Dermatome Anatomy

A dermatome is an area of skin that is mainly supplied by a single spinal nerve. There are eight cervical nerves (C1 being an exception with no dermatome), twelve thoracic nerves, five lumbar nerves and five sacral nerves. Each of these nerves relays sensation (including pain) from a particular region of skin to the brain. 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. A similar area innervated by peripheral nerves 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 (e.g. like pain or a rash) may indicate a pathology that involves the related nerve root. Examples include somatic dysfunction of the spine or viral infection. Referred pain usually involves a specific, “referred” location so is not associated with a dermatome. Viruses that hibernate[clarification needed] in nerve ganglia (e.g. Varicella zoster virus, which causes both chickenpox and herpes zoster) often cause either pain, rash or both in a pattern defined by a dermatome. However, the symptoms may not appear across the entire dermatome.

Following is a list of spinal nerves and points that are characteristically belonging to the dermatome of each nerve:[1] •C2 – At least one cm lateral to the occipital protuberance at the base of the skull. Alternately, a point at least 3 cm behind the ear. •C3 – In the supraclavicular fossa, at the midclavicular line. •C4 – Over the acromioclavicular joint.

•C5 – On the lateral (radial) side of the antecubital fossa, just proximally to the elbow. •C6 – On the dorsal surface of the proximal phalanx of the thumb. •C7 – On the dorsal surface of the proximal phalanx of the middle finger. •C8 – On the dorsal surface of the proximal phalanx of the little finger. •T1 – On the medial (ulnar) side of the antecubital fossa, just proximally to the medial epicondyle of the humerus. •T2 – At the apex of the axilla.

•T3 – Intersection of the midclavicular line and the third intercostal space •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 horizonal 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. •L3 – At the medial epicondyle of the femur.

•L4 – Over the medial malleolus.
•L5 – On the dorsum of the foot at the third metatarsophalangeal joint.
•S1 – On the lateral aspect of the calcaneus.
•S2 – At the midpoint of the popliteal fossa.
•S3 – Over the tuberosity of the ischium or infragluteal fold
•S4 and S5 – In the perianal area, less than one cm lateral to the mucocutaneous zone

Myotome
In vertebrate embryonic development, a myotome is a group of tissues formed from somites. These somites develop into the body wall muscle. Each myotome divides into a dorsal epaxial part and a ventral hypaxial part. The myoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls. The term “myotome” is also used to describe the muscles served by a single nerve root.[1] It is the motor equivalent of a dermatome. Each muscle in the body is supplied by a one or more levels or segments of the spinal cord and by their corresponding spinal nerves. A group of muscles innervated by the motor fibres of a single nerve root is known as a myotome.[2] The epaxial muscle mass loses its segmental character to form the extensor muscles of the neck and trunk of mammals. In fishes, salamanders, caecilians, and reptiles, the body musculature remains segmented as in the embryo, though it often becomes folded and overlapping, with epaxial and hypaxial masses divided into several distinct muscle groups. Clinical Significance

In humans myotome testing can be an integral part of neurological examination as each nerve root coming from the spinal cord supplies a specific group of muscles. Testing of myotomes, in the form of isometric resisted muscle testing, provides the clinician with information about the level in the spine where a lesion may be present.[3] During myotome testing, the clinician is looking for muscle weakness of a particular group of muscles. Results may indicate lesion to the spinal cord nerve root, or intervertebral disc herniation pressing on the spinal nerve roots. Myotome distributions of the upper and lower extremity are as follows;[4][5] •C1/C2-neck flexion/extension

•C3-neck lateral flexion
•C4-shoulder elevation
•C5-shoulder abduction
•C6-elbow flexion/wrist extension
•C7-elbow extension/wrist flexion
•C8-thumb extension
•T1-finger abduction
•L2-hip flexion
•L3-knee extension
•L4-ankle dorsi-flexion
•L5-great toe extension
•S1-ankle plantar-flexion
•S2-knee flexion