Electrodiagnostic examination Flashcards
electrodiagnostic examination
comprises:
- Nerve conduction studies
- EMG
Nerve conduction studies
tests conduction velocity and AP amplitude in motor, sensory and/or mixed nerves
uses electrodes to apply a small electrical stimulation to test nerve and muscle responses.
conduction velocity = distance/time
the rate at which nerves conduct electrical impulses can be calculated
EMG
examines spontaneous and voluntary activity in skeletal muscles
small needle electrode is inserted into the muscle and measures voluntary and spontaneous activity in the muscle
voluntary contraction as measured by recorded signals are known as motor unit potentials. These examine the amplitude, duration, phases and recruitment patterns and may show evidence of myogenic or neurogenic disease.
Nerve conduction studies of the upper limb
median, ulnar and radial nerve in the arms
Nerve conduction studies of the lower limb
sural nerve and superficial perineal nerves in the legs
EMG types
- Surface EMG
- Needle EMG
- Single fibre EMG
Needle EMG
axis of needle (recording electrode portion) is insulated from the shaft of the needle. the shaft of the needles acts as the reference electrode.
conventional EMG records from hemisphere radius of 1 mm.
recorded muscle activity can be targeted both visually (oscilloscope) and by ear (loudspeaker translates visuals into sound)
EMG considerations
muscle characteristics, age of the patient, patient cooperation, electrical specifications
EMG analysis
AP frequency, duration, amplitude, area and variability
motor unit diseases
associated with muscle wasting or weakness
7 sites of motor unit disease involvment
- pre upper motor neurons:
cortical neurons
corticospinal tracts - lower motor neurons
anterior horn neurons
peripheral nerve axons e.g. myelin sheath breakdown as with multiple sclerosis - NMJ e.g. myasthenia gravis
- Muscle membrane (sarcolemma) e.g. myotonia
- contractile elements e.g. muscular dystrophy
denervation
severing of the nerve
fasciculation
muscle twitch that is spontaneous, involuntary muscle contract and relaxation
EMG for diagnosis
distinguish between myopathic and neuropathic muscle wasting and weakness.
detect abnormalities such as chronic denervation or fasciculations in the muscle.
differentiate between focal and diffuse pathologies (in conjunction with nerve conduction studies)
evidence for pathophysiology of peripheral neuropathy
principle of selective vulnerability
motor neurone diseases affect only motor systems and not sensory or autonomic systems.
4 steps of EMG
- Insertional activity
- Spontaneous activity
- Motor unit potentials
- Recruitment and inference pattern
inference
greater contraction results in distortion and overlap of individual shape tracings to where they cannot be identified
spontaneous activity
activity at rest. in normal individual there should be a slight initial spiking activity where needle insertion has damaged small part of the muscle releasing K+ causing depolarisation. the muscle becomes quiescent very rapidly.
if reduced then could be linked to fibrotic conditions e.g. muscular dystrophy (replaced with collagenous material so signal is almost non existent).
if increases then could be linked to denervation and inflammatory processes.
voluntary activity
mild voluntary contraction induces isolated discharges of one or few motor units at a rate of 5 - 10 spikes per second.
force increases with two mechanisms of greater muscle force activity both occurring simultaneously
- recruitment of inactive units by size principle of motor unit recruitment
- more rapid firing of already active units
ultimately results in inference pattern.
normal voluntary EMG activity
appropriate motor discharge for effort input.
effort input is the examiner knowing the normally number of motor units that should be recruited for a given force exerted.
reduced EMG activity = ALS
amplitude increased, freq decreased
recruitment pattern is reduced in ALS = amyotrophic lateral sclerosis.
full inference is not detected. the amplitude of AP is much higher but frequency is drastically decreased.
increased EMG activity = limb girdle dystrophy
amplitude decreased, freq increased
exaggerated voluntary recruitment activity as in limb girdle dystrophy. minimal activity results in activation of 2/3 motor units in comparison to 1.
full recruitment pattern is established at maximal contraction but amplitude is smaller than in normal muscle.
muscle weakness (babinski sign)
diseases of premotor neurons and/or their axons characterised by spasticity, overactive tendon reflexes, abnormal plantar flexion (babinski sign)
distal weakness = neuropathy
proximal weakness = myopathy
more reliable diagnosis of muscle weakenss
fibrillation or fasciculation
fibrillation (denervation) (can be detected with voluntary movement)
only in lower motor neuron disorders + some myopathies + muscle inflammation generated by the muscle due to circulating ACh due to denervation supersensitivity.
fibrillation is invisible through skin but detected with needle EMG. can appear in healthy muscles.
confirmed by reproducible changes in at least 2 different areas of muscle is highly suggestive of lower motor neuron disorders and myopathic diseases.
denervation examples:
- acute injury
- chronic pressure and nerve entrapment
- death of alpha motor neurons in the anterior grey horn
- polyneuropathy e.g. diabetes
polyneuropathy
damage or disease affecting peripheral neurons in roughly the same areas on both sides of the body e.g. diabetes or guillain barre syndrome
denervation supersensitivity
@ site of lesion, the muscle fibres innervated remain viable and function BUT following a lesion (7 - 10 days) nAChR spread over the surface of muscle fibres known as denervation sensitivity and become so excitable that they discharge spontaneously causing fibrillation.
muscle with neurogenic disease
ALS and poliomyelitis
amplitude increased
frequency decreased
the number of motor units under voluntary control is reduced.
fibrillations present at rest as muscle undergoes denervation sensitivity.
number of MUAPs (frequency reduced) but MUAPS larger (amplitude increased) as denervated nerve fibres become re innervated by sprouts from viable neuron.
decrease in fineness of muscle control but allows for some strength.
muscle with myopathic disease
Muscular dystrophies
amplitude decreased
frequency increased
the number of muscle fibres is reduced as fibres shrink and become non functional.
fibrillation may or may not be present at rest.
number of MUAPs the same (frequency the same) but MUAP strength is reduced (amplitude decreased) and results in much weaker voluntary contractions.
fasciculations (involuntary)
slowly progressive diseases
involuntary discharge of motor neurons induing contraction of part or whole single motor unit occurring at irregular intervals
larger and more complex than fibrillation and visible at skin if occurring near surface of muscle.
characteristic of slowly progressive diseases of the motor neuron itself
myokymic discharges
e.g. multiple sclerosis or guillain barre syndrome
more complex bursts of repetitive discharges seen in muscles innervated by cervical nerves
