Muscular dystrophies

1. What is the inheritance of Duchenne's/Becker's MD?
2. What is the clinical presentation of MD?
3. What is seen on muscle biopsy and labs in MD?
4. What is the presentation of myotonic dystrophy?

Answers:
1. X-linked.
2. Prox muscle weakness, calf pseudohypertrophy, cardiomyopathy, mental retardation, Gower's sign, increased lumbar lordosis, abnormal MSR.
3. Biopsy shows no dystrophin (Duchenne) or decreased (Becker). Increased CPK.
4. Autosomal dominant, presents as infant. Distal weakness, hatchet face (wasting of temporalis and masseter), frontal balding, poor vision, hypertrichosis, mental retardation. Biopsy shows type I fiber atrophy and type II fiber hypertrophy.

Myopathy

1. What are seven different categories of myopathy?
2. What is the clinical presentation of myopathy?
3. What is seen on EMG in myopathy?
4. What is the utility of quantitative EMG for myopathy?
5. What is seen with rep stim and SFEMG in myopathy?
6. Which disorders show Type II fiber atrophy on biopsy?

Answers:
1. Dystrophic (Duchenne, Becker), congenital, metabolic (acid maltase deficiency), inflammatory (polymyositis, dermatomyositis), endocrine (thyroid, adrenal), toxic (alcohol, diuretics, vincristine), steroids.
2. Muscle atrophy, hypertrophy, abnormal MSR, weakness, hypotonia, gait abnormalities, or myotonia (painless delayed relaxation of muscles).
3. SNAP normal, CMAP low amplitude, EMG may show fibs, PSW, CRD, or myotonic discharges. Classic MUAPs are small duration, small amplitude due to loss of muscle fibers, but collateral sprouting may result in long duration large amplitude MUAPs. MUAPs may be unstable at the beginning of sprouting.
4. Used to calculate mean duration by using 20 MUAPs.
5. Rep stim may show a decremental response due to reduced safety factor in regenerating immature NMJs. SFEMG shows increased jitter, fiber density, and blocking.
6. Steroid myopathy, myasthenia gravis, deconditioning.

Single fiber EMG

1. When is SFEMG used?
2. What sorts of diseases cause SFEMG to be abnormal?
3. What is fiber density? What is normal density?
4. What is jitter? What is normal?
5. What is blocking? How long does it take to resolve?

Answers:
1. If rep stim of at least three muscles is normal and abnormal diagnosis is still suspected.
2. NMJ disorders, MND, peripheral neuropathy.
3. Represents the number of single fibers belonging to the same motor unit within the recording radius of the electrode. A density of 1.5 is normal, higher represents denervation and reinnervation.
4. Small variation between the interpotential discharges of two muscle fibers of the same motor unit. Normal is 10-60 u.
5. Abnormality that occurs when a single muscle fiber AP fails to appear, which occurs with jitter > 100 u. Resolves in 1-3 months, after reinnervation is completed.

High rate repetitive stimulation

1. What is the rate for HRRS? Is there any substitute for this?
2. What is the effect of HRRS?
3. What occurs as a result of HRRS in MG, LEMS, and botulism?
4. What is pseudofacilitation?

Answers:
1. 10-50Hz. May substitute maximal isometric contraction.
2. Accumulation of calcium in the cell.
3. MG shows a decrement, LEMS shows a 200-300% increment, and botulism shows a mild increment.
4. Pseudofacilitation is the progressive increase in CMAP amplitude with HRRS or voluntary contration in a normal patient, which results from synchronicity of muscle fibers (decreased temporal dispersion).

Post-activation facilitation and exhaustion

1. What is post-activation facilitation? When is it performed?
2. What is post-activation exhaustion? When is this test performed?

Answer:
1. PAF is a 10 second isometric contraction (or tetany-producting stim of 50 Hz) that is done if decrement is noted on LRRS.
2. Response seen as CMAP amplitude decreases, which occurs with a LRRS performed every minute for 5 mins after initial isometric contraction. This test should be used if there is no decrement with LRRS, but a neuromusc junction disorder is still suspected.

Repetitive nerve stimulation

1. What muscles are stimulated in repetitive stim?
2. What decrease in amplitude is significant?
3. What rate is used for low rate repetitive stim (LRRS)?
4. What happens if the safety factor is decreased in LRRS?
5. What is the amplitude change for LRRS in MG, LEMS, and botulism.

Answers:
1. ADM/APB, deltoid, trapezius, orbicularis oculi.
2. Greater than 10% decrease from 1st to 5th waveform.
3. 2-3 Hz.
4. Each stimulus causes the EPP amplitude to drop, so a reduced safety factor will result in the potential falling below the threshold for activation --> reduced MUAP amplitude.
5. All three disorders result in a >10% drop in amplitude.

Botulism

1. What is the mechanism behind botulism?
2. When does botulism occur after ingestion?
3. What is the presentation of botulism?
4. What does EMG show?
5. What is the treatment for botulism?

Answers:
1. Toxin blocks exocystosis of ACh from nerve terminal.
2. Begins 2-7 days after ingestion.
3. Bulbar symptoms first (ptosis, dysphagia, dysarthria), diarrhea, N/V, widespread paralysis or flaccidity, abnormal MSR, respiratory and cardiac dysfunction.
4. SNAP normal, CMAP reduced amplitude, unstable MUAP.
5. Trivalent ABE antitoxin, wait for collateral sprouting.