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Fig. 3. Early temporal quadrantic junctional loss on the left from early chiasmal involvement with temporal hemianopia on the right.

decussating fibers from the nasal retina of the other eye (which subserves the temporal field) is often signaled by a small upper temporal (junctional) defect (Fig. 3). This pattern of field loss, it is believed by some, is attributable that the nasal fibers of the contralateral eye that, after they have decussated, course anteriorly for a short distance into the optic nerve before looping back to join the fibres from the temporal retina to form the optic tract. Craniopharyngioma, which may often develop above the chiasm, often produces converse pattern of field loss, with an inferior quadrantic defect on one side, accompanying a complete temporal field loss on the other (Fig. 4).

More severe compression of chiasm and intracranial optic nerves results in the field defects crossing the midline—a serious sign that requires prompt action. Another common pattern of visual loss when the chiasm is prefixed and the suprasellar extension of the tumor is more laterally rather than centrally placed is a homonymous hemianopia, which is incongruous, indicating optic tract compression. The surgeon must beware the very prefixed chiasm which makes transcranial surgery risky and difficult, as the approach is blocked by the lack of 'normal' space below the chiasm and vitrually no space between the optic nerves and cartoids laterally. This situation can be anticipated on even poor quality imaging when, in the sagittal images, the mass of the lesion is directed backwards toward or even into the posterior fossa. Attempting tumor removal transcranially in this situation may lead to rapid surgical confusion and abandonment of the procedure with the tumor hiding behind the chiasm and carotids in the same way as a difficult craniopharyngioma.

Fig. 4. Temporal field defect on the right with a lower junctional defect on the left from a craniopharyngioma.

Occasionally patients present with homonymous or bitemporal defects affecting only the central few degrees of vision, the latter pattern being attributable to involvement of the decussating fibers from the maculae at the posterior inferior part of the chiasm.

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