“Why do our patients go blind? Acute blindness in companion animals” is the most recent veterinary webinar from Ron Ofri on ophthalmological themes with more to come later this year. The topics are always very clearly explained with great illustrations and this presentation maintains the very high standards that have been set previously.
Ron begins by emphasising the importance of the history and physical examination with three important questions.
- Is the onset of blindness sudden or gradual? This helps distinguish acquired from inherited causes.
- Preferential loss of night vision?
- Is the dog healthy? Looking for signs of neurological disease, and signs of systemic disease
Several sub-cortical reflexes are outlined. These are the pupillary light reflex (PLR) and the dazzle reflex. Surprisingly, if you are new to the discipline, or revising, in an animal with cortical blindness the PLR and dazzle reflex will be normal. Also, minimal stimulation can still elicit these reflexes, in animals with advanced cataracts and inherited retinal degeneration for example. The physiology for this is explained with a simplified line diagram of optic nerve pathways. This ‘simple guy’s approach’ continues with a table contrasting normal and abnormal ophthalmological examinations with expected PLR response.
The next part of this webinar examines how to localise the lesion, according to the preceding table, with the first question ‘Can light reach the retina?’ Problems such as blepharospasm, corneal oedema, hypopyon/hyphaema/ cataracts and various opacities are summarised. The second question asks if the retina is normal looking? Here progressive retinal atrophy (PRA), also known as outer retinal degeneration, and chorioretinitis are discussed.
Depending on the examination and PLR findings further conditions may be suggested and confirmed. Included are glaucoma, retinal detachment, proximal optic neuritis, diseases of the distal optic nerve, chiasm or proximal tract, and the condition known as SARDS. This is described in some detail at the end of the webinar. Finally, there is a comprehensive list of congenital lesions, metabolic causes, infectious causes, inflammatory CNS diseases, toxins, (e.g. lead) and finally forebrain disease (refer to neurology if you can!)
The remaining section of the webinar looks at acquired retinal diseases.
- Retinal haemorrhage
- Retinal inflammation
- Retinal detachment
This is an exceptionally good section crammed full of world-class illustrations. The first of these, retinal haemorrhage is a clinical sign, not a diagnosis. The underlying diseases that could account for the sign are listed and reference is made to a recent article from Cornell University, which demonstrated that diabetes, systemic hypertension, hypothyroidism, chronic renal disease and a few cases of Cushing’s disease and multiple myeloma accounted for 50 of 83 dogs. Beautiful retinal pictures of haemorrhage due to thrombocytopenia, hypertension and trauma continue, followed by suggestions for investigation and treatment.
Next is retinal inflammation containing information on anterior uveitis (which has numerous causes) and posterior uveitis (which is mostly infectious). The numerous causes of anterior uveitis are tabulated- getting on for 70! This is made less arduous by citing the most recent academic articles evaluating the condition retrospectively. Causes of feline uveitis are also listed –a more manageable 22 with just four considered leading causes. These are FIV, FeLV, FIP, and toxoplasmosis. In some parts of the world, fungi are important. A series of fundus photographs illustrate inflammatory signs before considering treatment.
Retinal detachment is a separation between the sensory retina at the retinal pigment epithelium (RPE) and underlying choroid. The retina usually remains attached at the ora serrata and optic nerve head. A histology slide demonstrates this very well, and I was amazed that an ultrasound image produced an almost identical picture. The pathogenesis can include serous detachment, traction detachment and rhegmatogenous detachment. These are all clearly explained and the latter is the most common.
Retinal detachment usually results in a blind eye developing acutely. A series of tests and results of ophthalmological examinations takes you through the diagnostic process. An extraordinary picture shows a detached retina moved up against the lens. The retinal vessels can be clearly seen without the need for an ophthalmoscope. Some aspects of treatment follow before moving on to the detailed description of SARDS I mentioned earlier.
SARDS is the condition known as Sudden Acquired Retinal Degeneration Syndrome. It results in acute blindness with negative electroretinography, but initially with a normal looking fundus. The pupils may respond slowly to white light, but no response to red and responsiveness to blue light, (using specialised equipment.)
It is a condition of older dogs (mean age 7-10 years) of which 60-90 % are female. It is most common in mixed breed dogs and small breeds. There is often weight gain, lethargy, polydipsia, polyuria, and polyphagia. These signs may precede blindness or occur simultaneously. Curiously polyphagia may increase in severity with other signs decreasing or disappearing completely.
Some of these signs suggest Cushing’s disease with similar haematological abnormalities but only 20% of dogs are diagnosed with Cushings, with only 2 SARDS cases reported in dogs with pre-existing Cushing’s. It has been suggested that the condition resembles autoimmune retinopathy in people and this condition is outlined. Currently, there are no peer-reviewed studies indicating a definitive cause in dogs.
It is possible to find purported ‘cures’ online but Ron cites two recent articles that state categorically that
- There are currently no proven safe and effective treatments available for dogs with SARDS (Oh et al, 2019)
- It is generally believed that vision loss with SARDs is permanent and that there is no treatment that can prevent or reverse SARDs related blindness (Komaromy et al, 2018)
I thoroughly recommend this webinar for its clarity and inspiration. I can imagine a whole generation of young colleagues interested in specializing in ophthalmology after watching Ron Ofri’s contributions. As I mentioned at the beginning more to come this year, including sudden blindness in cats. Worth waiting for!