Which nerve is involved and where is the site of the lesion?
a. ulnar nerve in the olecrenon groove
b. ulnar nerve next to the pisaform bone
c. median nerve in the arm
d. median nerve at the elbow
e. median nerve at the wrist
f. radial nerve at the mid-shaft humeral level
As is always the case with neural involvement, one must know something about the nerve of record, its configuration (mixed, motor, sensory), the route of the nerve and the site of the lesion. The key finding in this case is the paralysis of the flexor and abductor pollicis brevis. Both of these intrinsic muscles of the thumb are innervated by motor branches of the median nerve which emerges just distal to the transverse carpal ligament as the median nerve moves from the forearm to the hand through the carpal tunnel. The fact that we are dealing with a median nerve problem allows us to automatically exclude answers that suggest a problem with the ulnar or radial nerves. At this point we have three differential lesion sites that we must evaluate - median neuropathy at the arm, elbow or wrist. It's useful to apply proximal - distal or upstream downstream logic to answer the question. The general rule that we apply is: all motor and sensory function distal to the site of the lesion will be compromised. Put another way, structures proximal to the site of the lesion will be intact. If the median nerve was injured in the arm or elbow then we would expect all muscles in the anterior forearm and innervated by the median nerve (all except the flexor carpi ulnaris and the two ulnar heads of flexor digitorum profundus) to be paralyzed. We know this is not the case as the pronator teres, flexor digitorum superficiales, flexor digitorum profundus and the wrist flexors (flexor carpi radialis and ulnaris) which are all anterior forearm muscles and all innervated by the median nerve are intact. Since our examination revealed two intrinsic thumb muscles are paralyzed this means that the lesion must be somewhere between the motor points of the forearm flexors and the thumb. The only reasonable location of the lesion would therefore have to be at the wrist.
Now, it's conceivable that the lesion could be somewhere other than the wrist, however, given the choices available to us, this would be the most logical site of the lesion. Clinically, median nerve injuries are very common and often manifest as carpal tunnel syndrome, a repetitive stress injury.