Week 4 Muscle Diseases- Nelson

Question 1

Describe the genetic defect for spinal muscular atrophy.

Answer 1

a. Mutations affecting survival motor neuron 1 (SMN 1)
b. A gene on chromosome 5 that is required for motor neuron survival
c. Patients experience loss of motor neurons leading to muscle atrophy and weakness
d. Most common form = Werdnig-Hoffmann disease

Question 2

Compare and contrast the pathogenesis and clinical outcome of Duchenne and Becker muscular dystrophy.

Answer 2

-Duchenne → have little or no dystrophin, symptoms by age 5, duck-like gate, wheel chair dependent by 10-12 years

Becker → decreased amounts of dystrophin or a defective abnormal form dystrophin, later onset with milder symptoms, longer survival

Question 3

Describe the genetic defect for myotonic dystrophy

Answer 3

• Most common adult muscular dystrophy

- Increased CTG trinucleotide repeat sequences on chromosome 19 → effects chloride channel called CLC1

Question 4

Describe the genetic defect and clinical presentation of malignant hyperpyrexia (malignant hyperthermia).

Answer 4

• Mutations in the proteins that control levels of cytosolic calcium
• Marked hypermetabolic state triggered by certain inhalational anestetics
• Tachycardia, tachypnea, muscle spasms, etc.

Question 5

Compare and contrast the pathogenesis, clinical presentation, and pathologic findings of the 3 inflammatory myopathies.

Answer 5

• Dermatomyositis → immunologic injury and damage to small blood vessels and capillaries in the skeletal muscle (Skin + myositis). Discoloration of eyelids and Gottron papules → scaling erythematous eruption or dusky red patches over the knuckles, elbows, and knees. Clinical syndrome associated with malignancy!! Screen for malignancy!
• Polymyositis → similar but lacks skin involvement. Antibodies against histidyl t-RNA synthase caused by activated CD8+ cytotoxic T-cells.
• Inclusion Body Myositis → most common, begins with distal muscle involvement

Question 6

Compare and contrast the pathogenesis and key clinical associations of myasthenia gravis and Lambert-Eaton myasthenia syndrome.

Answer 6

• Myastenia gravis → autoimmune disease causing loss of function of the acetylcholine receptor (AChR), sometimes antibody against muscle specific tyrosine kinase. Associated with thymic abnormalities
• Lamber-eaton myasthenia → autoantibodies against presynaptic calcium channels, which block actelycholine release. Extremity weakness, rapid repetivitive stimulation of affected muscle increases the muscle response. Paraneoplastic syndrome.

Question 7

List the causes of elevated creatine kinase.

Answer 7

a. Myocardial infarct
b. Skeletal muscle disease such as inflammatory myopathies
c. Cerebrovascular accidents, head injury
d. *Total CK is helpful in the assessment of skeletal muscle inury in the absence of cardiac disease or other conditions that may cause increased CK.