Myotonic EMG Changes
Myotonic muscle (dystrophia myotonica; Chapter 10) responds to stimulation with high frequency action potentials. The discharge frequency diminishes as the seconds pass to create a whine, likened to a dive‐bomber of propeller‐driven vintage. A softer sound can be heard through a stethoscope over a contracting myotonic muscle. Complex repetitive discharges, a.k.a. pseudo‐myotonic, commence and end abruptly; they occur in chronic neuropathies and myopathies.
Hemifacial Spasm, Cramps, Myokymia and Stiff Person Syndrome
Hemifacial spasm (Chapter 13) is probably an example of ephaptic transmission, that is transmission between adjacent VIIth nerve fibres. EMG: bursts of normal motor unit discharges, without denervation.
Normal muscle cramps produce high frequency discharges. In myophosphorylase deficiency (McArdle’s disease; Chapter 10), cramps occur but these discharges are not found.
Myokymia (Chapter 13) refers to two facial phenomena:Quivering movements around the eye, common and invariably innocent.Worm‐like wriggling, persistent and typically around the chin – occurs in brainstem gliomas and MS.
In stiff person syndrome (Chapter 7), continuous motor unit activity is found simultaneously in opposing muscle groups, as one might expect from the stiffness.
Peripheral Nerve Conduction Studies
Five measurements are of value in neuropathies and peripheral nerve entrapment:
Motor conduction velocity (MCV) – normal values 41–49 m/s or greater
Sensory conduction velocity – normal 40–50 m/s
Distal motor latency (DML) – less than 4.4 m/s in median nerve, less than 3.3 m/s in ulnar
Sensory (nerve) action potentials (SAPs or SNAPs) – 2–15 mV, depending on nerve
Compound muscle action potentials (MAPs or CMAPs).
Nerve conduction studies use supramaximal stimulation, that measures conduction in fastest fibres – a blunt instrument compared with the finesse of EMG interpretation. Technique: see Figure 4.8.
Polyneuropathy
In axonal neuropathies, MCV is initially preserved but there is reduction in CMAP amplitude. SAPs are lower than normal. In demyelinating neuropathies, nerve MCV is markedly slowed. SAPs are lost or diminished (Chapter 10).
Entrapment Neuropathies
Hallmarks: increased distal motor latency such as in carpal tunnel syndrome, slowing of conduction across the site of entrapment with diminution of relevant sensory action potentials. Denervation when entrapment is severe.
Figure 4.8 Ulnar nerve conduction: nerve compression at elbow.
Source: Hopkins (1993).
F waves
These are low‐amplitude muscle responses to a peripheral stimulus produced by antidromic discharges of anterior horn cells. Prolonged latencies or disappearance occur in root lesions and polyneuropathies.
Hoffman (H) Reflexes
These are neurophysiological equivalents of a stretch reflex. Usually the tibial nerve is stimulated at the knee and contraction of gastrocnemius and soleus recorded: delayed when peripheral conduction is slowed, such as in polyneuropathies.
Neuromuscular Transmission
Repetitive Stimulation: Myasthenia and Myasthenic–Myopathic Syndromes
A muscle surface electrode records this. In myasthenia, responses decrease in amplitude. Also, a phenomenon known as jitter can also be recorded by single fibre studies.
In Lambert–Eaton syndrome the converse is seen – facilitation (increase) of motor responses with high frequency stimulation.
Cerebral‐Evoked Potentials
Evoked potentials record the amplitude and time for a visual, auditory or other sensory stimulus to reach the cortex. See Chapters 11 and 15.
Specialised Investigations
Various tests that may be unfamiliar to a newcomer to neurology are listed below:
| Serum copper & caeruloplasmin | Wilson’s disease & rare cord disease |
| CAG repeat assay | Huntington’s (Ch. 8) |
| Genetic tests | Neuropathies and ataxias (Chs 10, 17) |
| Antiganglioside antibodies | Acquired neuropathies (Ch. 10) |
| Antineuronal antibodies | Paraneoplastic syndromes (Chs 17, 21) |
| Anti‐endomysial & anti‐gliadin antibodies | Coeliac disease (Ch. 17) |
| Anti‐GAD antibodies | Stiff person syndrome (Ch. 8) |
| Anti‐acetylcholine receptor, anti‐MuSK antibodies et al | Myasthenic syndromes (Ch. 10) |
| Striated muscle antibodies, genetic tests | Myopathies, dystrophies (Ch. 10) |
| Aquaporin 4 antibodies | Devic’s (Ch. 11) |
| Voltage‐gated K channel antibodies et al | Autoimmune limbic encephalitis (Ch. 9) |
| Porphyrins, amino‐acids | Porphyrias, amino‐acid disorders |
| Leucodystrophies, various | V. long chain fatty acids, enzyme & genetic tests (Ch. 19) |