The L1 syndrome may manifest itself as one of several phenotypes. One of these is the HSAS syndrome.
Genetic defect: L1CAM (L1 cell adhesion molecule)
Inheritance: X-linked (affected males, carrier females)
Features of the HSAS syndrome:
(1) Hydrocephalus due to Stenosis of the Aqueduct of Sylvius (HSAS)
(2) spasticity with brisk tendon reflexes and extensor plantar responses, typically with a shuffling gait
(3) severe intellectual disability
(4) adducted thumbs (A)
(5) variable seizures
The hydrocephalus may appear in utero and is associated with elevated intraventricular pressure.
• The adducted thumb is due to a developmental defect in the extensor pollicis longis and/or brevis muscles.
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