Monilethrix: a clinicopathological illustration of a cortical defect

We describe two sisters aged 2 and 7 years, with monilethrix. The older sister was clinically less severely affected but, microscopically, the features of weathering and alternating nodes and internodes were more obvious. The younger girl had more severe alopecia, with shorter hair, but the microscopic features were less marked. Scanning-electron microscopy demonstrated only slight cuticular weathering in both cases, and evidence of breakage in the cuticle at points of no obvious cuticular pathology. Transmission-electron microscopy showed normal cuticle cells in cross-section, but an abnormal cortex. The abnormalities included areas of homogeneous non-fibrillar material, and a deviated axis of some microfibrils. We consider that these findings support the possibility that a cortical defect contributes significantly to the fragility of hair in monilethrix. This might explain why hair with marked beading can be stronger than hair from an affected sibling with less apparent abnormality.