Congenital Myasthenic Syndromes (CMS)

Types of CMS

Presynaptic CMS

This type of CMS is characterized by insufficient release of ACh (a chemical necessary for proper muscle function). It commonly manifests as CMS with episodic apnea (CMS-EA), which has its onset in infancy and causes weakness of the facial muscles and those involved in swallowing and talking, as well as episodes ofapnea, a temporary cessation of breathing.

Postsynaptic CMS — ACh receptor deficiency, fast-channel CMS

This type of postsynaptic CMS is caused by ACh receptors that are missing or don’t stay open long enough. Its symptoms vary from mild to profound. In infants, it may cause extreme weakness, feeding and respiratory problems, and delayed motor milestones (sitting, crawling and walking). Childhood and adult-onset cases often cause ptosis and fatigue, but usually don’t interfere with daily living.

Postsynaptic CMS — slow-channel CMS

This type of postsynaptic CMS is caused by ACh receptors that stay open too long. Infant-onset cases may result in extreme weakness, often leading to loss of mobility and respiratory problems in adolescence. Adult-onset cases may not be disabling.

Synaptic CMS

This type is caused by deficiency of acetylcholinesterase, an enzyme that breaks down ACh. It results in extreme weakness with feeding and respiratory difficulties from birth or early childhood. Weakness also causes delayed motor milestones, and often leads to reduced mobility and scoliosis (curvature of the spine).

For more, see Signs and Symptoms and Causes/Inheritance.