The visual evoked potential can save sight in patients with radiation-induced optic neuropathy

Radiation-induced optic neuropathy (RION) is one of the side effects of radiation therapy for brain and skull base tumors.^1–3 RION, a form of delayed radionecrosis of the anterior visual pathways, is characterized as a sudden, painless, and irreversible visual loss in one or both eyes following radiation therapy.^1,4,5 It evolves within months to years after external cranial irradiation.^1,4,5 The general initial presentation may be seen in one eye first. However, the fellow eye may also be affected throughout upcoming weeks to months.^1,2

As many other conditions may mimic RION, and for the fact that the interval time between the radiation therapy and the occurrence of visual symptoms and signs may lead to misdiagnosis, it is important to make the diagnosis of RION if the suspected patient has a previous history of head and neck or cranial radiation therapy after exclusion of all other etiological sources.^1,2

Since radiation therapy (RT) is a required part of the fundamental management of tumors originating from nasopharynx, paranasal sinuses, skull-base, brain stem, and midline intracranial structures,^1,2 RION may occur as a late complication of RT.^1–3

Even though the defined etiological factor of RION has not yet been understood, the proposed causes have been attributed to the ultimate outcome of endothelial and neural cell damage with resultant necrosis. ² RT-induced microangiopathy and demyelination are also among other anticipated causes of RION. ²

In view of the fact that the visual symptoms and clinical signs of RION happen to be obvious months or even years after RT, an early diagnostic test is required. ¹ This diagnostic test is critical as it may be late when visual symptoms and clinical signs are present later in life. ¹

Since many subjective tests fail to recognize RION at initial stages and for the fact that visual symptoms and clinical signs manifest very late, some objective tests such as electrophysiological tests (e.g. visual evoked potential) are of value. ¹ Visual evoked potential (VEP), a physiologic response of the occipital cortex to a sensory stimulus of vision, is very sensitive in detecting occult diseases.^1,4 In this context, it has been shown that VEP results may be abnormal months before the vision loss in patients with anterior visual pathway radionecrosis.^1,4 Accordingly, VEP results award the early evidence of optic neuropathy before the onset of visual symptoms and clinical signs.^1,4

According to the author’s experience and other scientists in the area of clinical electrophysiology of vision, a delay in the occurrence of P100 wave in VEP results (a delay in VEP latency) will be the first indication of the beginning of RION.^1,2,4 In addition, the amplitude of P100 wave is diminished in some cases.^1,4 Therefore, VEP abnormality is very imperative as an objective predictor of visual loss before the onset of RION.^1,4

Another vital matter is the proof that optic disc and retinal manifestations of RION are not initially noticed at the early stages. ¹ Consequently, it is essential to identify RION before the optic disc and retinal signs are clinically recognized in order to initiate the therapeutic interventions as soon as possible. ¹ This is even more central in the case of RION because the currently available therapeutic choices usually fail to reverse the already settled radiation damage to the optic disc and retina.^1,2

As a result, the author proposes to employ VEP before, during, and after RT for screening of all patients treated with radiotherapy for brain and skull base tumors in order to distinguish RION as early as possible.