As its name implies, congenital myasthenic syndromes (CMS) usually have a congenital (at or near birth) onset, but the disease can manifest in children and even in adults. The different types vary in the kind and degree of symptoms, but generally speaking, the earlier the symptoms appear, the more pronounced the disease is likely to be.
Presynaptic CMS commonly appears as CMS with episodic apnea (CMS-EA). It has its onset in infancy and causes ocular weakness — weakness in muscles controlling the eyes — which can cause droopy eyelids (ptosis). It also causes bulbar weakness (named for the nerves that originate from the bulblike part of the brainstem) making it difficult to talk, chew, swallow and hold up the head. This type is also characterized by episodes of apnea, a temporary cessation of breathing.
Postsynaptic CMS (ACh receptor deficiency, fast-channel CMS) has symptoms ranging from mild to extreme. In infants, it may cause severe weakness, feeding and respiratory problems, and delayed motor milestones (sitting, crawling and walking). Childhood and adult-onset cases often cause ptosis (droopy eyelids) and fatigue, but usually don’t interfere with daily living.
Postsynaptic CMS (slow-channel CMS) causes extreme weakness in infant-onset cases, often leading to loss of mobility and respiratory problems in adolescence. Adult-onset cases may not be disabling.
Synaptic CMS can cause 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).