The arm has too many muscles. It also has too many nerves.
The problem is that someone comes in with weakness or numbness and you need to think, “Where in the brain/spine/nerve root/bits of brachial plexus/terminal branch is the actual problem?!”
One of the ways to help suss this out is by testing the deep tendon reflexes.
- Only biceps reflex absent – might be problem with musculocutaneous nerve or C5
- Both brachioradialis and triceps absent – problem with the radial nerve
- Only triceps absent – potential problem with C7
Of course you should correlate the reflex findings with the sensory findings and motor strength (remember the good old ASIA exam for testing specific nerve roots) to help determine if it is a terminal branch issue or something higher up.
And don’t forget to grade those reflexes!
||Very brisk +/- Clonus
Generally upper motor neuron (UMN) lesions result in hyperreflexia while lower motor neuron (LMN) lesions result in hyporeflexia.
The median nerve provides sensory and motor innervation to the anterior compartment of the forearm and hand.
- Motor branches
- Pronator teres
- Flexor carpi radialis
- Flexor carpi sublimis
- Anterior interosseus (motor)
- Flexor pollicis longus
- Flexor digitorum profundus to 2nd & 3rd fingers
- Pronator quadratus
- Palmar cutaneous
- Sensory distribution: Skin over thenar eminence
- Terminal motor
- Abductor pollicis brevis
- Opponens pollicis
- Lumbricals: 1st & 2nd
- ± Flexor pollicis brevis: Also innervated by ulnar nerve
- Terminal sensory
- Sensory to palmar surface of thumb, 2nd, 3rd & lateral 1/2 of 4th finger
Physical exam for different muscles supplied by median nerve
- Flexor digitorum superficialis: patient flexes fingers at PIP joint against resistance
- Flexor digitorum profundus: patient flexes fingers at DIP joint against resistance
- Flexor pollicis longus (anterior interosseous nerve): flexes distal phalanx of thumb against resistance
- Abductor pollicis brevis: patient abducts thumb at right angles to palm against resistance
- Opponens pollicis: patient touches base of little finger with the thumb, examiner tries to pull apart
- 1st lumbrical interosseous: patient extends finger at the PIP joint against resistance with the MCP joint hyperextended and fixed
There are 4 main spinal cord syndromes, plus 2 sort-of-spinal-cord syndromes. Some are not common, but a favourite of examiners because they’re just so darn cool in that they really demonstrate where things cross (like in Brown-Sequard syndrome). Cauda equina syndrome is also an important one to know because if you see it acutely, the damage can actually be temporary (otherwise the person is stuck with permanent bowel and bladder dysfunction, which is really no fun).
ASIA stands for the American Spinal Injury Association. As part of their oh-so-standardized grading for spinal injuries, they devised the muscle groups you need to test corresponding to each nerve root.