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” 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.

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.

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

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 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.

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.

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 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

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 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.

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?

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.

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

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 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.

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.

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.

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 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?

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)

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….

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