DERMATOMES & MYOTOMES

DERMATOME

• a dermatome is an area of skin supplied by sensory neurons that arise from a spinal nerve ganglion. it is an area of skin that is mainly supplied by afferent nerve fibers from a single dorsal root of spinal nerve which forms a part of a spinal nerve.
• There are 8 cervical nerves (except C1 as it has no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these spinal nerves relay sensation from a particular region of the skin to the brain.

MYOTOME

• A myotome refers to a set of muscles innervated by a specific, single spinal nerve
• There are 31 spinal nerves.Each vertebrae has a spinal nerve. The nerves are categorized by the vertebra which house them. There are 8 cervical nerves, 12 thoracic nerves, 5 lumbarnerves, 5 sacral nerves,1 coccygeal nerve 16 of these 31 nerves has a specific myotome that controls voluntary muscle movement

HOW TO TEST?

dermatomes

we, as students, have resorted to looking at pictures of dermatomes & myotomes while performing a neurological examination. Although, its fine to do that as a student but when profession & work kicks in and you have to work with patients on a regular & professional level, one cant afford to keep looking at pictures everytime they get a patient.

Now we ask is there an easy way to learn them?

The answer is yes, there is.

EASY WAY TO LEARN DERMATOMES

One doesn’t need to learn the whole direction or pathway of a dermatome what you can do, instead, is learn few landmarks or points on body for a dermatome. Principle landmarks for dermatomes are as follows;

• C2- 3 cm behind the ear or 1 cm lateral to the occipital protuberance
• C3- supraclavicular fossa, at the midclavicular line or lateral aspect of neck
• C4- acromioclavicular joint
• C5- over deltoid
• C6- tip of thumb
• C7- tip of middle finger
• C8- tip of little finger
• T1- medial side of elbow, just proximal to medial epicondyle of humerus
• T2- medial upper arm, or apex of the axilla
• T3–T6, upper thorax
• T5–T7, costal margin
• T8–T12, abdomen and lumbar region(T10 at the level of umbilicus, T12 is at the midpoint of inguinal ligament)
• L1- groin, midway between key sensory points for T12 & L2
• L2- upper 1/3rd of anterior thigh
• L3- medial epicondyle of femur
• L4- lateral knee to medial malleolus
• L5- lateral lower leg to dorsum of big toe or dorsum of 3rd MTP
• S1- lateral aspect of the calcaneus or heel
• S2- medial aspect of posterior thigh or midpoint of popliteal fossa
• S3- over the ischial tuberosity
• S4-S5- perianal area

*dont forget to learn these points by identifying them on your own body*

EASY WAY TO LEARN MYOTOMES

Myotomes are easier to learn as compared to dermatomes. Do imitate the movements. They are as follows;

• 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- finger flexion
• T1- finger abduction
• L2- hip flexion
• L3- knee extension
• L4- ankle dorsiflexion
• L5- big toe extension
• S1- ankle plantarflexion
• S2- knee flexion
• S3-S4- anal wink

To make it even easier, here’s a myotome, dermatome dance video:

EXAMINATION

we test dermatomes & myotomes to assess sensory and motor function in central disorders like cervical radiculopathy and SCI, etc.

We are here going to talk about the examination done in SCI according to ASIA(click to find the scale).

For dermatomes, pain and light touch are the 2 types of sensation tested, why only these 2 sensations? To find out the answer, we need to look at the pathways that are responsible for them.

• light touch- dorsal column- medial lemniscus pathway
• if there is an injury in DCML, light touch will be impaired ipsilaterally
• pin prick- lateral spinothalamic tract
• if there is an injury in lateral spinothalamic tract, pin prick will be impaired contralaterally.

