Epidemiology and etiology
This optic neuropathy usually occurs in patients treated with radiotherapy for intracranial, skull base, or paranasal sinus tumors where the optic nerve is included in the radiation field. Radiation neuropathy can be caused by total radiation therapy exceeding 6000 cGy and a daily dose of approximately 200 cGy. In addition, radiation therapy to patients with pre-existing diabetic thyroid-related ophthalmopathy can also cause this disease. Notably, lower doses of radiation therapy given in conjunction with chemotherapy can also cause radiation optic neuropathy, probably because chemotherapy exacerbates the damage to the optic nerve from radiation therapy.
The exact pathogenesis is unknown, but it is assumed that radiotherapy induces damage to the vascular endothelium, which in turn causes vascular obstruction and necrosis. The main manifestation is retrobulbar optic neuropathy and, in rare cases, anterior segment optic neuropathy with optic papilloedema.
Clinical features
Symptoms
Acute, progressive vision loss in one or both eyes until total or substantial loss of vision. Visual loss usually occurs on average about 18 months after radiotherapy, but can also occur within the first year and has been reported to occur after 20 years.
Physical signs
Loss of visual acuity.
Visual field defects of the optic nerve or optic cross lesion type.
Initially normal appearance of the optic disc, followed by pallor.
Differential diagnosis
Recurrence of the primary tumor.
Optic nerve and optic cross prolapse secondary to empty saddle syndrome.
Radiation-induced arachnoiditis of the parsaddle tumor.
Diagnosis
The disease is clinically diagnosed in patients who have received a certain amount of radiation therapy, except for other causes of vision loss, with normal CT scans and no enhancement on enhancement scans. However, gadolinium-enhanced MRI shows significant enhancement of the optic nerve, optic cross and possibly the optic tract on T1-weighted images. When the visual function is stable, the enhancement subsides. T1- and T2-weighted images without enhancement show no abnormalities.
Treatment
Avascular necrosis is the cause of vision loss. Several treatments are available, including high-dose corticosteroids, alone or in combination with hyperbaric chambers. Their efficacy is questionable. Anticoagulation therapy is effective for radiation necrosis of brain tissue, but no studies have shown it to be effective for radiation optic neuropathy.
Prognosis
Despite all treatments, nearly half of the patients end up with no light perception. Even if some vision is preserved, it is less than 20/200.