Disorders of PNS

Question 1

How does a nerve conduction study help differentiate myelin vs. axonal pathology?

Answer 1

Myelin disorder (demyelination) --> conduction speed affected
Axonal disorder (axonal loss) --> amplitude affected

Question 2

Clinical features of distal median nerve lesion

Answer 2

Distal median nerve lesion affects wrist (like in carpal tunnel syndrome), as opposed to proximal lesion that would affect the forearm.

1st and 2nd lumbrical weakness –> index and middle finger claw with finger flexion

Thenar atrophy

Question 3

Clinical feature of ulnar neuropathy

Answer 3

3rd and 4th lumbrical weakness –> ring and pinky finger claw with finger extension

Question 4

Clinical features of radial neuropathy

Answer 4

Radial nerve innervates major extensors of hand/forearm –> wrist and finger drop (“Saturday night palsy”)

Question 5

Clinical features of peroneal neuropathy

Answer 5

Usually due to habitual knee crossing

Weakness in dorsiflexion and ankle eversion

Question 6

Compare clinical features of large fiber vs. small fiber polyneuropathies

Answer 6

Large fiber (motor/sensory for proprioception/fine touch) polyneuropathy would result in numbness, loss of sensation, gait imbalance, and muscle weakness

Small fiber (pain/temperature) polyneuropathy would result in burning/”pins and needles” sensation, dysautonomia

Question 7

Guillain-Barre Syndrome (GBS)/Acute Inflammatory Demyelinating Polyneuropathy (AIDP)

Answer 7

Acquire polyneuropathy, usually from Campylobacter (GI illness) or CMV infection, that results in Schwann cell destruction and following clinical features that peak acutely ~ 4 weeks:

1) Ascending motor weakness and sensory symptoms
2) Dysautonomia –> tachycardia, urinary retention, arrhythmia, loss of sweat, etc.
3) Increased CSF protein (“cytoalbumino dissociation”) but minimal EMG findings (since it’s acute?)

Treat with respiratory support, plasmapheresis, IVIG

Question 8

Chronic Inflammatory Demyelinating Polyneuropathy (CIDP)

Answer 8

Symptoms peak ~ 8 weeks

Elevated CSF protein with DEMYELINATING features

Frequent association with MGUS (need to be followed by heme/onc for progression to MM)

Question 9

What trunk does the suprascapular nerve come out of in the brachial plexus? What muscle(s) does it innervate?

Answer 9

Suprascapular nerve comes out of the upper trunk (formed by C5/C6) and innervates the supraspinatus and infraspinatus muscles responsible for abduction of the shoulder.

Question 10

Pathophysiology for Erb and Klumpke’s palsy

Answer 10

Erb’s Palsy due to lesion of upper trunk (C5, C6) usually due to neck laceration/trauma or from birth. Results in “waiter’s tip” with and sensory deficit over lateral arm (C5/C6 dermatome).

Klumpke’s palsy due to lesion of lower trunk (C7, C8) usually due to upward arm extension. Result’s in full “claw” hand and sensory deficit over medial arm and pinky finger (C8/T1 dermatome).

Question 11

Amyotrophic Lateral Sclerosis (ALS)/Lou Gehrig’s Disease

What other pathologies can present similarly to ALS?

Answer 11

Degenerative disease of upper and lower motor neurons of corticospinal tract.

Anterior horn degeneration –> lower motor neuron signs

Lateral horn degeneration –> upper motor neuron signs

Most die from respiratory failure

Usually sporadic but familial cases associated with superoxide dismutase mutation (SOD1)

Cervical disk impairment (C5-T1) can present like ALS. However, in ALS there are only motor deficits, while cervical disk impairment results in both motor and sensory deficits (corresponding myotome and dermatomes affected).