Now, to assess light touch(non-standardised)

• instrument- cotton wisp(non-standardised) or semmes- weinstein monofilament(standardised)
• procedure-S: Light touch to a small area of the patient’s skin.
• R: Patient says “yes” or makes nonverbal signal each time stimulus is felt.
• remember to perform the procedure on non affected limb too and compare

FOR STANDARDISED TEST

• instrument- Semmes-Weinstein monofilament
• procedure- S: Begin testing with fi lament marked 2.83, hold filament perpendicular to skin, apply to skin until filament bends (Fig. 9–7). Apply in 1.5 seconds, and remove in 1.5 seconds. Repeat three times at each testing site, using thicker filaments if the patient does not perceive thin ones (except for filaments marked 4.08, which are applied one timeto each site).
• R: Patient says, “yes” upon feeling the stimulus.

PIN PRICK

• Measures discrimination of sharp and dull stimuli, which indicates protective sensation
• Instrument- New or sterilized safety pin
• procedure-S: Randomly apply sharp and blunt ends of safety pin, perpendicular to skin, at the pressure that was necessary to elicit correct response on uninvolved side of body.
• R: Patient says “sharp” or “dull” after each stimulus.

TESTING MYOTOMES

C5- Shoulder abduction Ask the patient to raise both their arms till 90 degrees simultaneously as strongly as they can while the examiner provides resistance to this movement. Compare the strength of each arm.

C6- Elbow flexion Test the strength of lower arm flexion by holding the patient’s wrist from above and instructing them to bend their elbow. Provide resistance at the wrist. Repeat and compare to the opposite arm. This tests the biceps muscle. Test the strength of wrist extension by asking the patient to extend their wrist while the examiner resists the movement. Repeat with the other arm.

C7- Elbow extension Ask the patient to extend their forearm against the examiner’s resistance. Begin their extension from a fully flexed position because this part of the movement is most sensitive to a loss in strength. This tests the triceps. Note any asymmetry in the other arm.

C8- Finger Flexion Examine the patient’s hands. Look for intrinsic hand, thenar and hypothenar muscle wasting. Test the patient’s grip by having the patient hold the examiner’s fingers in their fist tightly and instructing them not to let go while the examiner attempts to remove them. Normally the examiner cannot remove their fingers. This tests the forearm flexors and the intrinsic hand muscles. Finger flexion is innervated by the C8 nerve root via the median nerve.

C8 & T1- Finger abduction- Test by having the patient abduct or “spread out” all of their fingers. Instruct the patient to not allow the examiner to compress them back in. Normally, one can resist the examiner from replacing the fingers.

L1 & L2 : Hip Flexion- test by asking the patient to lie down and raise each leg separately while the examiner resists. Repeat and compare with the other leg. This tests the iliopsoas muscles.

L3 Test extension at the knee by placing one hand on top of the lower leg to provide resistance. Ask the patient to straighten or extend the lower leg at the knee. Repeat and compare to the other leg. This tests the quadriceps muscle.

L4: Ankle Dorsiflexion Test by holding the top of the ankle and have the patient pull their foot up towards themselves. Repeat with the other foot. This tests the pre-tibial muscles(tibialis anterior, extensor hallucis longus, and extensor digitorum).

L5: Great toe extension Ask the patient to move the large toe against the examiner’s resistance up towards wall. This tests the extensor halucis longus muscle.

S1: Ankle plantar flexion. In standing rise up onto the forefoot. Repeat with the other foot and compare. This tests the gastrocnemius and soleus muscles.

S2: Test flexion of the knee by holding the knee from the side and applying resistance above the ankle and instructing the patient to pull the lower leg towards their hips. Repeat with the other leg.

IS THE EXAMINATION OF SENSATION SAME FOR CENTRAL AND PERIPHERAL NERVOUS DISORDERS?IF NOT,WHY?

No, examination is not the same for both of them due to regrouping of axons in the brachial plexus and lumbosacral plexus, thus, dermatomal testing is more appropriate in clients with CNS lesions. For PNS disorders, cutaneous innervation by brachial plexus is more appropriate.

REFERENCES

• Occupational therapy in physical dysfunction, Catherine A. Trombly
• Pedretti’s occupational therapy, practice skills for physical dysfunction
• www.emedicine.medscape.com

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Thank you for reading