Familial hemiplegic migraine (FHM) type 2 is a rare, autosomal dominant form of migraine with aura. Encephalopathy associated with FHM type 2 is uncommon, with limited case examples in the literature. We present a case of FHM type 2 associated with unilateral encephalopathy demonstrated on electroencephalogram (EEG) and a cytotoxic corpus callosum lesion on magnetic resonance imaging (MRI).

A 28 year old Irish woman was brought by ambulance to hospital after a weeklong history of progressive confusion, weakness and headache. She had a known history of hemiplegic migraine with the onset of headaches age 8 years, though not previously genetically confirmed, and there was a family history of FHM. She was not on regular medication and there was no history of substance misuse. On examination, she was drowsy with reduced responsiveness and failed to follow commands. She was apyrexial, had brisk reflexes and upgoing plantars. In hospital she had two witnessed, short-lived episodes of tonic posturing and unresponsiveness, followed by limb shaking. Our patient was discharged from the observation area of the neurology ward and improved with multidisciplinary input over a period of weeks.

Routine bloods, infectious and inflammatory panels were negative. Lactate was normal at presentation but elevated to 4.4 following episodes of tonic posturing. Electrocardiogram showed normal sinus rhythm. Urine toxicology screen was negative. CT Brain was unremarkable. Cerebrospinal fluid was acellular, with glucose and protein within normal limits. Neurophysiological testing revealed unilateral encephalopathy on EEG. MRI Brain showed a cytotoxic corpus callosum lesion. Genetics confirmed the pathogenic variant in the ATP1A2 gene.

FHM type 2 presenting with asymmetric encephalopathy confirmed by EEG is rare, having only been described in one case series previously. A cytotoxic corpus callosum lesion arising in this context, has not been previously described.