SESSION 5Dermatomes & myotomes

Forearm & hand

Introduction

• What is a spinal nerve, dermatome, myotome?

• Dermatomes and myotomes of the upper limb

• Testing function of dermatomes and myotomes

• Clinical importance of dermatomes

• Cubital fossa

• Carpal tunnel

• Some clinical notes on the forearm and hand

What is a spinal nerve?

• “Mixed nerve”afferent/sensory & efferent/motor,somatic & autonomic

• Runs between a specific vertebral level and the bodyhence “segmental nerve”

What is a dermatome?

• “Skin segment”

• An area of skin innervated by the cutaneous branches of a single spinal nerve

• Every spinal nerve except C1 innervates a dermatome

What is a myotome?

• The muscle equivalent

• A muscle mass innervated by the motor branches of a single spinal nerve

Learning dermatomes

• Be able to draw dermatomes on a blank diagram

• Be able to show the position of a dermatome on a person– OSCE!

Learning dermatomes

•Pictures in text books vary!

•LMS likes ‘Clinically Oriented Anatomy’ which likes the Foerster and Keegan & Garrett models

•Learn one but be aware that other representations exist

Dermatomes of the upper limb• The limbs receive their nerves from nerve

plexuses (e.g. brachial plexus)

• Spinal nerve roots join & split

• Terminal branches contain fibres from more than one spinal nerve“multi-segmental peripheral nerves”

Dermatomes of the upper limb

Dermatomes of the upper limb

• Dermatomes C5-T1 are supplied via branches of the brachial plexus

• OverlapNOT at the axial line

Cutaneous peripheral nerve vs. dermatome distribution

Median nerve, C6-8 Radial nerve, C6-8Ulnar nerve, C8 & T1

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C7

C8Dermatome distribution of the hand

Clinical relevance?!

• A nerve lesion proximal to the brachial plexus affects a spinal nerve and its individual dermatome and/or myotome

• A nerve lesion distal to the brachial plexus affects a multi-segmental peripheral nerve and its distribution

• Nerve lesions present with paraesthesia/anaesthesia and/or weakness/paralysis in their regions of innervation

Testing skin sensation

• Pain– pin prick

Temperature– test tube of hot water– something metal and cold

Light touch– wisp of cotton wool

• Areas where dermatome overlap is minimal:– C5; upper lateral aspect of arm– C6; pad of thumb– C7; pad of third finger– C8; pad of little finger– T1; medial aspect of elbow

Myotomes of the upper limbJoint and action Spinal nerve(s)

Shoulder abduction C5

Shoulder adduction C6, C7

Elbow flexion C5, C6 “C5 C6, pick up sticks”

Elbow extension C7, C8 “C7 C8, keep it straight”

Radio-ulnar pronation C7, C8

Radio-ulnar supination C6

Wrist flexion/extension C6, C7

Metacarpophalangeal/interphalangeal flexion/extension

C7, C8

Metacarpophalangealabduction/adduction

T1

Testing myotomes

• Selected joint movements against resistance– Symmetrical?– Particular myotomes affected?– Proximal/distal/general weakness?

• Muscle stretch reflexes– Absent/present/reduced/increased?

Muscle stretch reflexes

• Sudden stretching of a muscle usually causes rapid contraction of the muscle

• Tendon hammer

• Biceps jerk– C5, C6

• Triceps jerk– C7, C8

Clinical importance

• Nerve lesions– Sensory effects: paraesthesia, anaesthesia– Motor effects: weakness, paralysis

Clinical importance

• Referred pain – Injury to visceral structures can present

as vague pain in a distant area– Spinal nerves are mixed– Myocardial infarction• Myocardium is innervated by nerve fibres

from spinal nerves T1-T5• The brain perceives pain as coming from

the dermatomes of T1-T5• Pain felt in chest; referred to left arm

Summary

• Dermatome is an area of skin innervated by the cutaneous branches of a single spinal nerve

• Dermatomes and their spinal nerves are assessed using pain, temperature and light touch

• Myotome is a muscle mass innervated by the motor branches of a single spinal nerve

• Specific joint movements assess integrity of myotomes and their respective nerves

Some clinical notes on the forearm & hand

Cubital Fossa

Pulled elbowWhat: Subluxation of the radial headThe anular ligament tears

Pain:If free anular ligament is compressed between capitulum and radial head

Who: Children under 5 years of age

How: Pulling a child’s hand in pronated position (e.g. when child is pulled up a curb)

Treatment: Supination and flexion of the elbow joint(in a sling)

• History fall on outstretched hand; tender anatomical snuffbox

• Relatively poor blood supply, primarily from radial artery; artery enters distal pole of bone and passes proximally; fracture across the narrow waist can cause avascular necrosis aof proximal segment

• Fracture difficult to see on x-ray before bone resorption; “wrist sprain” misdiagnosis

• Without complication, healing takes 3 months

Scaphoid fracture

Carpal tunnelSidewallsConcavity of carpal bones

Floor Carpal bones

Roof Flexor retinaculum(prevents tendons from bowing)

Contents9 flexor tendons

4 FD profundus4 FD superficialis1 flexor pollicis longus

1 nerveMedian nerve

Carpal tunnel syndromeWhat:

Entrapment syndrome due to pressure on the median nerve in the carpal tunnel

Possible causes: Tenosynovitis, repetitive trauma, oedema, fractures, dislocation

Risk factors: RA, pregnancy, obesity

Clinical presentation:Sensory effect – paraesthesia/anaesthesia in lateral 3 ½ digitsMotor effect – progressive weakness in thumb; inability to oppose thumb

Treatment:Prevent cause of increased pressure, e.g. inflammationCarpal tunnel release

N.B.:Muscles innervated by the median nerve proximal to the carpal tunnel remain unaffected (e.g. FDS)Median nerve’s palmar cutaneous branch overlies flexor retinaculum i.e. does not pass through carpal tunnel, so central palm sensation is unaffected

Colles’ fracture

What: Transverse fracture of the distal 2cm radius

How does it occur: Fall on outstretched hand in pronationForced wrist dorsi-flexion

Clinical presentation: Dinner fork deformity“Posterior angulation just proximal to wrist”

Reason for dinner fork deformity:• Comminuted distal radial fragment

Dorsal displacement• Radial shortening

Ulnar styloid process often avulsedUlna projects further distally than radius

TenosynovitisCause:

Infection of the distal synovial sheath

Presentation:Subcutaneous inflammation, the digit swells and movement is painful

Infection spread:• Synovial sheath > common flexor sheath > carpal tunnel > forearm

Connection between synovial sheath andcommon flexor sheath?– NO: tendons of digits 2, 3 & 4– YES: tendons of digit 5

• Synovial sheath of FPL > forearm

• Ruptured inflamed sheath > hand compartment

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