia, pink mucous membranes
    • Abdominal palpation: comfortable
  • Ophthalmic examination
    • OU open and comfortable with no ocular discharge
    • STT-1 readings 22 mm/min OD and 24 mm/min OS
    • Menace response and dazzle reflexes absent OU
    • PLRs delayed or sluggish OU, weakPLR to red light but present to blue light (Iris-Vet chromatic PLR)
    • The pupils were very dilated at rest
    • Mild conjunctival hyperaemia OU (Figure 1)
    • Slit lamp examination: no corneal changes OU, multi-focal iridal pigmentation OD, nuclear sclerosis OU, mild anterior vitreous degeneration with mild pigment deposits OU, no aqueous flare OU
    • IOP: 17 mmHg OD and 20 mmHg OS
    • Unable to navigate in the room or negotiate obstacle course in photopic and scotopic conditions
    • Funduscopy: mild tapetal hyper-reflectivity OU (Figure 2), moderatedepigmentation of the non-tapetal fundus with dark degenerative areas at the junction with the tapetal fundus (Figure 3). The optic nerves were well myelinated with subjectively attenuated vasculature in the right eye and tortuous venules in the left eye.

fig.1

Figure 1. Izzy’s right eye, mild conjunctival hyperaemia and focal iridal pigmentation medially at 4 o’clock and laterally at 9 o’clock (RVC).

 

fig.2

 

Figure 2. Izzy’s left eye, note the pupil relatively dilated at rest and mild conjunctival hyperaemia (RVC).

fig.3

Figure 3. Izzy’s eyes at initial presentation after dilation with tropicamide, note the bright tapetal reflex OU (RVC)

 

fig.4

Figure 4. OD funduscopic appearance, note the hyperreflective tapetum lateral to the optic disc. The optic nerve is well vascularized and note the depigmentation of the non-tapetal fundus which appears grey (RVC).

 

Problem list

  • Polydipsia
  • OU Absent menace responses and absent dazzle reflexes
  • Dilated pupils
  • Funduscopic changes
  • Conjunctival hyperaemia

Differential diagnoses

  • Polydipsia
    • Primary: behavioural (pshychogenic), fever, encephalopathy, pain, neurologic disorder
    • Diabetes mellitus/insipidus, hyperadrenocorticism, renal disease, hypercalcemia, Addison’s disease
  • Absent menace responses with PLR deficit:
    • Degenerative disorders: retinal degeneration, Sudden acquired retinal degeneration syndrome (SARDS)
    • Inherited retinal disorder: progressive retinal atrophy (PRA)
    • Bilateral uveitis/ retinitis/chorioretinitis/retinal detachment
    • Optic nerve hypoplasia/ neuritis
    • Optic chiasm or lesion affecting the optic tracts to the level of the lateral geniculate nuclei:
      • Neoplasia: meningioma
      • Inflammatory/infectious CNS disease
      • Vascular: ischaemic necrosis of the optic chiasm
  • Absent dazzle reflexes OU:
    • Retinal degeneration
    • Optic nerve atrophy/cupped optic nerve head
    • Retinal detachment
  • Dilated pupils:
    • SARDS
    • Oculomotor nerve deficits (parasympathetic loss): external/internal ophthalmoplegia
    • Glaucoma
    • Retinal degeneration
    • Retinal detachment
    • Iris atrophy
    • Chronic uveitis
  • Funduscopic changes:
    • Tapetal hyperreflectivity: SARDS, retinal degeneration, PRA
    • Depigmentation of non-tapetal fundus: retinal degeneration, optic neuritis
  • Conjunctival hyperaemia:
    • SARDS
    • Conjunctivitis
    • Keratoconjunctivitissicca
    • Uveitis
    • Glaucoma
    • Orbital disease

Diagnostic plan

  • Full neurological examination
  • Biochemistry and haematology
  • +/- ACTH stimulation test, dexamethasone suppression test, urinalysis, abdominal ultrasound
  • Fundoscopic examination to look for retinal disease and optic neuritis
  • Electroretinography (ERG) to look for sudden acquired retinal degeneration (SARD)
  • +/-Magnetic resonance imaging (MRI)of the brain + cerebrospinal fluid analysis if the ERG was consistent with a normal retinal functionto look for optic neuritis or brain tumour

Results of investigations

  • Neurological examination (Figures 5-8): no change in mental status, no abnormal body posture, normal postural reactions, no abnormal gait, no cranial nerve deficits.

