Chiasmal Syndrome - Pathophysiology

Pathophysiology

Lee has divided optic chiasmal syndromes into anterior, middle and posterior locations3. Anterior chiasmal syndrome affects the junction of the optic nerve and chiasm. Middle chiasmal syndrome relates to the decussating fibers in the body of the optic chiasm while posterior chiasmal syndrome involves the caudal fibers.

The classic anterior chiasmal lesion affects the optic nerve fibers and the contralateral inferonasal fibers located in Wilbrand’s knee. This will produce an ipsilateral optic neuropathy, often manifested as a central scotoma, and a defect involving the contralateral superotemporal field. This is also known as a junctional scotoma. An alternative explanation for the contralateral field deficit has been provided by Horton1.

Middle lesions affecting the uncrossed temporal fibers are rare. These can result in a nasal or binasal hemianopia. Lesions in the body of the chiasm most commonly disrupt the crossing nasal retinal fibers. This leads to a bitemporal hemianopia. The field of vision may still be full when both eyes are open but stereovision will not be possible. However, if fusion of the images is lost, perhaps due to a preexisting phoria, binocular diplopia may result4.

Because macular fibers cross more posteriorly in the chiasm, they are damaged in posterior chiasmal syndrome. This leads to a smaller, paracentral bitemporal field loss. Because the temporal macular fibers have not been damaged, it is possible to preserve color vision and visual acuity. Posterior lesions may also involve the optic tract and cause a contralateral homonymous hemianopia.

Optic disc pallor may be apparent with an ophthalmoscope if the insult is longstanding. If the lesion does not affect the lateral uncrossed fibers, the pallor may take on a bow-tie configuration. This is due to loss of retinal ganglion cells nasal to the macula in the papillomacular bundle. Compressive lesions often cause headache and may compress the third ventricle leading to hydrocephalus. The most common tumors also cause pituitary gland malfunction.