Congenital muscular dystrophies (CMD)

Overview

Congenital muscular dystrophies (CMD) are a family of inherited conditions that show up at birth or within the first few months of life — though occasionally signs are subtle enough to go unnoticed until later in infancy or childhood. Bethlem myopathy, LAMA2-related CMD, LMNA-related CMD, SELENON-related myopathy, and Ullrich CMD are among the better-characterised subtypes, each covered in more detail elsewhere on this site.

Symptoms

Babies with CMD often feel 'floppy' due to low muscle tone, and tend to reach milestones like sitting and walking later than expected. Beyond that shared starting point, the specific pattern varies a great deal by subtype — possible features include muscle weakness from birth or in childhood, contractures at the ankles, hips, knees or elbows, breathing difficulties from respiratory muscle weakness, heart involvement, feeding difficulties, learning difficulties, seizures or other neurological features, and vision problems.

Muscle strength can actually seem stable or even improve slightly through early childhood in some subtypes, but in many forms weakness does progress over time, and respiratory weakness can be significant even when limb weakness looks mild. Some children walk, sometimes not until five or later; leg splints can help. A few who learn to walk independently later lose that ability as their growing body outpaces what their muscles can manage, while others never walk unaided and use a wheelchair from early on — outcomes really do need to be assessed individually rather than assumed from the diagnosis alone.

In some subtypes, differences in brain structure or function can cause learning difficulties or, less often, epilepsy; when present, this is usually evident early and tends not to worsen with age, though not every child with CMD experiences this.

Inheritance and cause

CMD arises from changes in any of more than 35 identified genes, which normally provide instructions for proteins that keep muscles (and sometimes the brain) working properly. When these genes are significantly altered, too little protein is made, or what's made doesn't function properly, leading to muscle weakness and the related symptoms above.

In the UK, the most common causes are LAMA2-related CMD and the COL6-related dystrophies (Ullrich CMD and Bethlem myopathy). Other subtypes are rarer, and for some the responsible gene hasn't yet been identified — something genetic research continues to work on. Inheritance itself depends on the subtype: some follow an autosomal dominant pattern (one changed copy is enough), others autosomal recessive (a changed copy from both parents), and occasionally there's no family history at all.

Getting a diagnosis

CMD is usually first suspected in a baby who seems unusually 'floppy' or weak — though since that description fits several different conditions, a proper diagnosis needs a series of tests. These commonly include a blood test for creatine kinase, electromyography, brain and/or muscle MRI, a muscle biopsy, and genetic testing — which is now the most decisive way to pin down the exact subtype, with biopsy findings (fibre size differences, fat or fibrous tissue replacing muscle, altered protein levels) supporting the picture.

Management and outlook

There's no cure for CMD, so care focuses holistically on function and quality of life for the person and their family. Exactly what that looks like depends heavily on the subtype. A multidisciplinary team, ideally led by a neurologist, should review progress regularly, watching in particular for breathing, heart or feeding complications early, since catching these promptly matters. Regular checks typically cover muscle strength, joint range of movement, weight, feeding and swallowing, and respiratory and cardiac function.

Additional tests can include spirometry (lung function), overnight sleep studies, and — for some subtypes — a yearly echocardiogram. If there's no existing relationship with a neurologist or specialist, a GP can help arrange one. Physiotherapy and orthotics access matters too: contractures are common (the hips especially, sometimes with dislocation), and some subtypes cause scoliosis needing specialist input. Orthotic devices help with joint movement and preventing deformity, while physiotherapy — including a tailored exercise programme — helps prevent or slow contractures from progressing.