Dermatomes & Myotomes

• Dermatomes

  Dermatomes = areas of the skin whose sensory distribution is innervation by the afferent nerve fibres from the dorsal root of a single spinal nerve root, which is that portion of a peripheral nerve that connects the nerve to the spinal cord
• Nerve Roots

  Nerve roots arise from each level of the spinal cord + many intermingled in a plexus to form different peripheral nerves

  This arrangement can result in a single nerve root supplying more than one peripheral nerve
• Dermatomes - Relaying Sensation

  There r 30 dermatomes that relay sensation from a particular region of the skin to the brain - 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves + 5 sacral nerves
• Dermatomes - Dysfunction

  Dysfunction or damage to a spinal root from infection, compression or traumatic injury can trigger symptoms in the corresponding dermatome
• Purpose of Dermatomes Testing

  Testing of dermatomes is part of the neuro examination

  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
• Dermatomes Light Touch Test

  Done w a pin and cotton wool

  Ask the patient to close their eyes + give the therapist feedback regarding the various stimuli

  Testing should be done on specific dermatomes + should be compared to bilaterally
• Dermatomes Pinprick Test

  For pain sensation

  Gently touches the skin w the pin, ask the patient whether it feels sharp or blunt

  During the review of systems, ask the patient to describe the pattern/distribution of sensory symptoms - tingling, numbness, diminished or absent sensation

  = provides w preliminary info to help guide the exam + to assist in identifying the dermatomes + nerves involved
• Myotomes

  Myotomes r part of the somatic nervous system = part of the PNS

  A myotome is defined as a group of muscles which is innervated by single spinal nerve root
• Myotome Distribution

  Most muscles in the limbs receive innervation from more than 1 spinal nerve root + r hence comprised of multiple myotomes

  E.g biceps brachial flexes the elbow

  It's innervated by the musculocutaneous nerve, which is innervated by C5, C6 + C7 nerve roots

  All 3 of these spinal nerve roots r associated w elbow flexion
• Purpose of Myotome Testing

  Testing of myotomes, in the form of isometric resisted muscle testing, gives info about the level in the spine where a lesion may be present

  Look for muscle weakness of a particular group of muscles

  Results may indicate lesion to the spinal cord nerve root or intervertebral disc herniation pressing on the spinal nerve roots

  The muscle strength grading scale, which assigns a rating to the degree of muscle weakness is often used
• Myotome Testing Technique

  Ask patient to perform a movement + hold an isometric contraction against resistance for a count of 5

  E.g. C5 = shoulder abduction

  = ask patient to raise both arms to the side of them simultaneously as strongly as they can while examiner provides resistance to this movement + compare the strength of each arm
• Clinical Relevance of Myotomes

  Myotomes r mainly useful for clinical evaluation in understanding the pattern of neurological deficit after a complex nerve injury

  Injury to each or a combo of spinal myotomes, lesions may be localised to tge spinal trunk or trunk level
• Dermatomes Distribution

  Dermatomes have a segmented distribution thru out the body

  Varying patterns for different ppl

  Some overlapping between neighbouring dermatomes may occur

  Cos spinal nerves exit the spine laterally, dermatomes associated w torso r distributed horiz

  In limbs, pattern is diff cos the shape of the limbs

  Dermatomes associated w limbs run vertically along the long axis of the limbs
• Importance of Dermatomes

  Radioculopathies = conditions in which a nerve root in the spine is compressed/pinched

  Pain from radioculopathies can follow one or more dermatomes

  E.g. sciatica - pain, weakness + tingling sensations
• Dermatomes vs Myotomes

  While dermatomes r areas of the skin (forms the connective tissue) a spinal nerve supplies sensation to, mytomes r the groups of skeletal muscles a spinal nerve supplies sensation to
• Testing Myotomes Function

  In the assessment of a suspected spinal cord lesion, the clinician can test myotomes function

  = help determine if there's spinal cord damage + where the damage is located
• Myotomes Power Ratings

  Myotomes r tested in terms of power + graded 0-5:

  0 = total paralysis

  1 = palpable/visible contraction

  2 = active movement, full ROM with gravity eliminated

  3 = active movement, full ROM against gravity

  4 = active movement, full ROM against gravity and moderate resistance in a muscle specific position

  5 = (norm) active movement, full ROM against gravity in a muscle specific position expected from an otherwise impaired person