Congenital myasthenic syndromes (CMS)

Overview

Congenital myasthenic syndromes (CMS) are a group of inherited conditions affecting neuromuscular transmission — the handover point where a nerve signal tells a muscle to contract. The result is 'fatigable' weakness that gets worse with repeated use. How CMS behaves — which muscles it affects, when it starts, how well it responds to treatment — depends heavily on exactly which gene is involved. CMS is a distinct condition from myasthenia gravis, an unrelated autoimmune condition covered separately.

Symptoms

Onset is usually at birth or in early childhood, occasionally not until early adulthood; as a rough pattern, earlier onset tends to mean more severe symptoms, and some presentations can be life-threatening.

Weakness that worsens with activity can affect specific muscle groups or be widespread. In infants this can delay milestones like sitting, crawling and walking; older children and adults may struggle with longer walks, stairs, or find themselves falling more easily. Eye involvement is common — drooping eyelids and weakened eye-movement muscles, sometimes needing extra head movement to compensate, and occasionally double vision.

Joint contractures can be present from birth or develop later, particularly affecting fine motor coordination, and some people develop scoliosis needing specialist monitoring (sometimes a brace, occasionally surgery). Weak breathing muscles can cause problems from an early age and need regular monitoring, since poorly managed respiratory issues can become life-threatening; frequent chest infections and shallow overnight breathing (which can cause morning headaches, daytime sleepiness and poor appetite) are both possible. Speech can sound nasal or unclear, and swallowing difficulties — coughing, choking, or a sensation of food sticking — can make eating and drinking harder; babies may show this early as a weak suck or cry.

Inheritance and cause

Around 35 genes are currently linked to CMS, most involved in building or maintaining the neuromuscular junction — the connection point between nerve and muscle. Depending on exactly where along that connection the fault sits, CMS is classified as pre-synaptic (at the nerve ending), synaptic (in the gap between nerve and muscle) or post-synaptic (on the muscle side) — post-synaptic forms are the most common, most often involving the CHRNE gene, followed by RAPSN and DOK7.

Most CMS is autosomal recessive (a changed copy needed from both parents); a minority — most often the 'slow channel' form — is autosomal dominant. Occasionally there's no family history at all.

Getting a diagnosis

A neurologist typically combines a physical exam, blood tests, repetitive nerve stimulation, and single-fibre EMG; because different CMS types produce different patterns of weakness, pinning down the diagnosis isn't always straightforward, and a muscle biopsy is occasionally needed. Genetic blood testing can identify the specific gene involved.

Management and outlook

Multidisciplinary care, usually led by a neurologist, is standard — a GP can help arrange access if there's no existing specialist relationship. Several medications can meaningfully improve muscle function, most commonly pyridostigmine, followed by 3,4-diaminopyridine, salbutamol, ephedrine or fluoxetine — but because a drug that helps one CMS subtype can worsen another, getting a genetic diagnosis before starting treatment really matters.

Breathing support ranges from monitoring through to non-invasive ventilation for nocturnal hypoventilation, depending on severity. Staying active with regular, gentle exercise supported by a physiotherapist helps maintain strength and slows contractures; swimming and walking are commonly recommended, and a wheelchair can help manage fatigue when needed. Speech and language therapy supports communication and swallowing, with dietitian input (including feeding tubes in severe cases) where nutrition is affected.

A number of medications — some antibiotics, muscle relaxants, heart drugs, and certain psychiatric medications among them — can worsen CMS symptoms, so any new prescriber needs to know about the diagnosis; a full list of medications to be cautious with is available via Myaware. Before any surgery, the anaesthetist and surgical team must be told about the CMS diagnosis in advance, with appropriate pre- and post-operative monitoring arranged.