BASICS
DESCRIPTION
Acute onset of visual loss which may be unilateral or bilateral in an individual with a history of having undergone radiation therapy usually for tumors involving or in close proximity to the optic apparatus (i.e., optic nerve, chiasm, or tract), including the orbit, parasellar region, and paranasal sinuses
ALERT
Acute onset of unilateral or bilateral visual loss in a patient with a history of radiation therapy to regions involving or adjacent to the optic apparatus (e.g., eye, optic nerve, optic chiasm, optic tract, paranasal sinuses, sella/parasellar regions) should prompt urgent referral to an ophthalmologist or a neuro-ophthalmologist in order to evaluate for the potential benefit of hyperbaric oxygen therapy and to exclude alternative etiologies (e.g., recurrent tumor) that warrant treatment to preserve or restore visual function.
EPIDEMIOLOGY
Incidence
• If <5,000 cGy total dose, radiation optic neuropathy (RON) rare
• If 5,000–6,000 cGy total dose, 5% 10-year actuarial risk of RON
• Majority of cases occur within 3 years of radiation therapy with a peak of 1.5 years.
Prevalence
Unknown, but extremely rare
RISK FACTORS
• Radiation therapy to regions involving or adjacent to the optic apparatus
• Patients with diabetes mellitus or who have received adjunctive chemotherapy may be at higher risk for developing RON.
GENERAL PREVENTION
• Limiting dosimetry of fractionated radiation therapy when possible to fractions of ≤200 cGy and total dosage to ≤5,400 cGy
– “Safe dose” of gamma knife radiosurgery adjacent to optic apparatus unclear but estimated around 8,000–10,000 cGy
PATHOPHYSIOLOGY
Unclear whether primary insult is secondary to radiation vasculitis or direct parenchymal injury to the optic apparatus secondary to the effects of radiation therapy or a combination of these two mechanisms
ETIOLOGY
Adverse effects of radiation either directly on the optic apparatus or on its vascular supply
COMMONLY ASSOCIATED CONDITIONS
Tumors (malignant or benign) in regions involving or adjacent to the optic apparatus
DIAGNOSIS
HISTORY
• Progressive painless unilateral or bilateral visual loss over the course of several weeks
• History of undergoing radiation therapy to anatomic regions involving or adjacent to the optic apparatus
• Second eye involvement, if it occurs, generally develops within few weeks of initial eye onset.
PHYSICAL EXAM
• Demonstrates features of an optic neuropathy: Decreased visual acuity (usually severe); decreased color vision; nerve fiber bundle-type visual field defects though may have temporal defects or homonymous defect if involvement of the optic chiasm or tract, respectively; afferent pupillary defect if unilateral or asymmetric optic nerve involvement
• Affected optic nerve usually normal in appearance or pale from prior compression by radiated lesion; occasionally edematous (if anterior optic nerve involved)
• Optic nerve pallor develops within 6–12 weeks if not present at onset from prior compression
• Possible concurrent radiation retinopathy
DIAGNOSTIC TESTS & INTERPRETATION
Imaging
• MRI orbits through chiasm with and without contrast with post-contrast fat suppressed images:
– MRI demonstrates segmental enhancement of the involved optic nerve(s), chiasm, or tract(s) which may be overlooked as involvement may be limited to a single acquired image slice.
Diagnostic Procedures/Other
Lumbar puncture to be considered to evaluate for meningeal carcinomatosis and serologic testing for paraneoplastic syndrome if MRI not typical for RON
Pathological Findings
Ischemic demyelination, reactive astrocytosis, endothelial hyperplasia, obliterative endarteritis, and fibrinoid necrosis of optic nerves
DIFFERENTIAL DIAGNOSIS
• Infiltrative optic neuropathy secondary to malignancy
• Compression of optic apparatus secondary to recurrent tumor or radiation-induced parasellar tumor
• Meningeal carcinomatosis with optic nerve involvement if history of malignancy
• Toxic optic neuropathy secondary to chemotherapy if previously administered
• Paraneoplastic optic neuropathy or retinopathy if history of malignancy
• Arachnoiditis
TREATMENT
ADDITIONAL TREATMENT
General Measures
• No therapy of uniformly accepted efficacy
• Hyperbaric oxygen therapy of potential benefit in selected cases though efficacy remains controversial
Nursing
Appropriate precautions for a blind patient in setting of bilateral visual loss if the patient admitted to hospital or other in-patient care facility
ONGOING CARE
FOLLOW-UP RECOMMENDATIONS
Referral for low vision evaluation and blind services if severe bilateral visual loss
Patient Monitoring
Serial clinical and visual field studies at a frequency determined by an ophthalmologist or a neuro-ophthalmologist
PATIENT EDUCATION
• Explain poor visual prognosis and lack of proven effective therapy for condition
• Instruct to call immediately for any visual changes
PROGNOSIS
Generally poor with most affected eyes having visual acuity <20/200 and 45% of eyes with no light perception in spite of attempts at therapeutic intervention
COMPLICATIONS
Permanent unilateral or bilateral blindness
ADDITIONAL READING
• Arnold AC. Ischemic optic neuropathy. In: Miller NR, Newman NJ, eds. Walsh and Hoyt’s clinical neuro-ophthalmology, 6th ed. Philadelphia: Lippincott Williams Wilkins, 2005:374–376.
• Levy RL, Miller NR. Hyperbaric oxygen therapy for radiation-induced optic neuropathy. Ann Acad Med Singapore 2006;35:151–157.
CODES
ICD9
377.34 Toxic optic neuropathy
CLINICAL PEARLS
• Acute onset of painless unilateral or bilateral visual loss in an individual with a history of radiation therapy to regions involving or adjacent to the optic apparatus should prompt strong consideration of RON if recurrent tumor excluded by brain and orbital imaging studies.
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