hese are a collection of congenital, progressive muscular disorder arising from the inheritance of defects in genes that are important for muscular function. They can be categorised according to their inheritance pattern, and the weakness pattern that they display:
This video from the team at Osmosis provides a great introduction
These are gene mutations on the X chromosome, and so are inherited in an x-linked pattern. The 2 main form are Duchenne’s and Becker’s. Duchenne’s is the more common and more severe, but they share similar characteristics.
Duchenne's Muscular Dystrophy
Caused by a mutation at the Xp21 locus, resulting in an absence of muscle cell membrane protein dystrophin, which helps anchor muscle cells to the extracellular matrix. It typically affects around 1 in 3000 live male births.
It presents with weakness of the proximal muscles. This is typically of the legs first, and starting at an age of 2-3 years. The presentation may be of an abnormal gait, falling frequently, and difficulty running, climbing stairs, or rising from sitting. The weakness is progressive. Consequences usually include flexion contractures, scoliosis, and patients are usually wheelchair dependent by the age of 12. Patients may develop pseudohypertrophy. This is fatty/fibrous replacement of certain muscle groups, commonly the calves. Most patients die by their 20s, due to cardiac or respiratory complications e.g. pneumonia.
Cardiac involvement results in myocardial degeneration. There can be a resulting dilated cardiomyopathy and heart failure, quoted as occurring in about 50%. Respiratory involvement arises from the progressive involvement of respiratory muscles. There is a restrictive ventilatory pattern (kyphoscoliosis), impaired cough, and frequent respiratory tract infection. There may also be impaired smooth muscle function which can contribute to bleeding in surgery (from vessels) and impair gut function.
Investigation will usually be triggered by a suspicious clinical history. A creatinine kinase level will be very elevated (10-100 times normal), but actually falls later in the disease as muscles atrophy. Diagnosis can be confirmed by a combination of:
Carrier status (in close relatives) can be assessed by genetic testing and is essential.
Becker's Muscular Dystrophy
This is essentially a milder form of the Duchenne form. There is a similar mutation but that results in abnormal or reduced dystrophin, rather than absence. The incidence is about 1 in 17,000 live births.
Symptoms are similar to Duchenne’s but the clinical course is slower. The average age of diagnosis is age 11, and patients are usually ambulant until their mid teens. Most patients survive into their 30s or 40s.
The muscular dystrophies have a significant impact on the conduct of anaesthesia. As the most severe and common form, much of this relates to Duchenne’s MD. Particular perioperative concerns include:
Postoperative respiratory dysfunction
A full and detailed anaesthetic history is important in these patients. The nature of the condition will be varied between patients, and the specific impact needs understanding. Indeed, in some case an exact diagnosis of the myopathy may not be clear at that time. Based on this history, further specific investigations may be needed include:
Bloods - baseline CK, FBC, U&Es
The underlying cardiac dysfunction may be unrecognised by the patient, and only unearthed by the stress of the perioperative period.
Specific considerations for anaesthetic conduct include:
Depolarising muscular blockade
There are increased numbers of extrajunctional acetylcholine receptors in these patients.
Use of DMB may result in fatal hyperkalaemia due to excessive activity
Non-depolarizing muscular blockade
Their onset is delayed and effect prolonged
They should be used very sparingly and at reduced doses (10-20% of normal) if essential
Atracurium or mivacurium may be preferred due to their predictable breakdown
Inhalational anaesthetic agents
They have been associated with rhabdomyolysis in patients with Duchenne’s
This relationship is unclear, but a TIVA technique with clean anaesthetic circuit is recommended.
Impaired gastric motility
Abnormal gut smooth muscle function can result in impaired gastric emptying
This can increase the risk of regurgitation
A prokinetic (e.g. metoclopramide) and an antacid may be used to reduce this risk.
Increased bleeding risk
Impaired vascular smooth muscle function can increase bleeding risk
There is also impaired platelet function
These adverse effects are more associated with general anaesthesia, and thus regional anaesthesia is a potentially preferable anaesthetic technique. Spinal changes (kyphoscoliosis) make make neuraxial technique challenging.
There is a relationship between muscular dystrophy and malignant hyperthermia. A high index of suspicion is needed, looking for any tachycardia or rise in end tidal CO2. Whilst the solidity of this relationship is not as strong as previously thought, TIVA is still generally employed.
Admission to a critical care environment postoperatively is essential in nearly all cases. Patients are at high risk of respiratory complications and need close monitoring for this. Patients with impaired cough and sputum clearance may benefit from physiotherapy.
Other Muscular Dystrophy
lthough rarer, these are worth having some knowledge of.
Emery-Dreifuss Dystrophy A muscular dystrophy with a primarily x-linked inheritance The genes involved code for certain muscle membrane proteins. Muscle wasting and weakness usually begins before adulthood. The biceps and triceps are commonly involved, and less frequently the distal leg muscles. Cardiac involvement is common, with adverse features including cardiomyopathy and conduction abnormalities.
Facioscapulohumeral dystrophy Is an autosomal dominant condition. It involves weakness of the facial muscles and shoulder girdle, as per the name. Onset of weakness is usually between later childhood and late teens. Life expectancy is normal with disease progression tending to be very slow, although there is some variability in severity.
Limb-girdle dystrophy There are several subtypes, with autosomal dominant and recessive patterns. This reflects the pattern of pathology, with several structural and non structural muscle proteins that may be involved. The symptoms primarily involve weakness in a limb girdle and proximal limb pattern. The autosomal recessive patterns tend to involve the pelvic girdle. Onset can be from childhood to teens.
Congenital muscular dystrophy This is a collective label for the rare forms of MD that are present at birth.
Myotonic dystrophy (Dystrophia myotonica) is another important condition which is discussed in a little more detail elsewhere (see here).
Links & References
Allman, K. Wilson, I (eds). Oxford handbook of anaesthesia (3rd ed). 2012. Oxford University Press
Beers, M. et al (eds). The Merck Manual (18th ed). Mitral regurgitation. 2006. Merck Research Laboratories.