Chapter 21 - Neuromuscular system disorders

Questions

Define the following terms:

neuropathy, myopathy, neuromuscular junction/myoneural disease, "dying back", demyelinative, Wallerian degeneration, epineurium, perineurium, endoneurium, Schwann cells, myelin, entrapment neuropathy, carpal tunnel, lateral femoral cutaneous neuropathy/meralgia paresthetica, polyneuropathy, Charcot-Marie Tooth, Lambert-Eaton myasthenic syndrome, paraneoplastic syndrome, myasthenia gravis, nerve conduction study, electromyography.
Neuropathy means a condition that is damaging nerves.
Myopathy is a condition that is damaging muscles.
Neuromuscular junction (myoneural) disease is a condition that is damaging the neuromuscular junction at the motor end-plate on muscles.
"Dying back" refers to a process in which the longest nerve fibers are injured and die.
Demyelinative refers to a process that damages the myelin sheath.
Wallerian degeneration happens to an axon after it has been cut off from its cell body. The distal end of the cut axon degenerates.
Epineurium refers to the connective tissue wrapping the nerve.
Perineurium refers to the connective tissue wrapping the fascicules within the nerve.
Endoneurium refers to the connective tissue wrapping each individual axon within the nerve.
Schwann cells are the cells that make myelin in the peripheral nervous system.
Myelin is a wrapping of cell membranes around an axon that speed up conduction.
Entrapment neuropathy is a condition where a nerve is caught between tougher tissues, contributing to chronic irritation and damage.
The carpal tunnel of the wrist is the most common site of nerve entrapment (in this case the median nerve).
Meralgia paresthetica is entrapment of the lateral femoral cutaneous nerve at the level of the inguinal ligament.
Polyneuropathy is generalized damage to peripheral nerves. The conditions that cause this usually affect the longest nerve first.
Charcot-Marie Tooth is a family of hereditary neuropathies that usually begin to show clinical signs in late childhood and adolescence and progress slowly.
Lambert-Eaton myasthenic syndrome is an autoimmune (and often paraneoplastic) condition that produces weakness.
A paraneoplastic syndrome refers to remote effects of a tumor. Most often it represents a condition in which antibodies directed at a tumor damage other tissues of the body.
Myasthenia gravis is a condition in which there is antibody-mediated destruction of the acetylcholine receptors producing weakness and fatigability of muscle.
A nerve conduction study is a test of speed and amplitude of conduction of peripheral nerve fibers.
Electromyography is a needle study in which the electrical activity of muscle fibers is recorded. It can detect damage to muscles and is sensitive to muscle fibers that have been disconnected from their nerves (denervated).


21-1. What modalities are conveyed by large, myelinated nerve fibers?

Answer 21-1. Large peripheral nerve fibers convey well-localized touch, pressure, vibration, joint position sense. They also comprise the axons of the alpha motor neurons.

21-2. What do small-diameter sensory nerve fibers convey?

Answer 21-2. Small fibers convey pain, temperature, very light touch and make up most of the autonomic nerve fibers as well.

21-3. What is entrapment neuropathy?

Answer 21-3. This is damage of a nerve due to focal pressure or irritation (such as carpal tunnel syndrome).

21-4. What are symptoms of polyneuropathy?

Answer 21-4. Distal, symmetrical (stocking, glove) sensory loss is most common and ankle jerk reflexes are usually lost early on. Some neuropathies are painful. It also may result in tissue damage if patients cannot detect injury.

21-5. What are the causes of polyneuropathy?

Answer 21-5. There are many causes - infection (leprosy, HIV, Lyme), diabetes, nutritional deficiency (thiamine, pyridoxine, B12), alcohol, toxins (eg, heavy metals), medications/drugs, hereditary (eg, Charcot-Marie Tooth), inflammatory (eg, lupus), AIDP (Guillain-Barre), CIDP

21-6. What are the potential causes of myopathy?

Answer 21-6. Muscle diseases (myopathies) may be metabolic, infectious, inflammatory, hereditary, drug or toxin-related.

21-7. What are the common symptoms of myopathy?

Answer 21-7. Myopathies usually result in proximal and symmetrical weakness (hip and shoulder girdle) before involving more distal muscles. There is no sensory loss (though muscles may be tender or cramp).

21-8. What effect do myopathies have on reflexes?

Answer 21-8. Reflexes are preserved until very late in myopathy.

21-9. What additional test would point to myopathy as a cause of weakness?

Answer 21-9. Many tests may help show muscles are damaged (EMG, muscle enzymes) but only biopsy and chromosomal analysis are likely to show the specific etiology.

21-10. What is the most common neuromuscular/myoneural junction disease?

Answer 21-10. Myasthenia gravis.

21-11. Who is most often affected by myasthenia gravis?

Answer 21-11. Most often young-adult females or late middle-aged males.

21-12. What are the symptoms of myasthenia gravis?

Answer 21-12. The symptoms are fatigue of muscles due to damage to acetylcholine receptors by antibody.

21-13. What blood test may be helpful in diagnosis of myasthenia gravis?

Answer 21-13. Autoantibodies to acetylcholine receptors are often found (although not always). These may be idiopathic or due to reaction to thymoma.

21-14. What regions of the body are most commonly affected by myasthenia gravis?

Answer 21-14. Myasthenia gravis usually affects extraocular muscles, though may be bulbar (swallowing, speech). Generalized myasthenia may affect diaphragm and other respiratory muscles causing respiratory insufficiency.

21-15. What is the treatment for myasthenia gravis?

Answer 21-15. Acetylcholinesterase inhibitors will strengthen muscles. Immune modulating therapy such as immunosuppressive medication, plasmapheresis or human immune globulin infusion often helps. Thymecotomy helps some patients.

21-16. What is Lambert-Eaton myasthenic syndrome?

Answer 21-16. It is an uncommon autoimmune condition characterized by weakness of hip girdle muscles with decreased reflexes. Autonomic instability is common. It often accompanies small cell cancer but can be idiopathic. There are usually autoantibodies against voltage-gated calcium channels. Strength increases with sustained or repeated contraction (this is different from myasthenia gravis).

21-17. What is the function of nerve conduction studies?

Answer 21-17. Nerve conduction studies evaluate the amplitude of response and speed of conduction along the fastest, largest nerve fibers. Damage to these fibers (especially the myelin sheath) will slow conduction, most severely in the area of damage.

21-18. What does electromyography evaluate?

Answer 21-18. EMG evaluates the health of muscles and can detect muscle fibers that have been denervated (either chronically or acutely).