Laboratory Studies
In any patient with bilateral central scotomas, serum B-12 (pernicious anemia) and red cell folate levels (marker of general nutritional status) need to be obtained. Other tests that could support the diagnosis of nutritional optic neuropathy are direct or indirect vitamin assays, serum protein concentrations, and antioxidant levels. Serologic testing for syphilis also should be completed.
Patients suspected of having a toxic optic neuropathy should have a CBC count, blood chemistries, urinalysis, and a serum lead level, particularly in those who have a coexisting peripheral neuropathy. The blood and urine also may be screened for other toxins if exposure to a particular one is not identified on history. On the other hand, if a specific intoxicant is suspected, one would try to identify it or its metabolites in the patient's tissues or fluids.
Imaging Studies
Although imaging studies yield normal results in toxic/nutritional optic neuropathy, they almost always are indicated, unless one is absolutely certain of the diagnosis. The most appropriate imaging study is an MRI of the optic nerves and chiasm with and without gadolinium enhancement. [10]
Other Tests
Formal visual field evaluation
Formal visual field evaluation, whether it is a static (Humphrey) or kinetic (Goldmann) field, is absolutely essential in the evaluation of any patient suspected of having toxic/nutritional optic neuropathy.
Central or cecocentral scotomata with preservation of the peripheral field are characteristic visual field defects of these optic neuropathies and are actually most prevalent in patients with these disorders. Rarely, patients may present with other defects, as mentioned below. Although the field defects do tend to be relatively symmetric, early on in a patient's presentation, the defect is usually more developed in one field than in the other field. Soft margins are another characteristic of these defects, which are easier to define/plot for colored targets, such as red, than for white stimuli. The anatomic basis of the cecocentral scotoma has yet to be established.
In ethambutol toxicity, central scotomas are the common visual field defect, but bitemporal defects [1] and peripheral field constriction have been reported. The field defect in amiodarone toxicity may be simply a generalized constriction of fields or cecocentral scotomas.
Optical coherence tomography
Optical coherence tomography (OCT), which is now commonly used to measure nerve fiber layer thickness in patients with glaucoma, can also be used to quantify such changes in patients with other optic neuropathies, like the one caused by ethambutol. [11, 12] As discussed above, early changes are not clinically apparent in patients on ethambutol. With OCT, one can clearly quantify the loss of retinal nerve fibers from the optic nerves of these patients as a sign of early toxicity from the drug, which might not yet be apparent on funduscopy. Therefore, in conjunction with visual field testing, it is an additional objective test available to monitor patients on ethambutol.
