At least four benign mimics of microscopic PNI exist, important in the differential diagnosis of microscopic PNI. Knowledge of these entities should help dermatopathologists to correctly distinguish them from PNI and avoid unnecessary additional treatment.

We propose the following definition for the minimum histopathologic criteria required to make a diagnosis of PNI: "In the presence of a malignancy, PNI may be diagnosed according to the observation of cytologically malignant cells in the perineural space of nerves. In equivocal cases, the observation of total or near-total circumferential involvement is supportive, as is the presence of perineural tracking in tangential sections and intraneural involvement."

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The clinical and pathologic data are presented on three patients who had clinical anoxia-ischemia in combination with serum hyperosmolality. The pathologic lesions consisted of perivascular necrosis, predominantly involving small vessels in the cerebral cortex. No intraluminal thromboembolic material was found in any of the vessels. The lesions are reminiscent of "no-reflow" lesions seen in experimental animals rendered ischemic.

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