Chiasmal syndrome
Visual field defects from optic chiasm lesions.
Chiasmal syndrome encompasses the signs and symptoms resulting from lesions of the optic chiasm, leading to various visual field impairments depending on the lesion's location. Pituitary adenomas are the most common cause, though the syndrome can also arise from cancer, multiple sclerosis, or neurofibromatosis.
Quick Facts
- Latin
- chiasma opticum
Facts from the source article.
Cause
Causes of chiasmal syndrome are divided into intrinsic and extrinsic categories. Intrinsic causes involve thickening of the chiasm itself and include gliomas and multiple sclerosis. Optic chiasm gliomas, usually astrocytic, are slow-growing tumors more common in children and often associated with neurofibromatosis type 1; they carry a worse prognosis if they extend into the hypothalamus. Treatment involves resection of the optic nerve, and the artifactual nature of Wilbrand's knee allows cutting the nerve at the chiasm junction without fear of additional field deficits. Extrinsic causes, which account for the vast majority of cases, involve compression by another structure. Common compressive etiologies include pituitary adenomas, craniopharyngiomas, and meningiomas. Pituitary tumors are the most frequent cause; visual field defects may be an early sign of non-functional adenomas, while functional adenomas typically present with hormonal disturbances such as Cushing's syndrome, galactorrhea, or acromegaly before compression occurs. Pituitary apoplexy, a hemorrhagic adenoma, can cause sudden visual loss. Craniopharyngiomas arise from Rathke's pouch remnants and approach the chiasm from behind and above; they may cause hydrocephalus if they extend into the third ventricle.
Anatomy
The optic chiasm forms where the two optic nerves unite. Nasal fibers from each nerve decussate to the opposite side, while temporal fibers continue ipsilaterally into the optic tract, ensuring that the left visual field is processed by the right brain and vice versa. The optic nerves contain axons from retinal ganglion cells; at the chiasm, 53% of nasal retinal axons cross the midline. These nasal fibers carry information from the temporal visual field, and temporal fibers transmit nasal field images. The two optic tracts emerge posteriorly, with most fibers synapsing in the lateral geniculate nucleus or pretectal nucleus. Macular fibers for central vision cross posteriorly in the chiasm. Inferonasal fibers pass more anteriorly, and superonasal fibers more posteriorly. Classical teaching described Wilbrand's knee, a loop of inferonasal fibers into the contralateral optic nerve sheath, but significant evidence now indicates this is an artifact from studying enucleated eyes with optic nerve atrophy. No visual field deficits have been observed in small series of optic nerve sections at the chiasm junction.
Pathophysiology
Optic chiasmal syndromes are classified by location as anterior, middle, or posterior. Anterior lesions affect the optic nerve junction and the contralateral inferonasal fibers, producing an ipsilateral optic neuropathy with a central scotoma and a contralateral superotemporal defect, known as a junctional scotoma. Middle lesions affecting uncrossed temporal fibers are rare and may cause nasal or binasal hemianopia; lesions in the body of the chiasm disrupt crossing nasal fibers, leading to bitemporal hemianopia. Although the visual field may appear full with both eyes open, stereovision is lost, and binocular diplopia may occur if fusion is disrupted. Posterior chiasmal syndrome damages macular fibers that cross posteriorly, causing a paracentral bitemporal field loss while preserving color vision and visual acuity if temporal macular fibers are spared. Posterior lesions may also involve the optic tract, producing a contralateral homonymous hemianopia. Longstanding lesions may cause optic disc pallor, which can appear as a bow-tie configuration if lateral uncrossed fibers are unaffected. Compressive lesions often cause headache and may compress the third ventricle, leading to hydrocephalus.
Management
When visual field tests point to chiasmal syndrome, an MRI is usually the next step. Contrast can highlight arterial aneurysms and makes most intrinsic chiasmal lesions stand out. If the MRI shows a mass, an endocrine panel can check for a pituitary adenoma. For patients already diagnosed with functional adenomas, visual field tests are a good way to screen for chiasmal involvement. These tests reveal field loss that stays on one side of the midline. Junctional scotomas cause ipsilateral optic disc neuropathy and a contralateral superotemporal defect. Bitemporal hemianopia, sometimes with a central scotoma, points to the body of the chiasm. A posterior chiasm lesion only affects the temporal sides of the central visual field.
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