Nonetheless, perimetry remains a very important diagnostic tool for following the course of disease in this region.
The very thin fibers of the papillomacular bundle are clustered together in the core of the optic nerve, and they represent the largest number of fibers in the so-called par-vocellular system. These axons project into the layers of the lateral geniculate body that contain neuronal somas, which are also quite small (see ■ Fig. 3.6). Parvocellular neurons transmit the encoded image information from the central-most portions of the visual field, which is characterized by high spatial resolution, good color perception, and stere-opsis. This subsystem of the afferent pathway is particularly susceptible to demyelinating or toxic damage, resulting in defects of the central visual field - central scotomas.
This central group of fibers from the papillomacular bundle occupies approximately 70% of the cross-sectional area of the afferent pathway in the optic nerve, chiasm, and optic tract. These fibers are also more sensitive to damage caused by space-occupying disorders, and their axonal transport mechanisms suffer, whether by direct mechanical compression of the fibers or impairment of their capillary blood supply (cf., ■ Fig. 3.3).
For this reason, it is generally the case that examination of the central 30° of the visual field is adequate for the detection of visual loss caused by the overwhelming majority of neuro-ophthalmic disorders that damage the anterior portions of the afferent pathway.
The fibers that arise in the nasal hemiretinas of both eyes (nasal to the fovea-bisecting vertical meridian) decussate in the chiasm to the contralateral side, while the fibers arising in the temporal hemiretinas remain ipsilateral as they enter the optic tracts (■ Fig. 3.4). During embryonic development, this process is thought to be controlled chemotac-tically, so that there are no sharply defined or straight lines at the interfaces between adjacent groups of fibers. The neural bundles originating in the nasal hemiretinas - and which decussate to the contralateral side within the chiasm - form short loops that protrude into the proximal contralateral optic nerve (by the inferior half of the decussating
Fig. 3.4. Schematic diagram of the course of the ganglion cell axons in the region of the chiasm and the corresponding visual field defects with their frequency of occurrence. The effect of the more common lesions within the various visual pathway segments in the chiasmal region on the central 30° of the visual field are diagrammed as examples. 1 Compressive optic neuropathies. 2 Partial prechiasmal lesion. 3 Anterior junctional syndrome with subtotal damage to anterior Wilbrand's knee, which carries the afferent signals from the contralateral inferior nasal retinal fibers, which correspond to superior temporal visual field quadrant. This pattern is often found with advanced levels of damage or even complete loss of function in the ipsilateral optic nerve. 4 Disease of the central chiasmal region. 5 Posterior junctional syndrome with damage to the posterior knee of Wilbrand, which carries signals form the ipsilateral superior nasal retinal quadrant and represents the inferior temporal quadrant of the visual field. This ipsilateral defect is associated with a homonymous hemianopia to the contralateral side. 6 Lesion of the optic tract (from Schiefer et al. 2004; see "Further Reading")
axons) and into the ipsilateral optic tract (by the superior half of decussating fibers) before completing the transition to the contralateral optic tract, where they are joined by the fibers originating in the corresponding contralateral temporal hemiretina (for further details, please see the "Anterior Junction Syndrome" section below). The chiasm measures 8 mm from the anterior to the posterior commissure, and is about 12- to 18-mm wide and 4-mm thick.
The chiasm is the locus of the greatest risk for catastrophic loss of vision. The sum total of visual information within the afferent pathway is concentrated into the small volume of about 1 cm3, where it is maximally susceptible to damage by focal disease processes. Consequently, comparatively small foci of disease in this region can produce rapid and complete, bilateral blindness. The space-occupying diseases of the sellar region are numerous (■ Fig. 3.5). Most common is the pituitary adenoma, which arises from just beneath the chi-asm, and which is the most common (and mostly benign) intracranial tumor (see Chap. 12). A point to keep in mind is that a tumor arising from the pituitary gland must rise a full 10 mm above the diaphragma sella before contacting the chiasm. After the advent of MRI scanning, it has become quite common for small adenomas to be detected long before they do visual damage, and their frequency is far greater than had been suspected.
Fig. 3.5. Structures of neuro-ophthalmic relevance and their relative positions in the perichiasmal region (modified from von Lanz et al. 2003; see "Further Reading")
Pituitary (posterior lobe)
Sella turcia m
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