What is the Dermatome Map?
The dermatome map shows how the spinal nerves live in relationship with specific areas of skin. This map may or may not aid in treating pain. Spinal nerves cause sensation to certain known areas of the skin. Because of these, dermatome maps have been created so that care providers can understand which nerves cause particular areas of pain.1 There are five sacral, five lumbar, twelve thoracic, and several cervical nerves, except for C1, that have a dermatome, or a section of skin that is supplied mostly by a just one spinal nerve. The skin can send sensations to the brain via the nerves. Dermatomes are disc-like in the abdomen and thorax. The limbs have longitudinal dermatomes.
When symptoms follow the pattern of the dermatome, such as with the presence of a rash or pain, a nerve root may be related. Referred pain is not dermatome-related. Viruses, such as the one that causes shingles/chickenpox, can be seen in the dermatome.2 Dermatomes differ from myotomes, which refer to the relationship between the muscle and spinal nerve. The Greek word, dermatome, means “skin cutting”. The dermatomes on the map are named for the supplying spinal nerve. The diagram of the map shows distinct dermatome areas but, in real life, people actually can have an innervation overlap.3 There are many free dermatome maps available online, and there is even an app for that.4
Levels of Principal Dermatomes
As noted before, dermatomes overlap, but there are principal dermatomes. One of the online maps shows that C5 is for the clavicles, as well as lateral parts of the upper limbs (which also include C6 and C7). C8 and T1 are for the medial sides of upper limbs. C6 is for the thumbs and hand (which includes C7 and C8), and C8 is also for the ring fingers and little fingers. T4 is for nipples and T10 is for the “level of umbilicus”. T12 is for the groin region.
L1-L4 are for the inner and anterior surfaces of the lower limbs, while L4-L5 and S1 are for the foot. L4 is also for the medial side of the large toe. S1, along with S2 and L5, is for the outer and posterior surfaces of the lower limbs. S1 is also for the lateral margin of the little toe and foot. S2-S4 are for the area of the perineum. It is said that the spinal nerves each provide sensation to an area of skin that is predictable. Therefore, if a patient has pain that radiates down the leg, for example, the S1 dermatome might indicate that there may be pinching caused by a herniated disc in the S1 spinal nerve root.5
Dermatome Maps of the Head
There are also dermatome maps that show areas of the head. On another dermatome map, available online, V1 is the area of the upper face, also known as the “Ophthalmic Division of Trigeminal Nerve”. V2 is the mid face, or the “Maxillary Division of Trigeminal Nerve”. Finally V3 is the lower face, otherwise referred to as the “Mandibular Division of Trigeminal Nerve”. C2 (“Occipital Protuberance”) impacts the back of the head and C3 (“Supraclavicular Fossa”) is part of the neck. On that map, C4 is in the clavicle region and C5 is the upper arms. Maps vary, though, as there is overlap in the sensory nerves.6
Research, Treatment, and the Dermatome Map
There has been research done to discover if the dermatomes map can be helpful in diagnosing patterns of radicular pain in patients. Patients with cervical and lumbar radicular pain were examined to see if there was coordination between the pain and the maps. The results found that most pain did not follow an expected dermatome in radicular pain, with the exception of S1’s nerve root.7 Even in massage therapy, dermatome maps may be a guide, but they are not absolute, and people vary from the maps and each other, due to the overlap that occurs between dermatome regions. During assessment, massage therapists can see if a patient has symptoms that are present throughout the entirety of a dermatome. If that is the case, the client may have a nerve root problem.8
Dermatome mapping of pain, weakness, numbness, and tingling can be helpful for isolating viruses that infect the nerves, such as shingles. Symptoms that impact dermatomes can indicate regions of the spine that are affected. C2-C8 (cervical) have nerves that affect the fingers, neck, scalp, and arms, T1-T12 (thoracic) affect the chest, abdomen, sternum, and pubic region, L1-L5 (lumbar) have nerves that impact the hips, front of the legs, and feet, and S1-S5 (sacral) affect the genitals, buttocks, and backs of the legs.9
Acupuncture also makes use of the dermatome map in treating patients, especially those with numbness.10 Research has shown that radicular pain that is only present in one or two dermatomes and is supplied by a single nerve root in the spine has more diagnostic accuracy for radiculopathy than traditional exam findings from the presence of weakness in the muscles, sensory change, change in the deep tendon reflex, and by performing a straight leg raise test.11