2Introduction What is a spinal nerve, dermatome, myotome? Dermatomes and myotomes of the upper limbTesting function of dermatomes and myotomesClinical importance of dermatomesCubital fossaCarpal tunnelSome clinical notes on the forearm and hand
3What is a spinal nerve? “Mixed nerve” afferent/sensory & efferent/motor,somatic & autonomicRuns between a specific vertebral level and the bodyhence “segmental nerve”Dorsal/ventral rootsCome together ~ intervertebral foramenSpinal nerves are paired structuresHence the term “segmental nerve”named according to vertebral level from which they emerge – e.g. C5, C6, L2.
4What is a dermatome? “Skin segment” An area of skin innervated by the cutaneous branches of a single spinal nerveEvery spinal nerve except C1innervates a dermatomeDerma and tomos (cutting)Somatic afferent, somatic sensory fibres.
5What is a myotome? The muscle equivalent A muscle mass innervated by the motor branches of a single spinal nerveSomatic efferent fibres.
6Learning dermatomes Be able to draw dermatomes on a blank diagram Be able to show the position of a dermatome on a personOSCE!
7Learning dermatomes Pictures in text books vary! LMS likes ‘Clinically Oriented Anatomy’ which likes the Foerster andKeegan & Garrett modelsLearn one but be aware that other representations existThe 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.
8Dermatomes of the upper limb The limbs receive their nerves from nerve plexuses (e.g. brachial plexus)Spinal nerve roots join & splitTerminal 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.
10Dermatomes of the upper limb Dermatomes C5-T1 are supplied via branches of the brachial plexusOverlapNOT at the axial line
11Cutaneous peripheral nerve vs. dermatome distribution Median nerve, C6-8Radial nerve, C6-8Ulnar nerve, C8 & T1multi-segmentalperipheral nervesImportant 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.C6C7C8Dermatome distribution of the hand
12Clinical relevance?!A nerve lesion proximal to the brachial plexus affects a spinal nerve and its individual dermatome and/or myotomeA nerve lesion distal to the brachial plexus affects a multi-segmental peripheral nerve and its distributionNerve lesions present with paraesthesia/anaesthesia and/or weakness/paralysis in their regions of innervation
13Testing skin sensation Painpin prickTemperaturetest tube of hot watersomething metal and coldLight touchwisp of cotton woolAreas where dermatome overlap is minimal:C5; upper lateral aspect of armC6; pad of thumbC7; pad of third fingerC8; pad of little fingerT1; medial aspect of elbow
14Myotomes of the upper limb Joint and actionSpinal nerve(s)Shoulder abductionC5Shoulder adductionC6, C7Elbow flexionC5, C6 “C5 C6, pick up sticks”Elbow extensionC7, C8 “C7 C8, keep it straight”Radio-ulnar pronationC7, C8Radio-ulnar supinationC6Wrist flexion/extensionMetacarpophalangeal/interphalangeal flexion/extensionMetacarpophalangealabduction/adductionT1When 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.
15Testing myotomes Selected joint movements against resistance Symmetrical?Particular myotomes affected?Proximal/distal/general weakness?Muscle stretch reflexesAbsent/present/reduced/increased?Particular myotomes affected – spinal nerve damage or more distal, peripheral nerve damage?
16Muscle stretch reflexes Sudden stretching of a muscle usually causes rapid contraction of the muscleTendon hammerBiceps jerkC5, C6Triceps jerkC7, C8Bicipital tendon.For triceps jerk test, hit just above olecranon process.
17Clinical importance Nerve lesions Sensory effects: paraesthesia, anaesthesiaMotor effects: weakness, paralysisRemember that the exact effect is dependent on the location of the lesion along the length of the nerve
18Clinical importance Referred pain Injury to visceral structures can present as vague pain in a distant areaSpinal nerves are mixedMyocardial infarctionMyocardium is innervated by nerve fibres from spinal nerves T1-T5The brain perceives pain as coming from the dermatomes of T1-T5Pain felt in chest; referred to left arm
19SummaryDermatome is an area of skin innervated by the cutaneous branches of a single spinal nerveDermatomes and their spinal nerves are assessed using pain, temperature and light touchMyotome is a muscle mass innervated by the motor branches of a single spinal nerveSpecific joint movements assess integrity of myotomes and their respective nerves
21Cubital Fossa Imaginary line between the M&L epicondyles Medial border of brachioradialis muscleLateral border of pronator teresFloorBrachialisSupinatorRoofBicipital aponeurosis, skin & fasciaMedian cubital veinMedial and lateral cutaneous nerves of the forearmContentsBicipital tendonBrachial artery, radial & ulnar arteriesMedian nerve(mnemonic from lateral to medial: tan = tendon, artery, and nerve) Bicipital aponeurosis protects underlying brachial artery and median nerve during median cubital venous accessMedian cubital vein connects the cephalic vein with the basilic vein.
22Pulled elbow What: Subluxation of the radial head The anular ligament tearsPain:If free anular ligament is compressed between capitulum and radial headWho:Children under 5 years of ageHow: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 dislocationRadial 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.
23Scaphoid fracture History fall on outstretched hand; tender anatomical snuffboxRelatively 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 segmentFracture difficult to see on x-ray before bone resorption;“wrist sprain” misdiagnosisWithout complication, healing takes 3 monthsAnatomical 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.
25Carpal 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 unaffectedThe 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?
26Colles’ fracture What: Transverse fracture of the distal 2cm radius How does it occur:Fall on outstretched hand in pronationForced wrist dorsi-flexionClinical presentation:Dinner fork deformity“Posterior angulation just proximal to wrist”Reason for dinner fork deformity:Comminuted distal radial fragmentDorsal displacementRadial shorteningUlnar styloid process often avulsedUlna projects further distally than radiusA 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)
27Tenosynovitis Cause: Infection of the distal synovial sheath Presentation:Subcutaneous inflammation,the digit swells and movement is painfulInfection spread:Synovial sheath > common flexor sheath > carpal tunnel > forearmConnection between synovial sheath andcommon flexor sheath?NO: tendons of digits 2, 3 & 4YES: tendons of digit 5Synovial sheath of FPL > forearmRuptured inflamed sheath > hand compartmentThe 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….