Fig.5-8

Figures 5-8. Izzy’s neurological examination. There were no cranial nerve deficits apart from absent menace response, no pain at palpation along the spine, no proprioceptive deficits and positive withdrawal (RVC).

 

  • Biochemistry: unremarkable apart from mildly decreased inorganic phosphorus 0.63 * (0.8 – 1.6 mmol/l) and raised cholesterol 7.13 * (3.2 – 6.2 mmol/l) with no clinical significance (Appendix 1)
  • Haematology: unremarkable with mild lymphopaenia- lymphocytes 0.51 * (1 – 4.8) consistent with stress leukogram or steroid-responsive
  • Further blood tests and abdominal ultrasound were not performed at this stage as not justified by the initial bloodwork results and the lack of systemic clinical signs.
  • An electroretinogram was performed under a short general anaesthetic after sedation with butorphanol 0.2 mg/kg i.v. (Alvegesic®, 10 mg/ml, Dechra) (Appendix 2). Izzy was anaesthetized in case the results of the ERG were normal she could undergo an MRI scan of the brain. A standard short protocol as described by Narfstrom et al., 2002 was performed.
    • Topical anaesthetic was applied to both eyes (proxymethacaine 0.5% minims, Bausch&Lomb), followed by placement of ground, reference and corneal electrodes.
    • Flat amplitudes OU indicative of absent retinal function (Figure 9)

fig.9

Figure 9. Results of ERG. Blue trace indicates OS recording and red trace for OD. There is no recorded b-wave in either eye consistent with a flat electroretinogram and absent retinal function (RVC)

fig.9b

 NORMAL ERG (RVC)

  • An MRI was not pursued after discussion with theowner.

Diagnosis

  • Suspected Sudden acquired retinal degeneration syndrome (SARDS)

Treatment

  • No treatment was recommended
  • A handout on ‘how to help your blind dog’ was given to the owners, including advice on how to deal with this condition.
  • The owner was advised to continue monitoring Izzy for development of systemic clinical signs including neurological signs in which case an MRI scan would be recommended.

Follow-up

  • Further blood tests to be pursued at referring vets if the clinical signs of polydipsia were not improving a month later.
  • Telephone update a month later: Izzy has not developed any further systemic clinical signs and has adapted well to her blind status; she was still drinking excessively.

Discussion

Sudden onset of blindness is frequently related to either ocular or CNS abnormalities. Following ERG, Izzy was diagnosed with likely SARDSa retinopathy with a yet unknown etiopathogenesis which leads to irreversible blindness. Laboratory studies have shown that this disease is characterized by antibody-mediated apoptosis of the photoreceptors with outer segments loss. Middle-aged dogs, mostly female neutered are affected by sudden loss of vision (Auten et al., 2018). Most commonly reported breeds were Dachshund, miniature Schnauzers, Pug and Brittany (Stuckey et al., 2013, Auten et al., 2018) with Labrador retriever significantly underrepresented (Auten et al., 2018). Dogs of 10-20 kg and 6-10 years of age were overrepresented in the same study.

These dogs also present with various systemic clinical signs including polydipsia, polyuria, lethargy and weight gain, with 56% of dogs having at least a systemic clinical sign in a recent study (Auten et al., 2018). About 50% of dogs have no dazzle reflexes and incomplete PLR. Only 6% of dogs had no PLR in one study (Auten et al., 2018). Conjunctival hyperaemia was reported in SARDS cases, with approximately 17% having mild bilateral hyperaemia. Funduscopic changes are usually absent in the initial phase, however tapetalhyperreflectivity and vascular attenuation was reported in 68% of cases presenting more than 5 weeks after the onset of blindness (Auten et al., 2018, Komaromy et al., 2016).Izzy presented for assessment two months after the sudden onset blindness. Therefore, she had increased tapetal reflectivity and subtle vascular attenuation at funduscopy.

