Peripheral Nervous System

Question 1

West Nile virus

Answer 1

• May infect lower motor neurons / anterior horn cells
• Type of arbovirus
• Most have no symptoms, but 20% have moderate to severe symptoms
• WNV can cause encephalitis or meningitis as well
  
  • WNV meningitis is characterized by presence of a “coarse tremor” in upper extremities

Question 2

Radiculitis

Answer 2

Inflammation of the spinal nerve roots

Question 3

In the cervical spine, disk herniation affects the __.

In the lumbar spine, disk herniation affects the __.

Answer 3

In the cervical spine, disk herniation affects the lower of the two vertebral nerves (ex, a C3-C4 herniation affects C4).

In the lumbar spine, disk herniation affects the nerve below the level of the joint (ex, an L4-L5 herniation affects S1)

Question 4

Spinal stenosis at the level of the cervical spine produces ___.

Spinal stenosis at the level of the lumbar spine produces ___.

Answer 4

Spinal stenosis at the level of the cervical spine produces symptoms of spinal cord compression at and below the level of the stenosis.

Spinal stenosis at the level of the lumbar spine produces cauda equina syndrome.

Question 5

Carpal tunnel syndrome

Answer 5

• Compression of the median nerve within the carpal tunnel of the wrist

Question 6

Cubital tunnel syndorme

Answer 6

• Compression of the ulnar nerve as it passes through the cubital tunnel of the elbow

Question 7

Axonopathies

Answer 7

Glove and stocking distribution

Affect peripheral nerves in order of length

Question 8

Myelinopathies

Answer 8

Demyelination

Affects peripheral nerves randomly

Question 9

Myasthenia gravis vs Lambert Eaton myasthenic syndrome

Answer 9

Question 10

In myasthenia gravis, the autoantibody target is ___. In Lamber Eaton myasthenia, the target is ___.

Answer 10

In myasthenia gravis, the autoantibody target is the acetylcholine receptor. In Lamber Eaton myasthenia, the target is the voltage-gated calcium channels.

Question 11

Diagnosing myasthenia gravis

Answer 11

• Screen for anti-AChR and anti-Muscle-specific tyrosine kinase (MuSK)
  
  • 90% of patients with systemic myasthenia have one, 50% of patients with ocular myasthenia have one
• Screen for thymoma
• Repetitive nerve stimulation test
  
  • Amplitude of action potential decreases w/ repetitive stimulation
• Ice pack test
  
  • If a patient has ptosis, leave an ice pack over eye for 2 min. Ptosis should go away if due to myasthenia gravis
• Endrophonium challenge
  
  • Inhibits AChE, should temporarily alleviate symptoms.

Question 12

Diagnosing Lamber-Eaton Myasthenia

Answer 12

• Screen for anti-VGCC
• Screen for neoplasia (especially small cell lung cancer)
• Repetitive nerve stimulation test
  • Amplitude of action potential increases w/ repetitive stimulation

Question 13

Long-term treatment of myasthenia gravis and Lamber Eaton myasthenia

Answer 13

1. Remove thymoma, if present
2. Treat cancer
3. Neutralize autoantibody w/ plasma exchange or IVIg
4. Long-term immunosuppression
   
   • Prednisone or steroid-sparing agents
5. For myasthenia gravis, pyridostigmine
6. For Lamber-Eaton syndrome, 3,4-diaminopyridine

Question 14

How does 3,4-diaminopyridine work?

Answer 14

It blocks the potassium channels on the pre-NMJ neuron, causing accumulation of positive charge as Ca2+ flows in through the free VACCs and facilitating ACh release to the NMJ.

Question 15

Hints that weakness may be due to autoimmune disease at the NMJ

Answer 15

Fluctuating weakness with diurnal variation, which is sometimes temperature-dependent.

Question 16

Findings on EMG

Answer 16

• Monitor at rest for fibrillation potentials, indicating irritability of the nerve likely due to nerve damage
• Monitor during action for motor unit potential analysis
  
  • Damaged and irritable nerves will have much more jagged and higher amplitude impulses, as opposed to the slow and orderly impulses in an organized rhythmic contraction

Question 17

Findings on Nerve conduction studies

Answer 17

• Amplitude correlates to # of functional neurons in the nerve
• Latency from initiation to peak. Surrogate for degree of nerve myelination.
• Repeat testing over different nerve segments, which may suggest very focal neuropathy that is likely due to compression.

Question 19

Deep tendon reflex and nerves

Answer 19

Question 20

A good way to track progress in patients being treated for myasthenia gravis

Answer 20

Monitoring the amount of ptosis in each eye

The muscles controlling ocular muscles are preferentially affected in myasthenia gravis. Ptosis and double vision are among the most common manifestations of the disease.