1. SESSION 5 Dermatomes & myotomes Forearm & hand

2. 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

3. 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”
Dorsal/ventral roots
Come together ~ intervertebral foramen
Spinal nerves are paired structures
Hence the term “segmental nerve”
named according to vertebral level from which they emerge – e.g. C5, C6, L2.

4. 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
Derma and tomos (cutting)
Somatic afferent, somatic sensory fibres.

5. What is a myotome? The muscle equivalent
A muscle mass innervated by the motor branches of a single spinal nerve
Somatic efferent fibres.

6. Learning dermatomes Be able to draw dermatomes on a blank diagram
Be able to show the position of a dermatome on a person
OSCE!

7. 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
The diagram shows the Keegan and Garrett model. can tell because it includes the whole 4th digit in the C8 dermatome and there is no T2 dermatome in the upper limb in this diagram.
The Foerster model seems to be based more on clinical findings, whereas the Keegan and Garrett model gels more with the concepts of embryological limb development.

8. 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 limbs are a bit more complicated than at the trunk because the limbs receive their nerves from nerve plexuses.

9. Dermatomes of the upper limb

10. Dermatomes of the upper limb
Dermatomes C5-T1 are supplied via branches of the brachial plexus
Overlap
NOT at the axial line

11. Cutaneous peripheral nerve vs. dermatome distribution
Median nerve, C6-8
Radial nerve, C6-8
Ulnar nerve, C8 & T1
multi-segmental
peripheral nerves
Important to understand that there is a difference between the distribution of multi-segmental peripheral nerves, and the distribution of dermatomes.
Using hand as an example, because it’s important to learn the distribution of the peripheral nerves in the hand, in addition to the dermatomes of the hand.
Purple area in digit 4 is there to show that digit 4 is an area of considerable overlap. This is one of the areas where a lot of the dermatome diagrams in the textbooks will differ from each other.
Another good example of a region of sensation that can be tested is the regimental badge area, on the lateral aspect of the brachium, innervated by the axillary nerve. C6 C7 C8
Dermatome distribution of the hand

12. 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

13. 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

14. 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
Metacarpophalangeal/interphalangeal flexion/extension
Metacarpophalangeal
abduction/adduction T1
When you’re thinking about myotomes, it’s important to consider the action that the myotome brings about on contraction.
You can see that most joint actions involve motor nerve fibres from two spinal nerves.
This is because most muscles of the upper limb consist of more than one myotome.

15. Testing myotomes Selected joint movements against resistance
Symmetrical?
Particular myotomes affected?
Proximal/distal/general weakness?
Muscle stretch reflexes
Absent/present/reduced/increased?
Particular myotomes affected – spinal nerve damage or more distal, peripheral nerve damage?

16. 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
Bicipital tendon.
For triceps jerk test, hit just above olecranon process.

17. Clinical importance Nerve lesions
Sensory effects: paraesthesia, anaesthesia
Motor effects: weakness, paralysis
Remember that the exact effect is dependent on the location of the lesion along the length of the nerve

18. 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

19. 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

20. Some clinical notes on the forearm & hand

21. Cubital Fossa Imaginary line between the M&L epicondyles
Medial border of brachioradialis muscle
Lateral border of pronator teres
Floor
Brachialis
Supinator
Roof
Bicipital aponeurosis, skin & fascia
Median cubital vein
Medial and lateral cutaneous nerves of the forearm
Contents
Bicipital tendon
Brachial artery, radial & ulnar arteries
Median nerve
(mnemonic from lateral to medial: tan = tendon, artery, and nerve) 
Bicipital aponeurosis protects underlying brachial artery and median nerve during median cubital venous access
Median cubital vein connects the cephalic vein with the basilic vein.

22. Pulled elbow What: Subluxation of the radial head
The 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)
Subluxation = partial dislocation
Radial head is displaced inferiorly.
Anular ligament = cuff of tissue; stabilises proximal radio-ulnar joint.
Children prone to this because of their incompletely developed radial heads and lax anular ligaments.

23. 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
Anatomical snuff box is the recess between the common tendon of AbPL&EPB and tendon of EPL;
SCAPHOID AND RADIAL ARTERY FORM ITS FLOOR.
Of course, if the fracture is misdiagnosed as a severe sprain, the risk of AVN is increased.
10-14 days, fracture is more evident on x-ray due to surrounding bone resorption.

24. 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

25. 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
The median nerve is the most vulnerable structure in the carpal tunnel, so anything causing increased pressure in the carpal tunnel will affect the median nerve.
More examples of muscles innervated by median nerve proximal to the carpal tunnel? (most in anterior forearm, except FCU and medial half of FDP)
Patients with carpal tunnel syndrome will have difficulty buttoning a shirt/blouse as well as using a comb. Why is this?

26. 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
A Colles’ fracture is the most common fracture of the forearm.
It is especially likely in women over 50 years of age. Why? (osteoporosis, decline in bone density, bones more liable to fracture)

27. 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
The classic case is piercing of the finger with a rusty nail.
Spread mainly depends on connection between the synovial membranes of the synovial digital sheath and the common flexor sheath.
Connections of the synovial sheaths to the common flexor sheath are subject to variation between individuals, but by enlarge….

28. Thanks for listening!