SESSION 5
Dermatomes & 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 body
hence “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
• Overlap
NOT at the axial line
Cutaneous peripheral nerve
vs. dermatome distribution
multi-segmental
peripheral nerves
M
Median nerve, C6-8
Radial nerve, C6-8
Ulnar nerve, C8 & T1
Dermatome
distribution of the hand
U
M
R
U
R
C6 C8
C7
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 limb
Joint 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
Metacarpophalangeal
abduction/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
What:
Subluxation of the radial head
The anular ligament tears
Pulled elbow
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)
Scaphoid fracture
• 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
Carpal tunnel
Sidewalls
Concavity of carpal bones
Floor
Carpal bones
Roof
Flexor retinaculum
(prevents tendons from bowing)
Contents
9 flexor tendons
4 FD profundus
4 FD superficialis
1 flexor pollicis longus
1 nerve
Median nerve
Carpal tunnel syndrome
What:
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 digits
Motor effect – progressive weakness in thumb; inability to oppose thumb
Treatment:
Prevent cause of increased pressure, e.g. inflammation
Carpal 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 pronation
Forced 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 avulsed
Ulna projects further distally than radius
Tenosynovitis
Cause:
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 and
common 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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