· test performed by stimulating a peripheral nerve at one point and recording the response at another
· response may be
· muscle contraction
· nerve action potential
· two possible types of injury
· affected in an all or none fashion
· transection or avulsion causes complete disruption to conduction
· lesser injuries may be electrophysiologically inapparent
· results in a graded slowing of conduction
· conduction is slowed in proportion to the length of nerve demyelinated until ultimately conduction stops
Polyneuropathy
· causes general slowing of conduction
Compression
· causes demylination in the largest fibres at the site of compression
· leads to focal slowing of conduction
Motor nerve conduction
· motor nerve stimulated
· muscle contraction measured
· APs are amplified by muscle fibres and measure 5-10 mV
Sensory nerve conduction
· sensory nerve stimulated
· action potential recorded
· proximally (orthodromic conduction)
· distally (antidromic conduction)
· APs are not amplified and are only 10-50 uV
Latency
· time between stimulus and detection of response
· sensory nerve latency reflects time taken for impulse to travel down nerve
· motor nerve latency reflects
· time taken for impulse to travel down nerve
· delay at end-plate
· time taken for contraction
Amplitude
· measure of total number of functioning axons in nerve
· loss of axons causes decrease in response amplitude
Conduction velocity
· calculated by distance / latency
· reflects only the function of the fastest conducting fibres
· fortunately most compressive or traumatic disorders affect the largest fibres
Requirements
· suitable muscle
· nerve supply of muscle able to be stimulated at 2 points along its course
Technique
· 2 surface electrodes over course of nerve
· needle or surface electrode over muscle
· measure stimulus-contraction interval proximally and distally
· called proximal and distal latencies
· can calculate conduction velocity
· use distance between electrodes and difference in latencies
· with one electrode, cannot detect conduction velocity
· can determine latency and compare to standard values
Applications
· median nerve and APB
· ulnar nerve and 1st dorsal interosseous
· peroneal nerve and EDB
Requirements
· nerve either
· close enough to skin to be picked up by surface electrode
· anatomically constant in position to allow needle electrode to be inserted next to nerve
Technique
· nerve stimulated distally by surface electrode
· detected proximally by surface or needle electrode
· stimulus-pickup interval (latency) and AP voltage measured
Applications
· median n (index digital n)
· ulnar n (little digital n)
· common peroneal n (n at ankle)
· late responses
· useful in investigation of nerve root or proximal nerve lesions where cannot use simple NCS
· involves stimulating a peripheral nerve and recording over a muscle
· signal travels centrally and is recorded on its return to the periphery
· round trip to spinal cord and back
H Reflex
· electrical equivalent of deep tendon reflex
· mixed peripheral nerve weakly stimulated
· selectively depolarises large myelinated sensory nerves that convey muscle stretch
· signal ascends to the dorsal root ganglion and then through the monosynaptic reflex arc
· depolarises the anterior horn cells
· causes secondary signal in the motor axons
· leads to muscle contraction
· characterised by
· having a consistent latency and
· being elicited by a stimulus so small that it does not cause a direct muscle contraction
· shortcomings
· can only be elicited in the tibial nerve and recorded over the gastrocnemius
F Response
· supramaximal stimulus is applied to a nerve
· motor nerves directly depolarised
· produces 2 impulses as axons conduct equally well in all directions
· orthodromic which travels to the muscle and produces the typical M response
· antidromic which travels back to the anterior horn cell and depolarizes the cell body
· recurrent discharge propagates back down the axon to the muscle
· shortcomings
· only muscles in which the round trip takes over 15 - 20 msec can be used
· does not measure sensory nerves
· overcomes problems of the F response and H reflex
· measures function of proximal parts of nerves
Technique
· small electrical stimulus applied repeatedly to a nerve
· 5 stimuli/second 1000 - 2000 times
· response recorded from brain or spinal cord
· all noise and background EEG activity eliminated by computer averaging
· very small responses ( 1 mV) can be measured
· low intensity signal selectively affects large myelinated sensory axons
· recording electrodes placed over the spinal cord and brain
· measurements can be taken from successively higher points on the neuroaxis
· individual nerves or roots can be studied with selective placement of the stimulating electrode
· used to assess spinal cord integrity during spinal instrumentation
Technique
· posterior tibial nerve stimulated
· scalp SEP used to monitor for surgical induced changes
· signal monitored is in the posterior column
· relatively immune to compromise of the anterior spinal artery
· motor control is located anteriorly in the descending corticospinal tracts
· measurement of potentials recorded from individual motor units during voluntary contraction
· each anterior horn cell and axon supplies a group of fibres
· known as motor unit
· normally each motor unit fires fairly synchronously
· myofibrils are distributed over a small area and so the potentials reaches the electrode at about the same time
· results in a single, brief, smoothly summated motor unit potential
· electrically silent at rest
· on mild contraction, motor unit potential seen
· bi- or tri- phasic
· smooth
· on maximal contraction, baseline obliterated
· interference pattern
· due to some units contracting and resting and others taking over
· individual myofibrils become smaller
· produces much smaller amplitude and does not summate smoothly
· typically produces small amplitude, polyphasic waveform
· usually of normal duration since the myofibres are no further apart than normal
Acute denervation
· initially electrically silent
· earliest evidence seen 1-2 wks after denervation
· corresponds to the time taken for the distal nerve stump to degenerate
· depolarises spontaneously
· gives rise to fibrillations (small brief discharges)
· positive sharp waves are similar related phenomena differing in shape
Reinervation
· each motor unit has many more myofibrils
· typically produces large amplitude and polyphasic waveform
· duration increased since the myofibres are spread out over a wider area
· pathognomonic "dive bomber" wave form
· abnormal spontaneous discharge that waxes and wanes
· twin-core needle inserted into muscle
· electrical potentials observed on oscilloscope
· with muscle at rest
· during full contraction
· normal areas in an abnormal muscle may have normal potentials
· must sample abnormal areas