BASICS

DESCRIPTION

• The optic chiasm is formed by the confluence of the intracranial optic nerves, is 12–18 mm in diameter, lies 10 mm above the sella turcica of the sphenoid bone, and contains the decussation of nasal nerve fibers from each optic nerve. Visual loss is often the initial symptom of a disorder involving the optic chiasm (1). Nearly 25% of all brain tumors occur in the area of the optic chiasm.

EPIDEMIOLOGY

Incidence

• Unknown for all chiasmal disorders

• It is difficult to measure all pituitary adenomas because many of them are asymptomatic. Pituitary tumors have been estimated to occur in 5 per 100,000 based on neurosurgical series.

Prevalence

• Prevalence of visual loss in patients with pituitary tumors varies widely (30–90%) depending on the source of the report.

RISK FACTORS

• Underlying risk factors for certain disorders involving the optic chiasm such as radiation therapy or genetic syndromes predisposing to tumor formation (neurofibromatosis and multiple endocrine neoplasia)

Genetics

• Genetic predisposition for certain types of disorders affecting the optic chiasm including multiple sclerosis and sarcoidosis

GENERAL PREVENTION

None

PATHOPHYSIOLOGY

• Typically compressive lesions around the sella turcica. The optic chiasm may also be directly affected by infiltration or inflammation. The crossing nasal retinal fibers are predisposed to compression resulting in temporal visual field loss.

ETIOLOGY

Conditions involving the optic chiasm can be broadly categorized as those that are intrinsic to the chiasm and those that are extrinsic (1,2). These include:

• Congenital

• Traumatic

• Iatrogenic (herniation of optic chiasm after surgery)

• Intrinsic tumors (glioma)

• Ischemia

• Inflammation

• Infection

• Extrinsic mass lesions (most common)

– Pituitary adenoma

– Craniopharyngioma

– Meningioma

COMMONLY ASSOCIATED CONDITIONS

• Hormonal abnormalities associated with pituitary dysfunction

DIAGNOSIS

HISTORY

• Hormonal dysfunction with galactorrhea, amenorrhea, decreased libido, and infertility

• Visual complaints, such as nonspecific visual blurring, double vision, visual field loss and decreased acuity, are the most common symptoms.

• Less commonly there may be

– Photophobia

– Loss of stereopsis

– Abnormalities in motion perception (Pulfrich phenomenon)

– Diplopia from compression of adjacent cranial nerves (III, IV, VI) within the cavernous sinus

• Headache and facial pain with cranial nerve V involvement. Headache may be abrupt and severe in patients with rapid expansion of a sellar lesion (see below).

• Diplopia from hemifield slide – variable vertical or horizontal diplopia that occurs due to disruption of latent heterophorias as images formed on corresponding portions of each retina fall into the affected nasal retina and are not detected

• Postfixation blindness (visual information from beyond the point of fixation falls into the affected temporal visual fields and is not seen)

• Oscillopsia from see-saw nystagmus (rare)

PHYSICAL EXAM

• Visual acuity and color vision are often normal early on in chiasmal dysfunction. As progression occurs, central vision may become affected.

• There may be a relative afferent pupillary defect with asymmetric involvement. The pupils may be sluggish with the progression of the deficit.

• Visual field loss is often the most important finding suggestive of a chiasmal syndrome and defects may include:

– Bitemporal hemianopia/quadrantanopia

– Unilateral central defect with contralateral hemianopia (junctional scotoma – which appears to be due to incomplete compression of the optic chiasm rather than due to the existence of Wilbrand’s knee – an anterior loop of nasal fibers at the junction of the optic nerves and the chiasm) (3)[B] or temporal visual field loss

– Homonymous hemianopia (from an optic tract syndrome)

• Pattern of visual field loss from a compressive lesion is dependent on the anatomic position of the optic chiasm (prefixed, normal, or postfixed) in relation to the sella turcica and the compressive lesion (4)[B].

• Optic disc pallor develops, primarily involving the papillomacular bundle and nasal retinal fibers, in a band (bow-tie) pattern that spares the inferior and superior portions of the optic disc. Pallor may take weeks to develop and the absence of optic disc pallor should not preclude an evaluation for a chiasmal disorder, especially with acute visual loss.

• See-saw nystagmus (rare) may occur in congenital achiasma or with the involvement of the brainstem and interstitial nucleus of Cajal.

DIAGNOSTIC TESTS & INTERPRETATION

Lab

Initial lab tests

• Hormonal levels (including prolactin) are typically performed for compressive lesions that may involve the pituitary gland or stalk.

Follow-Up & Special Considerations

• Following prolactin levels over time may be an important indicator as to the treatment efficacy of a prolactinoma.

Imaging

Initial approach

• Neuroimaging with CT or MRI. Imaging with MRI is generally better for assessing soft tissue, including the chiasm. Imaging should typically include contrast and should be targeted toward the sella.

Follow-up & special considerations

• MRI may show a heterogeneous signal with variable signal intensities suggestive of blood within a preexisting mass lesion (most commonly a pituitary adenoma) including within the optic chiasm (chiasmal cavernoma). When bleeding occurs acutely it may cause rapid expansion of a mass within or around the sella causing chiasmal compression and/or involvement of surrounding cranial nerves III, IV, V, and VI in the cavernous sinus.

• Rapid expansion of other intrasellar or parasellar lesions (such as expansion of a paranasal sinus mucocele) may cause similar symptoms.

• Pituitary apoplexy may cause shock and death, especially with the ensuing hypocortisolemia that may occur as the pituitary gland becomes necrotic.

• Symptoms of pituitary apoplexy include:

– Visual loss

– Severe headache

– Vomiting

– Meningismus

– Diplopia

• Pituitary apoplexy in the peripartum period (Sheehan syndrome) occurs with hypotension and hemorrhage that frequently accompany this period.

• Stimulatory agents, such as thyroid-stimulating hormone and gonadotropin-releasing hormone, have been thought to cause an increase in metabolic needs of an adenoma, leading to necrosis.

• Corticosteroid replacement therapy may be life saving in patients with pituitary apoplexy.

Diagnostic Procedures/Other

• Imaging of the retinal nerve fiber layer (including with optical coherence tomography) has shown decreased nerve fiber layer thickness, particularly nasally and temporally, in patients with chiasmal dysfunction (5)[C].

Pathological Findings

• Pathological findings depend on the cause of the chiasmal syndrome (for example granulomatosis inflammation from sarcoidosis).

• Chronically, optic nerve atrophy and thinning of the optic chiasm may be seen with any cause of a chiasmal syndrome.

• Pituitary adenomas are typically benign but can show high rates of mitoses in more aggressive and invasive tumors.

DIFFERENTIAL DIAGNOSIS

• Some optic neuropathies such as hereditary optic neuropathy (particularly dominant optic atrophy) and toxic optic neuropathy can cause visual field loss with cecocentral scotomas that can be difficult to distinguish from bitemporal visual field defects of a chiasmal syndrome.

• Congenitally anomalous optic discs (especially tilted optic discs) can cause temporal visual field loss; often these defects cross the vertical meridian.

• Retinopathy preferentially affecting the nasal retina

TREATMENT

MEDICATION

First Line

• Corticosteroids are used for acute pituitary compromise such as in pituitary apoplexy.

• Corticosteroids are typically used for inflammatory chiasmal syndromes, including those caused by multiple sclerosis (6)[C].

• Dopaminergic agents, including bromocriptine and cabergoline, may cause marked shrinkage of pituitary adenomas, primarily prolactin secreting tumors.

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