In keeping with our cranial nerve summer theme, I thought you might enjoy this interesting study about visual fixation testing in dogs.
What is Visual Fixation testing?
Visual fixation testing (VFT) is performed in humans with vestibular disease to help differentiate between central and peripheral lesions. For this test, patients are asked to fix their gaze on a target and then a bright penlight is shone into one eye while the other is covered. Patients with peripheral disease can suppress spontaneous nystagmus during testing, while those with central disease cannot. Exceptions were described such as those with focal cerebellar lesions, however the majority of humans with central lesions fail this test.
Visual fixation testing in dogs?
We clearly cannot ask dogs to focus their gaze, so would this work in veterinary patients? A recent study from the UK asked this question. They omitted the request to fixate and simply covered one eye while shining a bright focal light into the other eye. The slow phase of nystagmus was expected to increase in velocity with peripheral vestibular disease, therefore increasing the number of beats counted in 10 seconds compared to prior to VFT. Dogs with central disease were expected to have no change in beat frequency.
Results
· Almost ½ of the dogs had spontaneous nystagmus in both the peripheral and central group (48% and 44% respectively).
· During the VFT, an increase in beat frequency occurred in 33% of the peripheral group
· Only 1 case (6%) had an increase in beat frequency in the central group.
· The change in frequency was significantly different between central and peripheral vestibular disease, with more dogs showing an increase in beat frequency in the peripheral group.
· An increase in slow phase velocity was significantly associated with a peripheral diagnosis and this was significant even after the dogs with idiopathic disease were removed.
· Dogs with peripheral vestibular lesion localization had a 95% of having peripheral diagnosis with a neurologic examination alone. When accounting for the VFT, this probability increased to 99% (1% of central disease).
· Dogs with central lesion localization based on neurologic examination alone, had a 17% chance of peripheral disease. If they demonstrated an increase in beat frequency this increased their likelihood of peripheral disease to 52%.
Take away message
While only 1/3 of dogs with peripheral vestibular disease had an increase in beat frequency with the VFT in this study, this tool is one more easy option to attempt to localize vestibular disease more specifically. Central vestibular disease is frequently correctly localized based on the neurologic examination. On the contrary, 20% of dogs with central disease are diagnosed with peripheral vestibular disease. If a dog with suspected peripheral lesion localization has an increase in beat frequency during VFT, this can further support our lesion localization.
Reference: https://onlinelibrary.wiley.com/doi/epdf/10.1111/jvim.70182
Thanks for reading! I hope you enjoyed this “bright” TidBit Tuesday during this sunny summer day! Have a great rest of your week.