Hemimegalencephaly: involvement of cranial nerves and intracranial vessels

Aims: Hemimegalencephaly (HME) is a rare malformation characterized by hamartomatous overgrowth limited to one cerebral hemisphere. HME may occur isolated or be associated with other syndromes. Typical neuroimaging findings in HME include enlargement of the affected cerebral hemisphere with dysplastic and thickened cortex, abnormal signal intensity of the white matter, and ventricular anomalies. We aim to evaluate the MRI findings in a large cohort of children with HME focusing on morphological anomalies of cranial nerves and intracranial vessels.

Methods: All MR images were qualitatively evaluated for structural abnormalities.

Results: Twenty-five children were included (median age at MRI 0.8 years, range 1 day to 9.8 years; 13 males, 12 females). HME was isolated in 17. In eight children HME was associated with neurocutaneous syndromes. The ipsilateral olfactory bulb and trigeminal and optic nerves were enlarged in 4/19 (21%, 1 syndromic), 1/18 (6%, 1), and 1/25 children (4%, 1), respectively. Asymmetric dilatation was observed in the ipsilateral meningeal vessels in 14/25 patients (56%, 7), ipsilateral anterior and middle cerebral arteries in 8/25 (32%, 2), ipsilateral internal cerebral vein in 5/24 (21%, 2), and ipsilateral posterior cerebral artery in 5/25 (20%, 2). Ipsilateral enlargement of the cerebellum and middle cerebellar peduncles was present in 4/25 patients (16%, 4 and 3, respectively) and reduction in size of the ipsilateral cerebral peduncle in 6/25 (24%, 1).

Conclusion: In children with hemimegalencephaly, ipsilateral enlargement of other anatomical structures of the involved hemisphere is present. Olfactory bulb, several vessels, and hemicerebellum are the most common ones. Ipsilateral enlargement of the meningeal vessels, hemicerebellum, middle cerebellar peduncles, internal cerebral vein, and optic and trigeminal nerves were more common in syndromic HME.