Dermatomes

Original Editor - Lucinda Hampton

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Dermatomes

The term “dermatome” is a combination of two Greek words; “derma” meaning “skin”, and “tome”, meaning “cutting” or “thin segment”. Dermatomes are areas of the skin whose sensory distribution is innervated by the afferent nerve fibres from the dorsal root of a specific single spinal nerve root, which is that portion of a peripheral nerve that “connects” the nerve to the spinal cord.

Nerve roots arise from each level of the spinal cord (e.g., C3, C4), and many, but not all, intermingle in a plexus (brachial, lumbar, or lumbosacral) to form different peripheral nerves as discussed above. This arrangement can result in a single nerve root supplying more than one peripheral nerve. For example, the median nerve is derived from the C6, C7, C8, and T1 Nerve Roots, whereas the ulnar nerve is derived from C7, C8, and T1.

In total there are 30 dermatomes that relay sensation from a particular region of the skin to the brain - 8 cervical nerves (note C1 has no corresponding dermatomal area), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these spinal nerves roots. Dysfunction or damage to a spinal nerve root from infection, compression, or traumatic injury can trigger symptoms in the corresponding dermatome.

Table.1 Dermatomes

Nerve Root			Dermatome
Cervical	C2	Supply Skin of Neck	Temple, Forehead, Occiput
	C3		Entire Neck, Posterior Cheek, Temporal Area, Prolongation forward under Mandible
	C4		Shoulder Area, Clavicular Area, Upper Scapular Area
	C5	Supply the Arms	Deltoid Area, Anterior aspect of entire arm to base of thumb
	C6		Anterior Arm, Radial side of hand to thumb and index finger
	C7		Lateral Arm and Forearm to index, long, and ring fingers
	C8		Medial Arm and forearm to long, ring, and little fingers
Thoracic	T1		Medial side of forearm to base of little finger
	T2	Supply the chest and abdomen	Medial side of upper arm to medial elbow, pectoral and midscapular areas
	T3 - 6		Upper Thorax
	T5 - 7		Costal Margin
	T8 - 12		Abdomen and Lumbar Region
Lumbar	L1		Back, over trochanter and groin
	L2		Back, front of thigh to knee
	L3	Supply Skin of Legs	Back, upper buttock, anterior thigh and knee, medial lower leg
	L4		Medial buttock, latera thigh, medial leg, dorsum of foot, big toe
	L5		Buttock, posterior and lateral thigh, lateral aspect of leg, dorsum of foot, medial half of sole, first, second, and third toes
Sacral	S1		Buttock, Thigh, and Leg Posterior
	S2	Supply Groin	Same as S1
	S3		Groin, medial thigh to knee
	S4		Perineum, genitals, lower sacrum
Coccygeal	The dermatome corresponding with the coccygeal nerves is located on the buttocks, in the area directly around the coccyx.

History

The idea of dermatomes originated from initial efforts to associate anatomy with the physiology of sensation. Multiple definitions of dermatomes exist, and several maps are commonly employed. Although they are valuable, dermatomes vary significantly between maps and even among individuals, with some evidence suggesting that current dermatome maps are inaccurate and based on flawed studies.

The medical profession typically recognised two primary maps of dermatomes. Firstly, the Keegan and Garret Map (Fig.1) from 1948, which illustrates dermatomes in alignment with the developmental progression of the limb segments. Secondly, the Foerster Map from 1933, which portrays the medial area of the upper limb as being innervated by T1-T3, depicting the pain distribution from angina or myocardial infarction. This latter map is the most frequently used in healthcare and accounts for the dermatomes used in the American Spinal Cord Injury Association Impairment Scale (ASIA Scale). In recent years there have been few attempts at verifying these original dermatome maps. Lee et al conducted an in-depth review that examined the discrepancies among dermatome maps. They put forth an “evidence-based” dermatome map that combined elements of previous maps (Fig.3). Though the application of the term “evidence-based” may be somewhat questionable, their proposed map represents a systematic attempt to synthesise the most credible evidence available.

Purpose

Testing of dermatomes is part of the neurological examination. They are primarily used to determine whether the sensory loss on a limb corresponds to a single spinal segment, implying the lesion is of that nerve root (i.e., radiculopathy), and to assign the neurologic “level” to a spinal cord lesion.

Technique

Dermatome Testing is done ideally with a pin and cotton wool. Ask the patient to close their eyes and give the therapist feedback regarding the various stimuli. Testing should be done on specific dermatomes and should be compared to bilaterally.

• Light Touch Test - Light Touch Sensation - Dab a piece of cotton wool on an area of skin
• Pinprick Test - Pain Sensation - Gently touches the skin with the pin ask the patient whether it feels sharp or blunt

During the review of systems, asking the patient to carefully describe the pattern or distribution of sensory symptoms (e.g., tingling, numbness, diminished, or absent sensation) provides the therapist with preliminary information to help guide the examination and to assist in identifying the dermatome(s) and nerve(s) involved.

Light touch dermatomes are larger than pain dermatomes. When only one or two segments are affected, testing for pain sensibility is a more sensitive method of examination than testing for light touch.

Controversies

Dermatomes have a segmented distribution throughout your body. The exact dermatome pattern can actually vary from person to person. Some overlap between neighboring dermatomes may also occur. There exist some discrepancies among published dermatome maps based on the methodologies used to identify skin segment innervation.

In a clinical commentary, Downs and Laporte discuss the history of dermatome mapping, including the variations in methodologies employed, and the inconsistencies in the dermatome maps used in education and practice. [[Laporte C. Conflicting dermatome maps: educational and clinical implications. journal of orthopaedic & sports physical therapy. 2011 Jun;41(6):427-34.]]

Clinical Significance

Dermatomes are important because they can help to assess and diagnose a variety of conditions. Neurological screening of dermatomes helps to assess patterns of sensory loss that can suggest specific spinal nerve involvement. For instance, symptoms that occur along a specific dermatome may indicate disruption or damage to a specific nerve root in the spine.

• Nerve Entrapment
• Radiculopathy
• Spinal Cord Injury
• Herpes Zoster