result from somatic mutation in the prion protein gene or, more likely, from the spontaneous, random conversion of a normal prion protein to an abnormal prion protein and the subsequent expansion of the altered form. About 10% of CJD cases are familial and due to inherited mutations in the PrP gene.

CJD is uniformly fatal, presenting as a rapidly progressive dementia that results in death, usually in 8 months. The most commonly reported visual symptoms include diplopia, supranuclear palsies, complex visual disturbances, homonymous visual field defects, hallucinations, and cortical blindness. Occasionally, in the Heidenhain variant of CJD, patients present primarily with isolated visual symptoms.

Diagnostic testing includes MRI, electroencephalogram (EEG), lumbar puncture, and possibly brain biopsy. MRI diffusion-weighted images show typical high-intensity caudate and/or putamen lesions. The EEG shows characteristic periodic sharp wave complexes (PSWCs). CSF usually contains the 14-3-3 protein within the first 2 years of disease onset. Brain histologic studies reveal spongiform degeneration. No treatment is currently available.

Buono LM. Prion disease. Int Ophthalmol Clin. 2007;47(4):121–129.

Radiation Therapy

Several types of radiation therapy (RT) are currently available, and techniques continue to evolve and emerge with scientific advances. Traditional whole-brain RT, which is used to treat cerebral malignancies, is delivered in fractions over approximately 1 month. The technique of 3-dimensional conformal RT uses a computer to help concentrate radiation in a precise area; this concentration is accomplished through the use of complex asymmetric 3-dimensional shapes. Radiation surgery (radiosurgery) refers to linear accelerator or gamma knife techniques, which are distinct from traditional radiation therapy. Radiosurgery is typically given in a single sitting, using computer-based techniques to focus radiation at the desired regions. These techniques may treat malignancies, occasional inflammatory lesions, and vascular malformations.

Complications of RT directed at the CNS may take several forms and can occur years after therapy. Immediate complications include transient swelling of the involved tissue. Later complications of neuro-ophthalmic interest include radiation necrosis, cranial neuropathies, and ocular neuromyotonia (see Chapter 8). Radiation necrosis involves death of nervous system tissue with attendant edema. This complication may simulate the imaging appearance of recurrent neoplasm on traditional MRI or CT imaging. Occasionally, functional imaging techniques such as PET or magnetic resonance spectroscopy (MRS) are required to separate these entities radiographically; neoplasms generally display a hypermetabolic profile, whereas radiation necrosis is hypometabolic in nature.

Of greater concern to the neuro-ophthalmologist is radiation optic neuropathy (RON). This rare complication typically occurs months after radiation administration and produces subacute optic neuropathy. RON is in part dose-related, being more likely to occur with higher doses (often >5000 cGy). MRI studies of RON usually demonstrate enhancement of the optic nerve(s) on the postcontrast, T1-weighted sequences (Fig 14-24). Treatment is often ineffective, but anticoagulation, hyperbaric oxygen, and corticosteroids have all been used.

Figure 14-24 A, Axial and (B) coronal, T1-weighted, fat-suppressed MRI scans performed after gadolinium injection of a patient with radiation optic neuropathy. Radiation was performed for a pituitary adenoma, and the patient experienced vision loss OD > OS months after the radiation. MRI demonstrates enhancing optic nerves (arrows) in the prechiasmal

region. (Courtesy of Eric Eggenb erger, DO.)

Ocular neuromyotonia occurs months to years after radiation treatment. This uncommon syndrome produces episodic “spasms” of CN III, IV, or VI, causing recurrent short-lived episodes of diplopia. Although rare, this entity is important for the ophthalmologist because it responds to carbamazepine.

Barnett GH, Linskey ME, Adler JR, et al; American Association of Neurological Surgeons; Congress of Neurological Surgeons Washington Committee Stereotactic Radiosurgery Task Force. Stereotactic radiosurgery—an organized neurosurgerysanctioned definition. J Neurosurg. 2007;106(1):1–5.