Rigid Spine Syndrome Phenotype (RSMD1):
- Cause: Recessive mutations in SEPN1 (Selenoprotein N).
- Onset: First year of life.
- Symptoms:
- Hypotonia and weakness:
- Predominantly axial weakness.
- Neck flexors most affected → poor/absent head control in infancy.
- Joint hyperextensibility.
- Delayed motor development.
- Facial weakness: Common.
- Extraocular muscles spared.
- Kyphoscoliosis and spinal rigidity:
- Progressive, especially during adolescence.
- Often associated with respiratory insufficiency.
- Respiratory involvement:
- Progressive insufficiency typically develops in late adolescence or early adulthood.
- Intellectual performance: Normal.
- Prognosis:
- Most remain independently ambulant despite respiratory challenges.
