Kupferschmidt and colleagues [1] described two patients with acute intermittent porphyria who presented with cortical blindness. Magnetic resonance imaging showed bilateral occipital lesions, and the authors speculated that these lesions were caused by vasospasm-induced ischemia due to unopposed cerebral vasoconstriction resulting from a deficiency of nitrous oxide synthase, a major vascular dilator. The striking feature of the magnetic resonance findings in these cases and in those of acute intermittent porphyria described in the literature is that the lesions are bioccipital and partially or totally reversible [2]. These characteristics are typical of magnetic resonance findings seen in patients with hypertensive encephalopathy. It is also notable that vasospasm alone, often a sequela of subarachnoid hemorrhage, does not result in this clinical appearance.

Syndromes that appear to cause similar findings have been subsequently linked to hypertension. For example, a nearly identical magnetic resonance image is seen in cyclosporine toxicity and has been attributed to hypertension [3].

It has been suggested that the bioccipital distribution of the lesions in patients with hypertensive encephalopathy may be due to the distribution of adrenergic receptors. In hypertension, stimulation of the perivascular sympathetic nerves results in increased vascular resistance, which thus protects the brain [4]. In the vertebrobasilar system, however, breakthrough of autoregulation occurs because of sparse sympathetic innervation. Thus, the occipital lobes can be preferentially affected.

In the two cases reported by Kupferschmidt and colleagues, the blood pressure of the first patient was not noted, and the second patient was hypertensive. Previous patients in whom cerebral manifestations of acute intermittent porphyria were reported also had hypertension [2, 5]. In fact, hypertension has been noted to generally accompany the onset of neurologic symptoms in patients with acute intermittent porphyria. If hypertension is the cause of the lesions seen in these cases, the underlying cause would differ substantially from the vasospasm-induced ischemia suggested by Kupferschmidt and associates.