Most cases have bilaterally mydriatic pupils with diminished response to bright white light. Grozdanic et al., recommended testing chromatic pupillary light reflexes as a diagnostic tool to assess photoreceptor function. SARDS cases show a lack of response to bright (200kcd/m²) red light of 630 nm wavelength which stimulates cone photoreceptors (Grozdanic et al., 2007, 2013). The retinal ganglion cells are not initially affected by SARDS, therefore PLRs are present when stimulated with a bright blue light of 480 nm wavelength that corresponds to the spectral sensitivity of melanopsin. Dogs with retinal disorders show an absent response to red light but intact to blue light, on the contrary in case of optic nerve disorders there is an absent response to blue light and intact to red light. Izzy’s case demonstrated no response to red light and intact to blue light.

An MRI of the brain would have been useful to rule out central blindness, however this was finally not performed after discussion with the owner. Considering an absent retinal response on the ERG, SARDS would have been the most likely diagnosis. However, the owner was carefully counselled to continue monitoring Izzy in case further systemic or neurological clinical signs were developing in which case further investigations would be required.

Regarding treatment, several drugs have been trialled including steroids, intravenous immunoglobulins and immunosuppressive drugs. Although recent research has shown an immune-mediated pathogenesis for SARDS (Grozdanic et al., 2018) further immunosuppressive treatment has failed to show results (Young et al., 2018). Although no further testing was performed in Izzy’s case it is possible that the concurrent polydipsia was due to an atypical hyperadrenocorticism which has been suggested to occur as a physiologic response to stress due to sudden vision loss. This is usually not seen in dogs with optic neuritis. Izzy also showed mild lymphopenia due to stress leukogram.

The owners were advised that dogs with SARDS usually enjoy a good quality of life as the condition is not painful and they cope well with the loss of vision.

 

References (10)

Stuckey, J.A., Pearce, J.W., Giuliano, E.A. et al. (2013) Long-term outcomeof sudden acquired retinal degeneration syndrome in dogs.Journal of the American Veterinary Medical Association; 243:1425–1431.

Auten, C.R., Thomasy, S.M., Kass, P.H. et al. (2018) Cofactors associated with Sudden Acquired Retinal Degeneration Syndrome: 151 dogs within a reference population, Veterinary Ophthalmology, 21, Issue 3, 1-9

Grozdanic, S.D., Matic, M., Sakaguchi, D.S. et al. (2007) Evaluation of retinal status using chromatic pupil light reflex activity in healthy and diseased canine eyes. Investigative Ophthalmology and Visual Science; 48: 5178–5183.

Grozdanic, S.D., Kecova, H., Lazic, T. (2013) Rapid diagnosis of retina and optic nerve abnormalities in canine patients with and without cataracts using chromatic pupil light reflex testing. Veterinary Ophthalmology; 16: 329–340.

Grozdanic, S.D., Lazic, T., Kecova H. et al. (2018) Optical coherence tomography and molecular analysis of sudden acquired retinal degeneration syndrome (SARDS) eyes suggests the immune‐mediated nature of retinal damage, Veterinary Ophthalmology, 1-23.

Heller, A.R., Woerdt, A. van der, Gaarder, J.E., et al. (2017). Sudden acquired retinal degeneration in dogs: breed distribution of 495 canines, Veterinary Ophthalmology, 20, 2, 103-106.

Komaromy, A.M., Abrams, K.L., Heckenlively, J.R., et al. (2016) Sudden acquired retinal degeneration syndrome (SARDS) ‐ a review and proposed strategies toward a better understanding of pathogenesis, early diagnosis, and therapy. Veterinary Ophthalmology, 19:319‐331.

 

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