Dr. Sze suggests that the magnetic resonance imaging findings noted in our two patients with transient cortical blindness during an attack of acute intermittent porphyria were caused by hypertensive encephalopathy rather than by unopposed cerebral vasoconstriction resulting from a suggested deficiency of the theme protein nitric oxide synthase.

Although hypertensive encephalopathy cannot be excluded in some patients with acute porphyric attacks, we do not believe that this condition could fully explain our observations. First, papilledema and cerebral edema—both of which are common manifestations of hypertensive encephalopathy—were conspicuously absent in our patients and in those with similar clinical abnormalities described by others [1, 2]. Furthermore, intracerebral bleeding, another common finding in patients with accelerated hypertension, is not a feature of acute intermittent porphyria. Second, the main pathologic finding in patients with hypertensive encephalopathy—fibrinoid arteriolar necrosis and thrombosis, microinfarctions, and petechial hemorrhages—were not seen at autopsy in any cases of acute intermittent porphyria and cortical blindness [1]. Third, only the second of our patients was hypertensive (blood pressure, 170/100 mm Hg) at the time of hospitalization (because of sudden loss of vision) and during magnetic resonance imaging. These blood pressures are well below the generally accepted limits for hypertensive encephalopathy. In addition, no hypertensive cardiac symptoms or electrocardiographic signs were seen in either patients. Decreased nitric oxide production due to deficiency of hemoprotein nitric oxide synthase during porphyric attacks might therefore better explain the transient clinical and radiologic findings seen in our patients. This hypothesis should now be testable in a recently developed mouse model that uses porphobilinogen deaminase deficiency to show the typical neuropathy of human hepatic porphyria [3].