Making sense of Nerve conduction & EMG Drs R Arunachalam Consultant Clinical Neurophysiologist Wessex Neurological Centre Southampton University Hospital
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 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
Sensory nerve study SNAP = sensory nerve action potential
Motor nerve study CMAP = compound muscle action potential
F - waves
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
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?
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
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
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 /
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
Demyelination Long distal motor latency Slowing in intermediate segment
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 17:04:00 0.3 ms Step: Single 2 Average: Off Sig. Enhancer: Off 9.0.0 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 8.8 16.6 6.2 11.1 Amp mv 2.0 1.1 Area mvms 4.9 5.1 1 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
Demyelination Temporal dispersion
Demyelination Conduction block? Yes, in the forearm Yes, at entrapment site
Needle EMG
FASCICULATIONS New Folder\VP_20080123_19-22-51.exe New Folder\VP_20080122_19-00-21.exe Spontaneous discharge of an entire motor unit
FIBRILLATIONS New Folder\VP_20080122_19-06-44.exe Spontaneous discharge of a single muscle fibre
RECRUITMENT New Folder\VP_20080122_19-10-01.exe New Folder\VP_20080122_19-04-18.exe
CRD, Polyphasia New Folder\VP_20080122_19-12-13.exe
Carpal tunnel syndrome
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
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
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
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
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)
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
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
CASE STUDIES
Tingling fingers -?Carpal tunnel syndrome CTS high median brachial plexus C6 radiculopathy
Wasted hand? Cause Muscle disease Ulnar nerve Lower brachial Plexus C8/T1 root Anterior horn cell Sensory 15µV