Electrodiagnosis Course PDF

Electrodiagnosis Course Dr. Doha Al- Afifi  The nerve selected on the study depend on: The patient’s symptoms and signs and the differential diagnosis Motor, sensory and mixed nerve studies can be performed by stimulating the nerve and placing a recording electrode over a distal muscle, a...

Electrodiagnosis Course Dr. Doha Al- Afifi  The nerve selected on the study depend on: The patient’s symptoms and signs and the differential diagnosis Motor, sensory and mixed nerve studies can be performed by stimulating the nerve and placing a recording electrode over a distal muscle, a cutaneous sensory nerve or the entire mixed nerve respectively. Sensory conduction studies  Sensory nerve conduction studies(SNAP): In contrast to motor conduction studies, in which the CMAP reflects conduction along motor nerve, NMJ, and muscle fibers, in sensory conduction studies only nerve fibers are assessed. Because most sensory responses are very small (usually in the range of 1 to 50 µV), technical factors and electrical noise are more significant. For sensory conduction studies, the gain usually is set at 10 to 20 µV per division. For sensory studies, an electrical pulse of either 100 or 200 ms in duration is used, and most normal sensory nerves require a current in the range of 5 to 30 mA to achieve supramaximal stimulation. Sensory fibers usually have lower threshold than motor fibers. Sensory conduction velocity can be calculated with one stimulation  ANTIDROMIC STIMULATION Recoding electrode: Ring electrodes Digit 2 or 3. G1 metacarpophalangeal Joint and G2 distal interphalangeal joint. Stimulation electrodes: 13-14 cm Proximal from active ring electrode. Interelectrode distance :of 2.5 to 4 cm between two electrode.  Sensory nerve conduction studies(SNAP): Is a compound potential that represents the summation of all the individual sensory fiber action potentials. SNAPs usually are biphasic or triphasic potentials. For each stimulation site, the onset latency, peak latency, duration, and amplitude are measured. Unlike motor studies, a sensory conduction velocity can be calculated with one stimulation site alone, by taking the measured distance between the stimulator and active recording electrode and dividing by the onset latency. No NMJ or muscle time needs to be subtracted out by using two stimulation sites.  Onset Latency Is the time from the stimulus to the initial negative deflection from baseline for biphasic SNAPs or to the initial positive peak for triphasic SNAPs. Sensory onset latency represents nerve conduction time from the stimulus site to the recording electrodes for the largest cutaneous sensory fibers in the nerve being studied.  PEAK LATENCY The peak latency is measured at the midpoint of the first negative peak. Although the population of sensory fibers represented by the peak latency is not known (in contrast to the onset latency, which represents the fastest conducting fibers in the nerve being studied), measurement of peak latency has several advantages. There is practically no interindividual variation in its determination. In contrast, the onset latency can be obscured by noise or by the stimulus artifact, making it difficult to determine precisely  Amplitude The SNAP amplitude is most commonly measured from baseline to negative peak, but it can also be measured from the first negative peak to the next positive peak. The SNAP amplitude reflects the sum of all the individual sensory fibers that depolarize. Low SNAP amplitudes indicate a definite disorder of peripheral nerve.  DURATION Duration Similar to the CMAP duration, SNAP duration is usually measured from the onset of the potential to the first baseline crossing (i.e., negative peak duration), but it also can be measured from the initial to the terminal deflection back to baseline. The former is preferred given that the SNAP duration measured from the initial to terminal deflection back to baseline is difficult to mark precisely, because the terminal SNAP returns to baseline very slowly. The SNAP duration typically is much shorter than the CMAP duration (typically 1.5 vs. 5–6ms, respectively).  SENSORY CONDUCTION VELOCITY Unlike the calculation of a motor conduction velocity, which requires two stimulation sites, sensory conduction velocity can be determined with one stimulation site. Simply by dividing the distance traveled by the onset latency. Sensory conduction velocity represents the speed of the fastest, myelinated cutaneous sensory fibers in the nerve being studied.  Proximal sensory studies result in smaller amplitude potentials and often more difficult to perform, even in normal subjects, because of the normal processes of phase cancellation and temporal dispersion. PROXIMAL STIMULATION: NORMAL TEMPORAL DISPERSION AND PHASE CANCELLATION  TEMPORAL DISPERSION There is a normal variation in the size of individual sensory fiber action potentials, with larger fibers generally having larger amplitudes. Temporal dispersion occurs as these individual nerve fibers fire at slightly different times (i.e., larger, faster fibers depolarize before smaller, slower ones). Temporal dispersion normally is more prominent at proximal stimulation sites because the slower fibers progressively lag behind the faster fibers TEMPORAL DISPERSION  Special Considerations in Sensory Conduction Studies: Antidromic versus Orthodromic A. When a nerve is depolarized, conduction occurs equally well in both directions away from the stimulation site. A. Sensory conduction studies may be performed using either : Antidromic (stimulating toward the sensory receptor) or Orthodromic (stimulating away from the sensory receptor) techniques TYPES OF SENSORY CONDUCTION STUDY Antidromic Orthodromic  TYPES OF SENSORY CONDUCTION STUDY Antidromic Studies: higher amplitude Orthodromic Studies: more painful Orthodromic Orthodromic  EXAMPLE: MEDIAN NERVE When studying median sensory fibers to the index finger, one can stimulate the median nerve at the wrist and record the potential with ring electrodes over the index finger (antidromic study). Conversely, the same ring electrodes can be used for stimulation, and the potential recorded over the median nerve at the wrist (orthodromic study). Latencies and conduction velocities should be identical with either method, although the amplitude generally is higher in antidromically conducted potentials Antidromic Versus Orthodromic  WHY THE ANTIDROMIC TECHNIQUE IS SUPERIOR??????? Most important, the amplitude is higher with antidromic than with orthodromic recordings, which makes it easier to identify the potential. The antidromic technique is especially helpful when recording very small potentials, which often occur in pathologic conditions. Furthermore, because the antidromic potential generally is larger than the orthodromic potential, it is less subject to noise or other artifacts.  WHY THE ANTIDROMIC TECHNIQUE IS SUPERIOR??????? The SNAP amplitude is directly proportional to the proximity of the recording electrode to the underlying nerve. For most antidromically conducted potentials, the recording electrodes are closer to the nerve. For example, in the antidromically conducted median sensory response, the recording ring electrodes are placed on the finger, very close to the underlying digital nerves just beneath the skin from which the potential is recorded. When the montage is reversed for orthodromic recording, there is more tissue (e.g., the transverse carpal ligament and other connective tissues) at the wrist separating the nerve from the recording electrodes. This results in attenuation of the recorded sensory response, resulting in a much lower amplitude.  DISADVANTAGES OF ANTIDROMIC Since the entire nerve is often stimulated, including the motor fibers, this frequently results in the SNAP being followed by a volume-conducted motor potential. It usually is not difficult to differentiate between the two, because the SNAP latency typically occurs earlier than the volume-conducted motor potential. However, problems occur if the two potentials have a similar latency or, more importantly, if the sensory potential is absent. When the latter occurs, one can mistake the first component of the volume conducted motor potential for the SNAP, where none truly exists.  DISADVANTAGES OF ANTIDROMIC It is in this situation that measuring the duration of the potential can be helpful in distinguishing a sensory from a motor potential. If one is still not sure, performing an orthodromic study will settle the issue, as no volume conducte motor response will occur with an orthodromic study. In this case, the antidromic and orthodromic potentials should have the same onset latency.