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Dermatomes and myotomes

The map from spinal segments to skin and muscle. How dermatomes and key muscles are used to find the level of a spinal cord injury in the standard examination.

Updated 2026-10-025 sources

Depth:

Why it matters

Because each spinal nerve reaches its own patch of skin and its own muscles, testing sensation and strength at chosen places tells an examiner which segments of the cord work ^{[1, 2]}.

What it is

The sensory fibres of the spinal nerves connect with the skin in a topographically organised way, illustrated as dermatomes: regions of skin related to the spinal nerve that carries their sensory endings ^[1]. A myotome is the corresponding group of muscle functions for a segment; the standard examination tests one key muscle function for each of ten segments ^{[2, 3]}.

How it works

In the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), each side is tested at one key sensory point per dermatome and with one key muscle function per myotome ^{[2, 4]}:

Sensation is tested with light touch and pinprick ^[5].
The ten key muscle functions run from the elbow flexors (C5) to the ankle plantar flexors (S1) ^[2].

The results set the levels ^[3]:

The sensory level is the most caudal dermatome with intact sensation, sensation above it also being intact ^[5].
The motor level is the most caudal key muscle with a grade of at least 3, with motor function above it presumed intact ^[3].
Where no key muscle can be tested, as in C2 to C4, the motor grade is taken as normal when sensation in the matching dermatomes is intact ^[3].

When there is no sacral sparing, the zones of partial preservation are the most caudal dermatome and myotome on each side with partly preserved function; since the 2019 revision they also apply to incomplete injuries without sacral sparing ^[3].

How we see and measure it

Sensation is tested within a dermatome from distal to proximal along the limbs and from lateral to medial on the trunk ^[1]. Muscle strength is tested by having the person contract muscles against resistance ^[1].

ISNCSCI motor score^{[2, 4]}

\mathrm{MS} = \mathrm{UEMS} + \mathrm{LEMS},\quad \mathrm{UEMS} = \sum_{C5}^{T1} (R + L),\quad \mathrm{LEMS} = \sum_{L2}^{S1} (R + L)

Each of the ten key muscles is graded 0 to 5 on each side. The upper-limb motor score adds the five arm muscles (C5 to T1) on both sides, up to 50; the lower-limb score adds the five leg muscles (L2 to S1), up to 50.

Variables of ISNCSCI motor score
Symbol	Meaning	Unit
$\mathrm{UEMS}$	upper extremity motor score	dimensionless
$\mathrm{LEMS}$	lower extremity motor score	dimensionless
$\mathrm{MS}$	sum of the two motor scores	dimensionless

Valid when

A scoring convention for examination after spinal cord injury, done by trained examiners
For learning only: this site does not score or interpret anyone’s examination

Worked example^[2]

Full strength in all ten key muscles on both sides gives 50 + 50.

uems = 50 , lems = 50 → ms = 100

Common misconceptions

Misconception: Each muscle is supplied by exactly one spinal segment.

The examination uses one key muscle function per segment as a convention for testing; the key muscle is not the whole supply of that muscle ^[2].

Check yourself

What is the sensory level?

The most caudal dermatome with intact sensation, with all sensation above it intact ^[5].

Which key muscle function tests C5?

Elbow flexion ^[2].

References

Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 16.4 The Sensory and Motor Exams. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/16-4-the-sensory-and-motor-exams
Rupp R, Biering-Sørensen F, Burns SP, Graves DE, Guest J, Jones L, et al.. International Standards for Neurological Classification of Spinal Cord Injury. Topics in Spinal Cord Injury Rehabilitation. 2021;27(2):1-22. doi:10.46292/sci2702-1
Snider B, Kirshblum S, Rupp R, Schuld C, Biering-Sorensen F, Burns S, et al.. International Standards for Neurological Classification of Spinal Cord Injury: Case Examples Reinforcing Concepts From the 2019 Revision. Topics in Spinal Cord Injury Rehabilitation. 2025;31(3):1-14. doi:10.46292/sci24-00049
Kirshblum SC, Burns SP, Biering-Sorensen F, Donovan W, Graves DE, Jha A, et al.. International standards for neurological classification of spinal cord injury (Revised 2011). The Journal of Spinal Cord Medicine. 2011;34(6):535-546. doi:10.1179/204577211x13207446293695
Snider B, Kirshblum S, Rupp R, Schuld C, Biering-Sorensen F, Burns S, et al.. International Standards for Neurological Classification of Spinal Cord Injury: Classification Questions and Cases. Topics in Spinal Cord Injury Rehabilitation. 2025;31(3):77-88. doi:10.46292/sci25-00013

For learning only. This page does not diagnose, predict outcomes or recommend treatment. Corrections are welcome: how to suggest one.