Description

The age of onset varies from birth to adulthood. The main clinical symptoms are weakness (usually more severe in the face, neck flexors, and proximal limb muscles), hypotonia, and absence or reduction of deep tendon reflexes. Facial muscle weakness can cause difficulty in sucking, swallowing and speech disturbances. The respiratory muscles are often involved. NM is divided into 6 clinically overlapping subtypes, depending on the age of onset and the severity of motor and respiratory involvement. Typical NM (approximately 50% of patients) is a moderate form with onset in the neonatal period. Most patients have an active and independent life. Severe congenital NM (10-20%) is characterized by marked hypotonia and limitation of spontaneous movements. Survival after the neonatal period is rare. Intermediate NM (20%) is characterized by slow acquisition of gross motor development milestones and loss of independent walking and / or breathing before age 11. Adult-onset NM may be sporadic (<5%). The onset takes place between 20 and 50 years. This form is characterized by rapid progression. Mild NM or infantile-onset NM (10-15%) is characterized by symmetrical weakness in the dorsiflexor muscles of the ankle and drooping feet around age 10. Weakness progresses slowly. Amish type NM has been described in several families. It has a neonatal onset and life expectancy rarely exceeds 2 years.