Approach to unexplained vision loss
Patients are often referred for evaluation of unexplained central vision loss. The patient has a history of 20/20 best corrected (e.g. with glasses or contacts) vision in the past and now vision is perhaps 20/30, 20/40, or worse and the patient has noticed a change. There is no known history of lazy eye or “amblyopia” dating back to childhood.
In our office, the evaluation consists of :
1. clinical exam
2. OCT (retinal thickness measurement)
3. retinal angiogram
Different entities may affect the macula and may cause mild to moderate central vision loss. These entities may be difficult to see with clinical exam alone. There may be vitreomacular traction or cystic spaces in the fovea. There may be changes in the retinal pigment epithelium that are difficult to see clinically but more apparent on the angiogram. Foveal thinning may be apparent on an OCT. Adult onset foveomacular dystrophy or pseudovitelliform dystrophy may be present.
Often, however, we do not uncover any abnormalities on our exam. The macula looks good clinically, angiographically, and on the OCT.
Keep in mind how the visual system works. The cornea and lens are essentially the “lens of a camera.” The retina is the “film” in the camera. The optic nerve is the cable that carries the signal to the brain. One third of the brain is dedicated to vision, to working as a receiving and procesing station for the pictures taken by the eye.
So if the retina “checks out” we need to examine the rest of the system more closely.
With regards to the cornea, is there corneal swelling, clouding, scarring, or irregular curvature? I recommend a corneal topographic map or “corneal topo” that measures corneal curvatures and thicknesses. Many primary eye care providers have an “Orbscan” machine in their office that can perform such a scan to look for these problems.
Is the optic nerve and the receiving station in the brain (the visual cortex) functioning properly? If there is optic nerve “pallor” or paleness noted on examination then the optic nerve comes to attention as a possible culprit quickly, and appropriate referral to a neuro-ophthalmologist can be made. However, early on in the course of some problems affecting the optic nerve, the optic nerve may “appear” fairly normal on dilated exam. An indirect and easy way to test the function of the optic nerve and visual cortex is a visual field test.
As a test of central vision, I recommend a Humphrey 10 degree field Sita Standard with foveal thresholds on or equivalent central visual field test. If the patient has some complaints regarding peripheral visual field, I would also recommend a Humphrey 24 degree field Sita Standard with foveal thresholds on. If the complaints are mainly to do with central vision, it’s important to do the ten degree field as sometimes a ten degree field may show a subtle central scotoma that is missed by a thirty degree field.
If either or both visual fields show an abnormality, the patient should be referred to a neuro-ophthalmologist for evaluation. It is preferable that the neuro-ophthalmologist see the patient before any MRI of the orbit and brain is ordered, and that, if needed, the neuro-ophthalmologist order the MRI. Based on the clinical suspicion, the neuro-ophthalmologist may order the MRI with special parameters (gadolinium, fat suppression, special cuts, etc.) and the neuro-ophthalmologist is in the best position to order the correct MRI scan and properly interpret the findings. In rare situations the neuro-ophthalmologist may also order an electrical test of the optic nerve and visual cortex known as a VEP, for Visual Evoked Potential.
A final test is a multifocal ERG that tests the electrical function of the macula (the central retina). Rarely patients may have a macula that appears normal clinically, on the angiogram, and on the OCT, yet has diminished electrical function. Such a situation may represent a cone dystrophy, which is a poorly understood group of entities where the “cone” cells in the retina lose function.
If the visual field, corneal topography, retinal exam, retinal angiogram, and OCT are all normal, then a multifocal ERG may be ordered. The multifocal ERG is a specialized instrument and is located at the University of Oklahoma/McGee Eye Institute. If appropriate Dr. Dahr may arrange for a multifocal ERG to be performed there.