Diagnosis:
The diagnosis is made based on clinical features supported by neurophysiology (NCS/EMG)
Findings on investigations:
Neurophysiology (NCS/EMG)
SNAP:
- More sensitive than CMAP
- Normal conduction velocity and distal latency
- Decreased amplitude in affected nerve (may be normal initially).
CMAP:
- Indicates severe injury
- Decreased amplitude
- Conduction block distal to Erb’s point (i.e. amplitude is reduced at Erb’s point and increased distally) may occur
- Slowing of conduction velocity at Erb’s point indicates demyelinating lesion
F-wave: nonspecific
EMG:
- Fibrillations and Positive Sharp Waves PSW in denervated muscles
- If reinnervation has occurred: MUAP shows decreased recruitment, long duration, increased amplitude, polyphasia
- Paraspinal muscles are normal (dorsal rami supply these)
Upper trunk:
SNAP: affected amplitudes
- Lateral antebrachial nerve
- Median nerve to 1st digit
- Radial
CMAP:
- Musculocutaneous nerve to Biceps
- Suprascapular nerve to supraspinatus
- Axillary nerve to deltoid
EMG:
- Involvement of: supraspinatus, biceps, pronator teres, deltoid, brachial
Middle trunk:
SNAP: affected amplitudes
- Median nerve to 3rd digit and 4th digit
CMAP:
- Radial nerve to extensor digitorum communis
EMG:
- Involvement of: Latissimus dorsi, teres major, extensor digitorum communis, pronator teres, flexor carpi radialis
Lower trunk:
SNAP:
- Ulnar nerve to 5th digit
- Medial antebrachial
CMAP:
- Ulnar nerve to adductor digiti minimi
- Median nerve to abductor pollicis brevis
EMG:
- Involvement of: abductor digiti minimi, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum
Lateral Cord:
SNAP: affected amplitudes
- Lateral antebrachial nerve
- Median nerve to 1st digit
CMAP:
- Musculocutaneous nerve to biceps
EMG:
- Involvement of: Biceps, pronator teres, flexor carpi radialis, Pectoralis
- Sparing of: Suprapinatus, infraspinatus, levator scapulae
Posterior Cord:
SNAP: reduced amplitudes in
- Radial
CMAP:
- Axillary nerve to deltoid
- Radial nerve to extensor carpi ulnaris
EMG:
- Involvement of: Latissimus dorsi, teres major, deltoid, radial muscles
Medial Cord:
SNAP:
- Ulnar nerve to 5th digit
- Medial antebrachial nerve
CMAP:
- Ulnar nerve to abductor digiti minimi
- Median nerve to abductor pollicis brevis
EMG:
- Involvement of: Ulnar muscles, flexor digitorum superficialis, flexor pollicis longus, abductor pollicis brevis,
Investigations to consider:
- CT neck and thorax: cervical rib
- MRI brachial plexus,
- NCS/EMG: rule out radiculopathy
Causes of brachial plexopathy:
Compressive/traumatic/radiation:
- Neurogenic thoracic outlet syndrome (a type of cervical rib syndrome)
- Radiation induced plexopathies (radiation plexopathy)
- Erb’s palsy
Immune:
- Acute brachial plexus neuritis a.k.a. Parsonage-Turner syndrome formerly brachial neuralgic amyotrophy
Infectious:
- Herpes Zoster plexitis, neuritis and ganglionitis
Neoplastic:
- Carcinomatous brachial plexopathy: lung cancer (e.g. pancoast’s tumor), breast cancer
Hereditary:
- Heredofamilial brachial plexopathy a.k.a. hereditary brachial plexus neuropathy a.k.a. hereditary neuralgic amyotrophy HNA, rare