A 7-year-old boy was born at term following an uneventful pregnancy, with no family history. Birth examination showed a flat, alopecic, xanthomatous lesion in the left temporo-occipital scalp, suggestive of nevus sebaceous ( Fig. 58.1 ). Neurological examination revealed asymmetry of spontaneous movements with preferential use of the left arm, as well as mild motor delay. In the first year of life, he presented with language delay, oppositional behavior, and poor social interaction. Griffith developmental scale at the age of 14 months revealed a total developmental score of 60 (mild mental impairment). Seizures began at 6 months, with clusters lasting 2 to 3 minutes, characterized by sudden extensions of the upper limbs and trunk.

Nevus sebaceous. Xanthomatous and alopecic lesion of the left posterior scalp.

Awake EEG showed continuous left temporo-occipital epileptiform abnormalities ( Fig. 58.2A ). Ictal video-EEG demonstrated periodic diffuse slow waves with superimposed fast activity lasting 20 to 30 seconds, associated with subtle ocular elevation ( Fig. 58.2B ).

(A) Awake EEG shows continuous left temporo-occipital ( rectangle) slow waves and epileptiform abnormalities, with sporadic contralateral activity. EEG , Electroencephalogram. (B) Left temporo-occipital ( arrows ) periodic slow wave complexes ( arrows ) were associated with subtle eye elevation.

Brain MRI showed left cerebral overgrowth with diffuse cortical dysplasia ( Fig. 58.3 ).

Hemimegalencephaly. Brain MRI, (A) axial T2 and (B) coronal T2 show left hemimegalencephaly with asymmetric cerebral enlargement and gray-white blurring.

At the age of 2 years, the patient underwent left temporooccipital resection. Histopathology documented FCD type Ia in the occipital region and hippocampal sclerosis type 1 with preserved cortical organization in the temporal lobe. During the next 5 years of follow-up, he was seizure-free without medications (Engel class Ia).

Custom next generation sequencing (NGS) panel on DNA extracted from the brain specimen and nevus sebaceous revealed a somatic KRAS gene mutation c.35G>T (p.Gly12Val), which was absent in blood.