Introduction:
Radiation optic neuropathy is post-radiotherapy optic nerve damage, a delayed complication resulting in sudden complete loss of vision. Radiation injury of the optic nerve and chiasm is the most common complication of conventional external beam radiotherapy.
Radiotherapy is the mainstay of treatment of tumors originating in the head, face, and neck regions, especially the nasopharynx, paranasal sinuses, skull base, brain stem, midline intracranial structures like hypophyseal fossa and pineal glands, cerebellum and cerebral hemispheres. Depending upon the site and tumor staging, radiotherapy has a single definitive treatment protocol or can be used in adjunct with chemotherapy and surgery.
It usually occurs in almost all irradiated patients when the irradiation doses exceed sixty grays. Most of the symptoms become evident around six to twenty-four months after radiotherapy. However, most cases are diagnosed within three years of the post-radiotherapy period.
It typically presents with acute painless, rapidly progressive monocular vision loss or sudden complete vision loss. The initial presentation may be limited to one eye, but the other may also be affected with time.
What Are the Causes of Radiation Optic Neuropathy?
Causes of optic neuropathy:
Rapid Onset:
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Demyelinating.
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Inflammatory.
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Non-arteritic ischaemic.
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Arteritic ischemic.
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Traumatic.
Gradual Onset:
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Infiltrative.
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Compressive.
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Hereditary.
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Radiation.
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Toxic or nutritional.
What Are the Signs and Symptoms of Radiation Optic Neuropathy?
Symptoms of optic neuropathy include
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Pain.
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Altered color vision.
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Uthoff's symptom.
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Flashes.
Signs include:
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Decreased visual acuity.
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Afferent pupil defect.
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Optic disc swelling.
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Optic disc pallor.
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Abnormal contrast sensitivity.
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Abnormal flicker perception.
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Altered depth perception.
Central visual acuity is significantly reduced in most affected cases. In addition, the presence of field defects consisting of central scotomas, nerve fiber bundle defects, junctional scotomas, or bitemporal hemianopsia is strongly suggestive of chiasmal or optic nerve injury.
What Is the Pathogenesis of Radiation-Induced Optic Neuropathy?
Principally radiation-induced optic neuropathy is a delayed white matter disease characterized by tissue necrosis. The tissue injury, including the late necrosis, is likely related to the radiation-generated free radicals. Although free radicals are normally involved in physiological functions such as cell proliferation, differentiation, and inflammation, excess production of free radicals may lead to pathological stress in tissues with deficient antioxidant defense mechanisms.
In addition, fibrogenesis may be induced when the damage level rises to levels where the oxidative stress response is transiently overwhelmed. Additionally, repeated or chronic stress may result in abnormal radical concentrations that further intensify the fibrotic process by enhancing the production of reactive oxygen species.
The primary site of cellular damage is still controversial. Still, the radiation-induced optic neuropathy development process probably involves the depletion of neuroglial progenitor cells and the vascular endothelium in a time-dependent manner.
What Are the Diagnostic Tests to Be Carried Out?
Imaging plays a crucial role, but computerized tomography (CT) scans are usually reported to be within normal limits. Likewise, the unenhanced T1 and T2-weighted MRI scans. Therefore, T1-weighted contrast-enhanced magnetic resonance imaging (MRI) is the gold standard imaging modality for radiation optic neuropathy which yields a marked segmental gadolinium enhancement along the optic nerve.
Although the contrast enhancement of the affected nerve or chiasm is widely considered as the main radiological findings yet, it is not pathognomonic for radiation optic neuropathy as optic neuropathies of other causes, optic neuritis, optic gliomas and other infiltrative lesions such as granulomatous involvement need to be excluded for an accurate diagnosis. However, contrast-enhanced MRI is usually recognized as sufficient for radiation optic neuropathy diagnosis in the presence of appropriate history and physical examination. In addition, electrophysiological tests such as visual evoked potential (VEP) can also be helpful in the earlier diagnosis of radiation optic neuropathy months before the settlement of visual symptoms.
What Is the Management of Radiation Optic Neuropathy?
The risk of radiation optic neuropathy is also associated with radiotherapy (RT)
delivery method, total dose, and fractionation parameters. Implementing intensity-modulated radiation therapy (IMRT) to routine radiotherapy practice enabled radiation oncologists to deliver
higher doses to the target volumes more safely and precisely as a single fraction ablative dose or multiple fractions with lower per fraction doses.
The main goal of fractionated RT is the provision of enough time intervals to neighboring healthy tissues to repair the sublethal damage and, therefore, to increase their tolerance to higher tumoricidal doses. Considering the fractionated RT, the IMRT appears more advantageous than the other three-dimensional conformal RT techniques for reducing the risk of radiation optic neuropathy.
Treatment with hyperbaric oxygen (HBO) therapy is the latest therapeutic addition for management that has come up recently. HBO is inhaling a hundred percent oxygen inside a hyperbaric chamber pressurized to greater than 1 atmosphere (atm). Each session usually takes place in a hyperbaric multiplace chamber and lasts 75 to 90 mins, during which 100 % oxygen is inhaled through a facemask.
It uses oxygen as a drug by dissolving it in the plasma and delivering it to the tissues independently of hemoglobin. For various organ systems, HBO is known to promote new vessel growth in areas with reduced oxygen tension due to poor vascularization and, in addition to that, promotes wound healing and recovery of radiation-injured tissue. In addition, intravitreal bevacizumab (anti-VEGF, i.e., anti-vascular endothelial growth factor - monoclonal antibodies) can be used and has been seen to be very beneficial in a few cases as it improves vision and reduces hemorrhage as well as optic disk edema (angiographic leakage).
Anticoagulation drugs may be recommended to manage cerebral radionecrosis, myelopathy, or plexopathy. High doses of corticosteroids have proven to be ineffective in the treatment of radiation optic neuropathy.
Conclusion:
Radiotherapy-induced optic nerve dysfunction is an important cause of sudden visual loss, especially in patients with head and neck cancers receiving radiotherapy treatment. Treatment with hyperbaric oxygen (HBO) therapy and intravitreal bevacizumab are the latest therapeutic options for management that have come up recently.
