A Woman With Arm Weakness After Treatment for Breast Cancer

A 68-year-old woman with left breast cancer presented with some discomfort in the left shoulder and weakness with difficulty raising the arm for 6 months after she had been treated with chemotherapy and radiation.

Examination revealed weakness in the left deltoid 3/5, spinati 4/5, biceps 4+/5, and brachioradialis 4/5; other muscles of the arms and legs were normal. Sensory examination showed a patchy sensory deficit in the lateral forearm. Reflexes were 2+, except for the left biceps and brachioradialis where they were absent. The rest of the neurologic examination was normal.

What is the Differential Diagnosis?

This patient with a history of left breast cancer developed weakness in the left upper extremity, particularly in muscles innervated by the C5–C6 roots and the upper trunk of the brachial plexus. The differential diagnosis includes a radiculopathy, a plexopathy caused, for example, by a brachial neuritis, but the progressive course and previous history of cancer and radiation suggest that the etiology would be either neoplastic invasion or a postradiation plexopathy.

Neoplastic invasion of the plexus affects mainly the lower trunk, whereas radiation affects mainly the upper trunk. Although the patient had some discomfort, she had no severe pain, also favoring a diagnosis of radiation plexopathy because, in carcinomatous plexopathy, pain can be significant.

An EMG Test was Performed

Motor Nerve Studies

Nerve and Site	Latency(ms)	Amplitude (mV)	Conduction Velocity (m/s)
Median Nerve L.	Normal ≤ 4.2	Normal ≥ 6	Normal ≥ 50
Wrist	3.9	8	–
Elbow	7.9	8	51

Ulnar Nerve L.	Normal ≤ 3.6	Normal ≥ 8	Normal ≥ 50
Wrist	3.2	9	–
Below elbow	6.8	8	53
Above elbow	9.5	8	50
Axilla	12.3	8	50
Erb’s point	15.4	8	47

Nerve and Site	Latency (ms)	Amplitude (mV)	Conduction Velocity (m/s)
Ulnar Nerve R.	Normal ≤ 3.6	Normal ≥ 8	Normal ≥ 50
Wrist	2.8	16	–
Below elbow	5.9	16	58
Above elbow	7.8	16	63
Axilla	11.2	13	60
Erb’s point	13.5	12	61
Musculocutaneous Nerve L.
Erb’s point	4.2	0.5 ^a	–
Musculocutaneous Nerve R.
Erb’s point	3.8	8 ^a	–