Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy.
DMD affects 1 in 3500-5000 newborn boys.
DMD is caused by a fault (mutation) in a gene (the dystrophin gene), this stops the body from making a protein which is also called dystrophin. The protein dystrophin is important in muscle fibres and its absence results in muscle weakness. This weakness worsens over time because muscle cells break down and are eventually lost.
The dystrophin gene is found on the X-chromosome, therefore Duchenne usually only affects boys (girls have two X chromosomes so if one of these is unaffected it can compensate for the faulty one). Boys have one X chromosome therefore if that single copy of the gene is faulty then they will show the clinical features of DMD. Girls who have one affected and one unaffected gene are called carriers and do not usually display symptoms. DMD runs in families; a carrier mum has a 50:50 chance of having a son who is affected.
Most common early features of DMD are difficulty with walking, they might walk later than their peers. Typically they have enlarged calf muscles. This is ‘pseudohypertrophy’ as muscle cells are broken down and replaced by fat cells (giving the appearance of enlarged “muscular” calves). Boys with DMD develop difficulties with running, jumping and climbing stairs due to progressive muscle weakness. A classical sign of DMD is the “Gower signs” (see image below) – ask a child to lie flat on his back on the floor and ask the child to stand up quickly. A boy with Duchenne will roll onto his tummy and uses his hands and arms to climb up his body in order to stand up. Speech delay, learning and behavioural difficulties can also be a clinical feature of DMD. A Gower’s test is illustrated in pREMS-gait.
Videos of abnormal gaits due to muscle disease are available.
The graphic below demonstrates Gowers test - the child has difficulty getting to the upright position due to proximal weakness.
The first step in diagnosis of DMD is a blood test for creatine phosphokinase (CPK); a muscle enzyme. Boys with DMD have very high levels of CPK (10-100 times normal). If the CPK is normal then DMD can be excluded. If the CPK is high then a referral to paediatric neurology (or Neuromuscular Disease centre) is required asap. The diagnosis is confirmed by genetic testing, which analyses the DNA in order to identify the specific mutation in the dystrophin gene. In most cases this gives the diagnosis, only rarely these days a muscle biopsy is needed for the diagnosis of DMD.
Typically boys with DMD lose the ability to walk between the ages of ten and fourteen. In their late teens they progressively lose the strength in their upper body, leading to difficulties raising their arms for example for brush their teeth or for feeding. The disease also affects the heart and respiratory muscles and eventually boys with DMD will need help with breathing. Life span is reduced in DMD due to cardiac or respiratory failure but in recent years steroids have significantly slowed down the disease progression and delayed the onset of respiratory complications.
While there is still no cure for DMD, it is one of the conditions where there is substantial active research and where several potential new therapies are currently being tested in clinical trials. It is also a condition for which experts have established internationally approved care guidelines that can make a big difference to the quality of life and life expectancy of a boy with the disease (http://www.treat-nmd.eu/care/dmd/diagnosis-management-DMD/)
Becker muscular dystrophy
This is a milder form of muscular dystrophy. It is related to Duchenne muscular dystrophy as both result from a mutation in the dystrophin gene. The difference is that in Becker muscular dystrophy there is still some dystrophin being produced and expressed in the muscles (in Duchenne there is no dystrophin); this results in milder symptoms and signs. The disease onset is later and there is a much slower rate of progression. Loss of ambulation (walking) may not occur until the patient is in his fifties. Patients with Becker muscular dystrophy however are still at risk to develop cardiomyopathy even without a significant skeletal muscle weakness.
The vast majority are due to lack of vitamin D, resulting from poor sunlight exposure or dietary inadequacy, but can also occur with malabsorption (e.g coeliac disease or inflammatory bowel disease)
Clinical features include bow legs, joint (metaphyseal) swelling, bossed forehead and limb pain and irritability. Proximal myopathy also occurs with difficulty getting up from the floor and inability to jump.
The diagnosis is confirmed by bone chemistry, parathyroid hormone levels and 25OH vitamin D levels.