Optic Chiasm, Chiasmal Syndrome
The optic chiasm (from the Greek for “crosspiece”) is the anatomical location where the two optic nerves, having left the eyes, join together before then separating once more as the two distinct optic tracts, thus forming an anatomical “X.” Chiasmal syndrome is the name given to the group of symptoms and signs that occur together as a result of lesions affecting the optic chiasm.
This optic chiasm measures 15 mm in width and 3.5 mm in height  and has several important structures located adjacent to it; below the optic chiasm, in a bony excavation called the sella turcica, lies the pituitary gland, and either side of it are found the venous cavernous sinuses, which transmit the internal carotid artery and several important cranial nerves. The frontal lobe of the brain sits above the chiasm, and the floor of the third ventricle lies behind. The optic chiasm is significant because it is here that the visual signals coming from each retina decussate, and so from this point onward, there is representation from both eyes on both sides (i.e., left and right sides) of the visual pathway. The nasal optic nerve fibers, carrying signals from the nasal retinae (and hence temporal hemi-fields of vision), cross or decussate to the other side (contralateral) at the optic chiasm, while the temporal fibers, which carry signals from the temporal retinae (and hence nasal hemi-fields of vision), continue on the same side (ipsilateral). In anatomically normal individuals, approximately 50 % of the axons (i.e., those from the nasal retina) decussate across the midline to join the uncrossed fibers traveling toward the brain from the temporal retina. These fibers exit the chiasm to form the left and right optic tracts, which carry signals from the right and left hemi-fields of vision, respectively. In certain conditions, the proportion of crossing fibers at the chiasm is significantly different from the 50 % value observed in normals; in humans with albinism, a greater number of fibers from temporal retina decussate and project contralaterally .
Pathology in any of the key anatomical structures that lie adjacent to the optic chiasm (see above) has characteristic effects on the optic chiasm, giving rise to a unique constellation of visual signs and symptoms, collectively known as the chiasmal syndrome. Depending on the particular pathology involving these adjacent structures, and the consequent relative location of damage to the chiasm, the exact features seen in any chiasmal syndrome can help anatomically locate the site of the lesion.
A visual field defect may be the earliest sign of a lesion at the optic chiasm , and the classical visual field defect seen in chiasmal syndrome is the loss of the temporal hemi-fields of vision in both eyes (so-called bitemporal hemianopia). This is caused by a lesion in the body of the chiasm which damages the crossing nasal retinal fibers. Any lesion of the chiasm (such as a tumor, vascular lesion, traumatic lesion, etc.) may cause this, and there are subtle variations in the extent of the bitemporal hemianopia that can help give clues as to the causative lesion. The exact field defect will vary according to the type of lesion and the precise individual anatomical relationship of the chiasm to the structures that surround it. The most common cause of such a field defect is a tumor of the pituitary gland . The visual acuity may also be reduced, as may the color vision on testing with pseudoisochromatic plates, with a tendency to have more severe red-green deficits and milder blue-yellow losses .
Lesions of the anterior chiasm can cause a so-called junctional scotoma, presenting with a central field loss in one eye and a superotemporal field loss in the other – this is classically said to be due to a small bundle of inferonasal nerve fibers that briefly loop forward in the contralateral optic nerve, called Wilbrand’s knee. More recently, the existence of Wilbrand’s knee has been questioned as an artifactual error, relating to the fact that the cadaveric specimens on which the observation of this anatomical feature had been based had always suffered the loss of one eye before death , and investigations have found no clinical perimetric evidence to support the existence of this anatomical feature in human beings .
Lesions of the posterior chiasm may cause an uneven field loss in both eyes of the visual field on the same side, due to damage to the optic tracts (i.e., an incongruous homonymous hemianopia).
Compression of the optic chiasm may also cause atrophy of the nasal nerve fibers, with the superior and inferior nerve fibers being relatively spared – so-called “bow-tie” optic atrophy – as well as an unusual type of nystagmus (an involuntary movement of the eyes) called “seesaw” nystagmus . Loss of the temporal visual fields can lead to postfixation blindness and the inability to keep the eyes both facing in the same direction, a phenomenon known as hemi-field slide .
The optic chiasm also lies close to a venous space (the cavernous sinus) where the nerves that control eye movement pass – lesions in this area can damage these nerves and cause further problems with the eyes moving together, resulting in diplopia (double vision).
Anyone presenting with the above features of chiasmal syndrome should be investigated with urgent neurological imaging, in order to promptly diagnose and, if necessary, treat a potentially life-threatening cause.
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