Assessment of Vision Loss without Electrophysiology

As a medical retina and electrodiagnostics subspecialist, I often receive referrals from my colleagues for patients with unexplained vision loss. These patients often present with poor visual acuity and sometimes with suboptimal visual field, but a detailed examination, including imaging, does not find anything wrong with the eye. Sometimes electrodiagnostics (ERG, VEP, etc.) is required to clinch the diagnosis, but here is some subtle information you can glean from the Spectralis viewing system.

Get data on refraction

Uncorrected refractive error is a frequent cause of vision loss. However, in a busy ophthalmology clinic, auto-refraction or subjective refraction is only sometimes performed on our patients. Fortunately, on the Spectralis OCT, under image information, the information on the focus of the machine is recorded automatically, and this can be used to estimate the spherical refraction of the eye.

I was consulted on the case of a 19-year-old girl with mild uveitis. Also, in her most recent attendance, she found a 2D difference in refraction between her two eyes; one eye is around 1mm shorter than the other. There was no previous refraction data, and the primary ophthalmologist was concerned about an orbital mass causing the shortening of the axial length.

As this patient had multiple previous OCTs, I reviewed her previous scans and, using the data on scan focus, showed that the difference in refraction was long-standing and not a cause for concern.

Undiagnosed keratoconus is a well-documented cause of vision loss without obvious signs. A corneal tomography scan is beneficial if this condition is suspected.

Assess the effect of cataract and ocular media using OCT image quality

The effect of cataracts and ocular media opacity on vision can be estimated by looking at the image quality of the fundus. The easiest way of doing this is to look at the quality of the OCT image centring on the fovea.

‍

In a well-performed OCT with dilated pupils, one would expect the image quality to be 30dB or more. If a patient manages to have a high-quality scan despite co-existing cataracts, it is less likely that the cataract is the cause of vision loss.

Recently I was referred a 70-year-old man with visual acuity of 6/9 right and 6/30 left and bilateral nuclear sclerotic cataracts. According to the referral, the cataract is worse in the left eye, and cataract surgery was suggested. However, the OCT image quality was measured at 33dB right and 37dB left, suggesting minimal image degradation by cataracts. As a result, I suspect a posterior segment pathology and the OCT was analysed in detail - read on to find out more.

Look at the nerve fibre layer and ganglion cell layer on OCT macula scans

Usually, one would perform a specific parapapillary nerve fibre layer scan to assess the nerve fibre layer. However, useful information can be gleaned on the status of the optic nerves using the OCT macula volume scans. Loss of the macular ganglion cells is a subtle but important cause of vision loss.

‍

Furthermore, using the Spectralis segmentation tool, normal subjects' ganglion cell volume should be more than 0.4mm³.

Going back to the case discussed above, his OCT shows loss of the ganglion cell layer in the left eye - as below:

Left optic neuropathy was diagnosed, and the cataract surgery was postponed until a neuro-ophthalmology work-up was completed.

Summary

Here I demonstrated three useful techniques on the Spectralis viewing platform to help assess patients with vision loss of unclear cause. I hope you find this useful. A future post will address the use of electrophysiology in assessing patients with vision loss.

About Dr Leo Sheck

Dr Sheck is a RANZCO-qualified, internationally trained ophthalmologist. He combined his initial training in New Zealand with a two-year advanced fellowship in Moorfield Eye Hospital, London. He also holds a Doctorate in Ocular Genetics from the University of Auckland and a Master of Business Administration from the University of Cambridge. He specialises in medical retina diseases (injection therapy), cataract surgery, ocular genetics, uveitis and electrodiagnostics.