How Much Can Results Vary Between Two Different OCT Scans?

​Here is My Question:
I had an OCT scan done a few months ago, a year after sudden loss of vision in my left eye. The results showed that the RNFL thickness was 82 in my left eye and 92 in my right eye. 

I saw a neuro-ophthalmologist earlier this month and that OCT scan showed a thickness of 82 in my left eye and 86 in my right eye. She told me that my first OCT scan looked as if I had had at least mild optic neuritis but the newest scan shows that both nerves are thin, making the diagnosis of optic neuritis less likely. She hypothesized that my thin RNFL could be congenital but I don't think so as I have really good vision, better than 20/20 and optic nerve hypoplasia varies from total blindness to near normal vision. I can't find any research that shows you can have better than normal with ONH. 

I'm wondering if I may have a subclinical attack in my right eye between both scans, accounting for the 6 point difference between each scan in my right eye. Is it normal to have two different OCT scans vary that much without the eye undergoing damage? 

I also had a VEP test done and the low-frequency latencies were 111 in my left eye and 113 in my right eye. This, I guess, is within normal range. I did, however, take an oral steroid pack and my last pill was the day before the exam. Could oral steroids have any bearing on the VEP test? I didn't even think to mention that I had been taking them as I was really sick and I just wanted to get through my appointment and go home. 

Answer:
I would say that the diagnosis of optic neuritis is a clinical diagnosis and that the thinning of the OCT helps confirm the diagnosis, but not necessarily enough to make the diagnosis.

If you had acute onset painful vision loss in one eye (pain worse with eye movement) and your eye exam, MRI orbits and other exam findings were all consistent with optic neuritis, then that is probably still the diagnosis.

The thinning in the unaffected eye could be due to subclinical optic neuropathy which may occur in the setting of multiple sclerosis or other demyelinating diseases of the central nervous system associated with optic neuritis (MS is the most common cause however).
 
I don’t know what the normal values are for the lab where you had the VEPs performed (those values could be normal in both eyes but different labs have different values for normal ranges (so maybe they are both slightly prolonged in both eyes?).
 
The pattern of the thinning on the OCT might be helpful in distinguishing optic nerve hypoplasia versus optic neuritis too. But again, I would make the diagnosis of optic neuritis based more on the history of your vision loss, the eye findings at the time of the sudden vision loss and the MRI findings at that time too. 
 
It is hard to give an answer without seeing the detailed OCT data however and I think your case is complicated enough that I cannot adequately answer  your question just based on the information offered so far.
 
Benjamin Osborne, MD
Associate Professor of Neurology and Ophthalmology
Director, Neuromyelitis Optica (NMO) Clinic
Director, Neuro-Ophthalmology Clinic
Associate Director of the NIH/Georgetown Neurology Residency Program
Medstar Georgetown University Hospital
3800 Reservoir Road, NW 7PHC
Washington, DC 20007