Thursday, 4 November 2010

MRCP revision battle 42.2: Mononeuropathies

Mononeuropathies are lesions of individual nerves.  The main causes of mononeuropathies are trauma or entrapment.

If 2 or more nerves are affected it is called mononeuritis multiplex. Causes may be remembered by the mnemonic DRAWS PLC:
  • diabetes
  • rheumatoid
  • amyloid
  • wegeners
  • sarcoid/SLE
  • PAN
  • leprosy/lyme disease
  • carcinomatosis/churg strauss

Important individual mononeuropathies to be familiar with include:

Key nerves of the arm: RUM - radial nerve C5-T1, ulnar never C6-T1 and median nerve C7-T1

Radial Nerve
  • Origin: C5-T1
  • Route: radial groove in humerus then anterior to the lateral epicondyle
  • Sensory supply: lateral 3.5 fingers in back of hand; only small area at base of thumb unique
  • Motor supply: tricipes and extensors of wrist/fingers plus suppinators
  • Classical presentation: 'Saturday night paralysis' - fall asleep with arm across back of chair, compress radial nerve in its groove and bruise it so awake with wrist drop.

Ulnar Nerve
  • Origin: C6-T1
  • Route: posteriomedial aspect of humerus then adjacent to ulna.  Runs superficial to flexor retinaculum into hand.
  • Sensory supply: medial 1.5 fingers
  • Motor supply: interossei, hypothenar eminence, medial 2 lumbicals
  • Classical presentation: claw hand/unable to cross fingers 

Median Nerve
  • Origin: C7-T1
  • Route: medially, goes through carpal tunnel
  • Sensory supply: lateral 3.5 fingers palmar aspect of hand plus nail beds of these fingers
  • Motor supply: pronator, LOAF = 1st and 2nd lumbricals, opponens pollicis, abductor pollicis brevis, flexor pollicis brevis (=thenar enimence)
  • Classical presentation: carpal tunnel syndrome 

Carpal Tunnel Syndrome is so common it deserves its very own battle...