A dermatome is an area of skin that is mainly supplied by afferent nerve fibers from a single dorsal root of spinal nerve which forms a part of a spinal nerve. There are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these nerves relays sensation from a particular region of skin to the brain. A dermatome also refers to the part of an embryonic somite. Along the thorax and abdomen the dermatomes are like a stack of discs forming a human, each supplied by a different spinal nerve. Along the arms and the legs, the pattern is different: the dermatomes run longitudinally along the limbs. Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints. An area of skin innervated by a single nerve is called a peripheral nerve field.
Clinical significance
A dermatome is an area of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Symptoms that follow a dermatome may indicate a pathology that involves the related nerve root. Examples include somatic dysfunction of the spine or viral infection. Certain skin problems tend to orient the lesions in the dermatomal direction. In referred pain, sensory nerve fibers such as that from dermatomes may come together at the same spinal cord level as the general visceral afferent fibers such as that from the heart. When the general visceral sensory fiber is simulated, the central nervous system does not clearly discern whether the pain is coming from the body wall or from the viscera, so it perceives the pain as coming from somewhere on the body wall, e.g. left arm/hand pain, jaw pain. So the pain is "referred to" the related dermatomes of the same spinal segment. Viruses that lie dormant in nerve ganglia, often cause either pain, rash or both in a pattern defined by a dermatome. However, the symptoms may not appear across the entire dermatome.
Important dermatomes and anatomical landmarks
Following is a list of spinal nerves and points that are characteristically belonging to the dermatome of each nerve:
T4 – Intersection of the midclavicular line and the fourth intercostal space, located at the level of the nipples.
T5 – Intersection of the midclavicular line and the fifth intercostal space, horizontally located midway between the level of the nipples and the level of the xiphoid process.
T6 – Intersection of the midclavicular line and the horizontal level of the xiphoid process.
T7 – Intersection of the midclavicular line and the horizontal level at one quarter the distance between the level of the xiphoid process and the level of the umbilicus.
T8 – Intersection of the midclavicular line and the horizontal level at one half the distance between the level of the xiphoid process and the level of the umbilicus.
T9 – Intersection of the midclavicular line and the horizontal level at three quarters of the distance between the level of the xiphoid process and the level of the umbilicus.
T10 – Intersection of the midclavicular line, at the horizontal level of the umbilicus.
T11 – Intersection of the midclavicular line, at the horizontal level midway between the level of the umbilicus and the inguinal ligament.
T12 – Intersection of the midclavicular line and the midpoint of the inguinal ligament.
L1 – Midway between the key sensory points for T12 and L2.
L2 – On the anterior medial thigh, at the midpoint of a line connecting the midpoint of the inguinal ligament and the medial epicondyle of the femur.
