A 70-year-old woman developed pain in the right arm and shoulder with some neck discomfort. Four days later, she presented with a rash on the arm and could no longer raise the arm. Past medical and social histories were unremarkable.
Examination revealed an erythematous vesicular rash in the outer aspect of the right arm, and she had weakness of the right deltoid of 2/5 ( Fig. 15-1 ). Supraspinatus was 5/5 and brachioradialis was 3/5. Biceps strength was 3/5; triceps, forearm, and hand muscles were normal. The left arm and leg strength were normal. The right biceps and brachioradialis reflexes were absent; the triceps was normal. Reflexes in the legs and left arm were normal. Sensory examination showed diffuse hypoalgesia in the right lower arm; the affected dermatome was hard to localize. The rest of the neurologic examination was normal.
Fig. 15-1
Notice the patient’s inability to raise the right arm ( A ) and the characteristic rash, which is better seen in ( B ).
What is the Differential Diagnosis?
This woman had weakness in muscles innervated by C5–C6 roots and upper trunk of the brachial plexus with decreased brachioradialis and biceps reflexes. These can be caused by a focal lesion, such as a tumor in the plexus, brachial neuritis, a herniated disk, or other lesion in the cervical spine. However, the acute onset accompanied by a rash is very suggestive of a herpes zoster (HZ) infection.
An EMG Test was Performed
Motor Nerve Studies
Nerve and Site
Latency(ms)
Amplitude (mV)
Conduction Velocity (m/s)
Median Nerve R.
Normal ≤ 4.2
Normal ≥ 6
Normal ≥ 50
Wrist
4.1
10
–
Elbow
7.0
10
62
Ulnar Nerve R.
Normal ≤ 3.7
Normal ≥ 8
Normal ≥ 50
Wrist
3.7
9
–
Below elbow
6.9
9
53
Above elbow
9.2
9
52
F-Wave Studies
Nerve
Latency (ms)
Normal Latency ≤ (ms)
Median nerve R.
26.4
30
Ulnar nerve R.
27.6
30
Sensory Nerve Studies
Nerve
Onset Latency (ms)
Normal Onset Latency ≤ (ms)
Peak Latency (ms)
Normal Peak Latency ≤ (ms)
Amp (μV)
Normal Amp ≥ (μV)
Conduction Velocity (m/s)
Normal Conduction Velocity ≥ (m/s)
Median nerve R.
2.4
2.6
2.9
3.1
25
20
54
50
Ulnar nerve R.
2.3
2.6
2.8
3.1
18
13
52
50
EMG Data
Muscle
Insrt Activity
Fibs
Pos Waves
Fasc
Amp
Dur
Poly
Pattern
Cervical paraspinals R.
Inc
2+
2+
None
Norm
Norm
Few
Full
Infraspinatus R.
Norm
None
None
None
Norm
Norm
None
Full
Rhomboids R.
Norm
None
None
None
Norm
Norm
None
Full
Deltoid R.
Inc
2+
2+
None
Lg
Inc
Few
Red
Biceps brachii R.
Inc
2+
2+
None
Lg
Inc
Few
Red
Brachioradialis R.
Inc
2+
2+
None
Lg
Inc
Few
Red
Extensor digitorum comm. R.
Norm
None
None
None
Norm
Norm
None
Full
Flexor carpi radialis R.
Norm
None
None
None
Norm
Norm
None
Full
Flexor carpi ulnaris R.
Norm
None
None
None
Norm
Norm
None
Full
First dorsal interosseous R.
Norm
None
None
None
Norm
Norm
None
Full
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