Ron Ofri DVM PhD DECVO always delivers practical and information laden webinars and his January offering covering acute blindness in companion animals was no exception. This webinar was not filled with ‘difficult to digest’ complicated diagnostic pathways deemed necessary to diagnose these cases of acute onset blindness. Instead Ron kept it simple by delivering a logical and straightforward approach.
Firstly Ron advises asking three simple questions:
1. Was the animal’s blindness sudden or gradual in its onset?
2. Was there a preferential loss of night vision?
3. Is the animal well otherwise?
The answers to these questions should give some insight into what the underlying cause might be. Inherited causes of blindness are more likely to be associated with a gradual onset of blindness and are more likely to affect night vision. Some causes of blindness can also be secondary to a number of systemic and neurological diseases, so ascertaining whether an animal is otherwise healthy is crucial in order not to miss any underlying causes.
Checking the direct and consensual PLR alongside the dazzle reflex are also a given when performing an ocular examination in these patients. However Ron reminded us that even if these reflexes are present, this does not imply that the affected animal has vision. Ron explains there needs to be very little afferent input for the presence of a PLR , meaning this reflex will not be dulled even in the presence of advanced cataracts or retinal degeneration.
Localising the lesion is the key to diagnosing the underlying cause for a patient’s blindness and Ron explained how to do this in a very simple way. He advises using a grid system which can be found within the webinar and four separate scenarios can be identified using this grid system:
1. Normal ocular examination and normal PLR
2. Normal ocular examination and no PLR
3. Abnormal ocular examination and normal PLR
4. Abnormal ocular examination and no PLR
Lesions can then be localised dependant on which category your patient falls. For example a patient with a normal examination and normal PLR is more likely to have metabolic conditions such as hepatic encephalopathy or hypoglycaemia. They may also be suffering from neurological disease and referral to our neurology colleagues would be wise. However if a patient has an abnormal examination but a normal PLR then it is likely there is some form of opacity preventing light getting to the retina and may include conditions such as cataracts or corneal oedema. Underlying retinal disease must also be ruled out. Diseases consistent with an abnormal examination and no PLR include glaucoma, retinal detachment and optic neuritis. Finally the patient with a normal examination and an absent PLR is likely to be suffering from diseases of the distal optic nerve, chiasm or proximal tract. They may also be suffering from SARDS (sudden acquired retinal degeneration syndrome).
Ron discussed acquired retinal diseases in much greater depth including retinal haemorrhage, retinal inflammation, retinal detachment and SARDS which, in my opinion, is a fascinating syndrome. Patients presenting with SARDS are usually middle aged moderately overweight females who present with acute blindness and will often have blood work and clinical signs highly suggestive of cushings disease. However classic cushings disease is only diagnosed in 20% of these patients and only two known cases of SARDS have been diagnosed in dogs with pre-existing cushings disease. Also in patients diagnosed with SARDS the systemic signs associated with the disease such as PUPD and polyphagia usually disappear over time. Sadly however the blindness associated with SARDS remains permanent. Ron also discussed a really interesting comparison between dogs with SARDS and humans with an autoimmune retinopathy which usually affects moderately overweight middle aged women and presents in a very similar way to dogs with SARDS.
As always Ron’s webinar delivers really practical advice on how to approach patients presenting with acute blindness in a calm and logical way which will undoubtedly offer confidence to vets working up these challenging cases.