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Background electrical noise
a. Larger in sensory potentials because of low amplitude of signal (low signal-to-noise ratio)
b. Background noise from voluntary muscle activity needsto be minimized (high amplitude relative to signal)
Stimulus artifact is reduced by
- Preventing electrode or sweat “bridging”
- Minimal duration and intensity of stimulus (just enough to obtain supramaximal response)
- Minimizing distance between stimulator and nerve to allow supramaximal depolarization with minimal possible current, reducing overstimulation and muscle artifact
Important factors in stimulation
1) Axon size: larger axons are stimulated more easily
2) Myelin: myelinated axons are stimulated more easilythan unmyelinated axons
Compound muscle action potential (CMAP) conductionaffected by
nerve conduction time, neuromuscular junction transmission, and the muscle.
So in sensory exam Only nerve axons are assessed (as compared with motorconduction studies, which measure conduction alongmotor nerve, neuromuscular junction, and muscle)
Disadvantages of antidromic conduction study inhand:
muscle artifact and volume conductionfrom lumbricals (with ring electrodes)
Placement of electrodes and recording
1) Two recording electrodes (G1 and G2) are placed over the nerve, 3.5 to 4 cm apart
2) G1, the active recording electrode, is placed closer to stimulator
3) Ground electrode: relatively large and placed between stimulating and recording electrodes
- Conduction velocity Can be determined with one stimulation site (ascompared with the motor conduction velocity thatrequires two stimulation sites)
- Compound potential that is summation of individual nerve action potentials, may be biphasic or triphasic.
- Usually more sensitive to both generalized and focalnerve disease than motor nerve conduction studies (e.g., entrapment mononeuropathies such as carpal tunnel syndrome)
- SNAP abnormalities may be first abnormalities of a neuropathic process such as generalized sensorimotor neuropathy (earlier than motor studies) and may be selectively involved in sensory neuropathies.
- Proximal sensory studies result in smaller amplitudepotentials and are difficult to perform
- - Temporal dispersion and phase cancellation withproximal stimulation normally occur with SNAPs
Antidromic technique (compared with orthodromic technique)
- Amplitude of the responses is higher inantidromically conducted potentials because therecording electrodes (e.g., ring electrodes) are closer to underlying sensory nerves
- Useful when recording small potentials in neuropathic conditions
- Because a mixed motor-sensory nerve is stimulated, SNAP is usually followed by, and may be mistaken for, a volume-conducted motor response
Reason for tempral dispersion and phase canellation in proximal SNAP
- Lag time between faster and slower conductingfibers is exaggerated with proximal stimulation,leading to increased duration and decreased amplitude.
- Phase cancellation occurs from overlap of negative phase of asingle-fiber sensory action potential and positivephase of another sensory action potential.
Note: Onset latency is not affected by temporal dispersion and phase cancellation and can be used to measure conduction velocity.
Why SNAPs potentially may be abnormal in lower lumbar or upper sacral radiculopathies
Because the DRG of lower lumbar and upper sacral segments may be inside the spinal canal and axonal injury related to a compressive radiculopathy in these segments may be at or distal to the DRG, SNAPs potentially may be abnormal in lower lumbar or upper sacral radiculopathies
- Generally causes reduced amplitudes
- Relative preservation of conduction velocity and distal latency (unless severe enough to affect fastest and largest fibers)
- Hyperacute axonal loss: normal sensory responsesif assessed before wallerian degeneration
- Conduction block: reduced amplitudes across thesite (proximal sensory responses may be absent)
- Focal slowing: slowed conduction velocity across the site
- Focal slowing may not be appreciable with SNAPs (as seen in motor responses), Sensory amplitudes may decrease because of phase cancellation
- Lateral antebrachial cutaneous sensory --> upper trunk or lateral cord distribution
- Superficial radial sensory --> middle trunk or posterior cord distribution
- Medial antebrachial cutaneous --> lower trunk or medial cord distribution
CMAP in myopathy
CMAP may be normal in myopathy until disease isadvanced and there is loss of muscle tissue
Temporal dispersion and phase cancellation in CMAP study is more common in .......
acquired demyelination with orwithout conduction block (less common ininherited demyelination)