MUSCULAR DYSTROPHIES: TYPES
Muscular dystrophies are a varied group of genetic disorders affecting the skeletal, smooth, and even cardiac muscles.
Duchenne’s and Becker’s Muscular Dystrophies
Duchenne’s and Becker’s muscular dystrophies are a result of an X-linked recessive mutation in the dystrophin gene. In Duchenne’s muscular dystrophy (DMD), dystrophin in usually absent; in Becker’s muscular dystrophy (BMD), dystrophin is partially functional. Dystrophin is found in skeletal, smooth, and cardiac muscles, as well as the brain. It maintains muscle membrane integrity. DMD is more severe and more common than BMD. The onset of DMD begins in early childhood, with the majority presenting by age 4. Life expectancy is in the 30s; death results primarily from respiratory failure. In BMD, the onset of symptoms is in the teens or twenties, and patients can live into their early 40s.
In both DMD and BMD, there is progressive weakness and wasting of proximal muscles. Presenting symptoms in DMD include a waddling walk and difficulty climbing stairs. The classic Gower maneuver describes using both arms to assist in getting from a sitting to a standing position. The disease affects the heart in 90% of DMD and BMD patients, although there is no correlation between the severity of skeletal muscle and severity of cardiac muscle involvement. Echocardiogram may be normal or show wall motion abnormalities. Dilated cardiomyopathy is seen as the disease progresses. Cognitive impairment is also often seen in affected patients.
Limb girdle dystrophy is similar to DMD and encompasses a heterogeneous group of disorders. Anesthetic considerations are similar to those with DMD/BMD.
Myotonic dystrophy is characterized by progressive muscle weakness. Aspiration occurs secondary to smooth muscle weakness, and cardiac arrhythmias contribute to increased mortality. Mitral valve prolapse is present in about a fifth of patients. Succinylcholine is contraindicated and can lead to sustained contractions that are not responsive to nondepolarizing muscle relaxants. Other triggers for myotonia include hypothermia, shivering, and mechanical/electrical stimulation. Myotonic reactions respond to treatment with phenytoin or quinine.
Myotonia congenita has two forms: one with autosomal recessive and the other with autosomal dominant inheritance. It is characterized by uncontrolled temporary muscle excitability due to mutations in the chloride channel gene. Succinylcholine can lead to severe masseter muscle spasm in these patients. Dantrolene is usually effective. Patients should be kept normothermic because shivering can trigger myotonia.
MUSCULAR DYSTROPHIES: ANESTHETIC CONSIDERATIONS
DMD and BMD require careful preoperative evaluation, with close attention paid to the cardiopulmonary systems. Preoperative workup should include an EKG and echocardiogram. Patients are at risk of aspiration due to involvement of the bulbar and laryngeal muscles. Postoperatively, DMD patients are at risk of respiratory compromise. Scoliosis may further compromise respiratory status. Pulmonary function ...