Dermatome
Dr M Idris Siddiqui
Dermatome
• The surface of the skin is divided into specific areas
called dermatomes, which are derived from the
cells of a somite.
• These cells differentiate into the following 3 regions:
• (1) Myotome , which forms some of the skeletal
muscle;
• (2) Dermatome, which forms the connective tissues,
including the dermis; and
• (3) Sclerotome, which gives rise to the vertebrae.
Dermatome
• A dermatome is an area of skin in which
sensory nerves derive from a single spinal
nerve root
• A dermatome is defined as ‘a strip of skin
that is innervated by a single spinal
nerve‘.
• The area of skin supplied by a single
spinal nerve is called a dermatome.
Dermatome
• The dermatomes along the arms and legs
differ from the pattern of the trunk
dermatomes, because they run
longitudinally along the limbs.
• The general pattern is similar in all
people, but significant variations exist in
dermatome maps from person to person.
Dermatome
• On the trunk, adjacent dermatomes overlap
considerably, so that interruption of a single spinal
nerve produces no anaesthesia; the same applies to
the limbs, except at the axial lines.
• The line of junction of two dermatomes supplied
from discontinuous spinal levels is demarcated by
an axial line, and such axial lines extend from the
trunk on to the limbs.
• In the upper limb the anterior axial line runs from
the sternal angle across the second costal cartilage
and down the front of the limb almost to the wrist.
Dermatome
• Clinical Significance
• Dermatomes are useful to help localize
neurologic levels, particularly in
neuropathy.
• Dermatomes are clinically important and
necessary for assessing and diagnosing
the level of spinal cord levels.
Dermatome
• The dermatomes lie in orderly
numerical sequence when traced
distally down the front and proximally
up the back of the anterior axial line
(C5, 6, 7, 8 and T1) and these
dermatomes are supplied by the
nerves of the brachial plexus.
Dermatome
• In addition, skin has been
‘borrowed’ from the neck
and trunk to clothe the
proximal part of the limb
(C4 over the deltoid muscle,
T2 for the axilla).
Map of the dermatomes
• The dermatomes have been mapped by
sensory examination after sectioning of
dorsal nerve roots, electrical stimulation of
dorsal roots, or by recording
somatosensory evoked potentials elicited
by cutaneous stimuli. They are also
revealed by the pattern of lesions in cases
of herpes zoster (shingles).
Dermatomes of the extremities
• The organization of dermatomes in
the limbs is more complex than that
of the dermatomal distribution in
the trunk as a result of the limb buds
and corresponding dermatomes
being "pulled out" during early
embryologic development.
Dermatomes of the extremities
• The medial, intermediate, and lateral
supraclavicular nerves from the cervical
plexus supply the dermatomal distribution to
the root of the neck, upper pectoral, deltoid,
and the outer trapezius areas.
• The posterior divisions of the upper 3 thoracic
nerves supply the region over the trapezius
area to the spine of the scapula.
• The brachial plexus gives rise to most of the
rest of the cutaneous innervation of the upper
extremity.
Dermatomes of the extremities
• Contrary to the considerable
overlap of the dermatomes of the
trunk, the overlap between the
peripheral nerves of the limbs
(upper and lower extremities) is
not extensive.
Spinal Component
Skin Distribution
Third and fourth cervical nerves
Limited area of skin over the root of the neck, upper
aspect of the pectoral region, and ashoulder
C5 dermatome
Lateral aspect of the upper extremities at and above
the elbow
C6 dermatome
The forearm and the radial side of the hand
C7 dermatome
The middle finger
C8 dermatome
The skin over the small finger and the medial aspect of
each hand
T1 dermatome
The medial side of the forearm
T2 dermatome
The medial and upper aspect of the arm and the
axillary region
Clinically useful guidelines
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C1 -----No skin supply
C2 -----Occipital region, posterior neck and
skin over parotid
C3 -----Neck
C4 ----- Infraclavicular region
(to manubriosternal junction),shoulder and
above scapular spine
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C5 --- Lateral arm
C6 -----Lateral forearm and thumb
C7 -----Middle fingers
C8 -----Little finger and distal medial
forearm
T1 -----Medial arm above and below elbow
T2 -----Medial arm, axilla and thorax
T3 -----Thorax and occasional extension to
axilla
T4 -----Nipple
T7 -----Subcostal angle
T8 -----Rib margin
T10 ----Umbilicus
T12 ----Lower abdomen, upper buttock
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L1 ----Suprapubic and inguinal regions, penis,
anterior scrotum (labia), upper
buttock
L2 ----Anterior thigh, upper buttock
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L3 ----Anterior and medial thigh and knee
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L4 ----Medial leg, medial ankle and side of
foot
L5 ----Lateral leg, dorsum of foot, medial sole
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S1 ----Lateral ankle, lateral side of dorsum and
sole
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S2 ----Posterior leg, posterior thigh, buttock,
penis
S3 ----Sitting area of buttock, posterior
scrotum (labia)
S4 ----Perianal
S5 and Co, Behind anus and over coccyx
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Clinically Important Dermatomes
• Upper extremity
• C6 - Thumb
• C7 - Middle finger
• C8 - Little finger
• T1 - Inner forearm
• T2 - Upper inner arm
Other
• C2 and C3 - Posterior head and
neck
• T4 - Nipple
• T10 - Umbilicus