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Toxic/Nutritional Optic Neuropathy Workup

  • Author: Andrew A Dahl, MD, FACS; Chief Editor: Hampton Roy, Sr, MD  more...
 
Updated: May 09, 2016
 

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.

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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]

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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.

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Contributor Information and Disclosures
Author

Andrew A Dahl, MD, FACS Assistant Professor of Surgery (Ophthalmology), New York College of Medicine (NYCOM); Director of Residency Ophthalmology Training, The Institute for Family Health and Mid-Hudson Family Practice Residency Program; Staff Ophthalmologist, Telluride Medical Center

Andrew A Dahl, MD, FACS is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, American Intraocular Lens Society, American Medical Association, American Society of Cataract and Refractive Surgery, Contact Lens Association of Ophthalmologists, Medical Society of the State of New York, New York State Ophthalmological Society, Outpatient Ophthalmic Surgery Society

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Hampton Roy, Sr, MD Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences

Hampton Roy, Sr, MD is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, Pan-American Association of Ophthalmology

Disclosure: Nothing to disclose.

Additional Contributors

Andrew W Lawton, MD Neuro-Ophthalmology, Ochsner Health Services

Andrew W Lawton, MD is a member of the following medical societies: American Academy of Ophthalmology, Arkansas Medical Society, Southern Medical Association

Disclosure: Nothing to disclose.

Acknowledgements

Robert C Sergott, MD Professor of Ophthalmology, Department of Ophthalmology, Thomas Jefferson University; Consulting Surgeon, Wills Eye Hospital, Children's Hospital of Philadelphia

Disclosure: Nothing to disclose.

Brian R Younge, MD Professor of Ophthalmology, Mayo Clinic School of Medicine

Brian R Younge, MD is a member of the following medical societies: American Medical Association, American Ophthalmological Society, and North American Neuro-Ophthalmology Society

Disclosure: Nothing to disclose.

Aftab Zafar, MD Consulting Staff, Department of Ophthalmology, St Mary's General Hospital

Aftab Zafar, MD is a member of the following medical societies: Canadian Medical Association, Canadian Ophthalmological Society, College of Physicians and Surgeons of Ontario, Ontario Medical Association, and Royal College of Physicians and Surgeons of Canada

Disclosure: Nothing to disclose.

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