88 Neuroanatomy: Draw It to Know It
Referred Pain
Here, we will draw a map for referred pain, which is the
physiologic process whereby internal organs manifest
with body surface pain. Since Sir Henry Head wrote
about this subject in the late 1800s and early 1900s,
many accounts of the dermatomal distribution of vis-
ceral pain have been published; however, the true
pathophysiologic mechanism of visceral pain remains to
be determined. It results either from direct intermin-
gling of visceral and somatic aff erent fi bers or from indi-
rect somatic fi ber sensitization. Also, it either occurs
peripherally (ie, in the peripheral nerves) or centrally
(ie, in the spinal cord). Regardless of the exact pathophys-
iologic mechanism of referred pain, it is clear that vis-
ceral organs refer pain to their related dermatomal
distributions.
Using this rule, let’s consider the somatotopic map of
referred pain for a few important organs. Draw the trunk
and upper left arm. Indicate that innervation of the dia-
phragm comes from C3, C4, and C5, which cover the
neck, shoulders, and upper lateral arm. We remember
this innervation pattern by the mnemonic: C3, C4, C5
keeps the diaphragm alive! In accordance with the vis-
ceral-dermatomal rule we have established, diaphrag-
matic pain is felt in the neck and upper shoulder: the
dermatomal distribution of these cervical levels. Next,
let’s consider the visceral map for the heart. Show that,
generally, the T1 to T5 spinal nerves innervate this
organ; the upper thoracic spinal nerves cover the chest
and medial upper arm and forearm. Again, in accordance
with the referred pain principle, myocardial ischemia is
commonly felt along this upper thoracic dermatomal
distribution: the left side of the chest and inside of the
left arm. Note that classic cardiac pain does not extend
into the fi ngers, which are supplied by the C6 to C8
spinal nerves. Finally, consider the appendix, which is
innervated by T10. Appendicitis is fi rst felt as a vague,
painful sensation at the umbilicus — the dermatomal
distribution of T10. Only later, when the appendicitis
worsens, does the pain become somatic, at which time it
moves to the right lower quadrant.^12