Making sense of Nerve conduction & EMG

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John O’Neal’
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1 Making sense of Nerve conduction & EMG Drs R Arunachalam Consultant Clinical Neurophysiologist Wessex Neurological Centre Southampton University Hospital

2 EMG/NCS EMG machine For the assessment of patients with neuromuscular diseases Extension of the neurological examination Directed history and examination followed by NCS and EMG, amended during exam in light of findings

Anterior Horn cell Motor neurone disease Polio Neuromuscular diseases Radiculopathy Disc / Osteophytes Root avulsion Tumour / infiltration Infection DRG Neuronopathy/ganglionopath y Plexopathy Trauma

3 Anterior Horn cell Motor neurone disease Polio Neuromuscular diseases Radiculopathy Disc / Osteophytes Root avulsion Tumour / infiltration Infection DRG Neuronopathy/ganglionopath y Plexopathy Trauma Neoplastic / Radiation Diabetic Neuromuscular junction Acquired / congenital Myasthenia gravis LEMS Botulism Neuropathy Focal/entrapment Polyneuropathy Sensory / Motor / Mixed Axonal / Demyelinating Acquired / Inherited Mononeuritis multiplex Myopathy Congenital Inflammatory Metabolic Endocrine Muscular dystrophies

4 Sensory nerve study SNAP = sensory nerve action potential

6 Motor nerve study CMAP = compound muscle action potential

8 F - waves

9 Goals of NCS / EMG Localisation Nerve neuronopathy root plexus nerve Characterisation Pathophysiology Motor / sensory Fibre size Axonal / Demyelinating Disease specific NMJ pre-synaptic post-synaptic Temporal course Acute Subacute Chronic Muscle Severity assessment / Prognosis

10 Typical referral questions Is this carpal tunnel syndrome / ulnar neuropathy? Is this radiculopathy or entrapment neuropathy? Is this a brachial plexopathy? What is the cause of this patient s wrist drop? Is the nerve lesion improving or recoverable, or is surgery required? Does the patient have a peripheral neuropathy?

11 What is normal? Conduction velocity Upper limb >50m/s (<38 in demyeln) Lower limb >40m/s Distal latency Median 3.5ms (prolonged>4.5) Ulnar 3.0ms (prolonged>4.0) F-latency Upper limb 30ms Amplitudes Tricky

12 Demyelinating vs. Axonal lesion DEMYELINATING loss of / damage to myelin results in slow conduction often reversible AXONAL loss of excitable tissue results in loss of amplitude often irreversible

13 Axonal vs. Demyelinating Amplitude N/ Conduction block Amplitude Morphology N Velocity N/ Distal latency N/ F-latency N/ Temporal dispersion Velocity / Distal latency / F-latency /

14 Demyelination Slowed conduction sensory or motor Reduced amplitudes sensory or motor Prolonged distal latencies F-wave abnormalities prolonged minimum F-wave latency reduced F-wave persistence absent F-waves chrono-dispersion of F-waves multiple A-waves Temporal dispersion & Conduction block Reduced motor unit firing on EMG

15 Demyelination Long distal motor latency Slowing in intermediate segment

16 Demyelination Right Rec: APB, Stim: Wrist, Elbow, Axil... Switch: N-R Stim: 1 Rate: Non-Rec Median Level: MNC Record 0.0 ma 5 ms Dur: #7 17:04: ms Step: Single 2 Average: Off Sig. Enhancer: Off Recording Site: A1 Abductor pollicis brevis 58.1mA 1 mv A2 100mA Stimulus Site A1: Wrist A2: Elbow A3: Axilla A4: Supraclav fossa Lat1 ms Dur ms Amp mv Area mvms mv Segment A3 1 mv Wrist-Elbow Elbow-Axilla Axilla-Supraclav fossa Dist mm 220 Diff ms 7.8 CV m/s 28 Long distal motor latency A4 1 mv 2-10kHz Conduction block? No temporal dispersion

17 Demyelination Temporal dispersion

18 Demyelination Conduction block? Yes, in the forearm Yes, at entrapment site

19 Needle EMG

20 FASCICULATIONS New Folder\VP_ _ exe New Folder\VP_ _ exe Spontaneous discharge of an entire motor unit

22 FIBRILLATIONS New Folder\VP_ _ exe Spontaneous discharge of a single muscle fibre

24 RECRUITMENT New Folder\VP_ _ exe New Folder\VP_ _ exe

25 CRD, Polyphasia New Folder\VP_ _ exe

27 Carpal tunnel syndrome

30 Typical findings in CTS Initial changes are mainly sensory Bifid response from ring finger Median slower than ulnar amplitude Motor changes occur later distal motor latency amplitude in severe cases

31 Ulnar neuropathy at elbow Sensory findings digit V amplitude ulnar dorsal cutaneous amplitude CAP (mixed nerve response) Motor findings Slowing across the elbow Conduction block across the elbow (drop in amplitude) EMG changes in hand & forearm muscles

32 Ulnar neuropathy at wrist Sensory findings digit V amplitude Normal ulnar dorsal cutaneous amplitude Normal CAP (mixed nerve response) Motor findings distal motor latency Normal conduction forearm and elbow EMG changes confined to the hand

33 Brachial plexus Sensory changes amplitude - postganglionic Often no significant change Motor nerve conduction Can be normal Reduction in amplitude - severe cases F-wave abnormalities Diagnosis based on EMG abnormalities Knowledge of brachial plexus anatomy helps localise the lesion

34 Thoracic outlet syndrome Affects the lower trunk / medial cord Sensory changes In ulnar territory Often medial antebrachial nerve is involved Motor changes Often in median nerve territory EMG abnormalities C8/T1 innervated muscles (median>ulnar)

35 Cervical radiculopathy Sensory nerve conduction Normal despite significant symptoms Preganglionic lesion Motor nerve conduction Mostly normal Occasionally F-wave abnormalities Reduction in amplitude - rare, severe cases Diagnosis based on EMG abnormalities Knowledge of myotomes help localise the level Normal EMG does not exclude root disease

36 Peripheral neuropathy Often length dependent Feet affected first Usually symmetrical Sensory nerve conduction Reduced amplitudes Reduced velocities Motor nerve conduction Reduced amplitudes Reduced velocities F-wave abnormalities

37 CASE STUDIES

38 Tingling fingers -?Carpal tunnel syndrome CTS high median brachial plexus C6 radiculopathy

39 Wasted hand? Cause Muscle disease Ulnar nerve Lower brachial Plexus C8/T1 root Anterior horn cell Sensory 15µV

A/Professor Arun Aggarwal Balmain Hospital

A/Professor Arun Aggarwal Balmain Hospital Nerve Conduction Studies Test to evaluate the function of motor / sensory nerves Evaluate Paraesthesia (numbness, tingling, burning) Weakness of arms and legs