Lec 7 Late Response PDF

Late Response Dr. Dany Alphonse Anwar Habib Lecturer of physical therapy for neuromuscular disorders and its surgery MUST University Mission and Vision of the College of Physical Therapy – Misr University for Science and...

Late Response Dr. Dany Alphonse Anwar Habib Lecturer of physical therapy for neuromuscular disorders and its surgery MUST University Mission and Vision of the College of Physical Therapy – Misr University for Science and Technology (MUST) Intended Learning Outcomes of the Course (ILOs) A- Knowledge and Understanding A3: Realize the principles of examination specific to each treatment procedure A4: Identify basics of patient’s safety and safety procedures during practice B- Intellectual Skills Not applicable Intended Learning Outcomes of the Course (ILOs) - continued C- Professional &Practical skills: C2: Conduct physical therapy examination using electrodiagnostic testing techniques C4: Examine patients using electrodiagnostic testing to determine the need for consultation or referral with other members of the health team D- General and transferable skills: Not applicable Required Reading: pp: 36 - 46  By the end of this lecture, the student should have a basic understanding about F- Response and H-Reflex Topics will be discussed in this lecture: 1- F-Response 2- H-Reflex Late Responses  Nerve conduction studies are most often used to assess distal nerve segments, with routine stimulation seldom done above the elbow or knee. Few studies can be easily performed to assess the more proximal nerve segments (plexus and roots). In the arm, surface stimulation can be performed proximally in the axilla and at Erb’s point, although technical factors limit these studies, especially at Erb’s point.  In the electromyography (EMG) laboratory, two late responses, the F response and the H reflex, are used routinely to study the more proximal nerve segments. Each has its advantages and limitations  they travel the entire nerve segment from distal to proximal and back. Thus, they are most useful when routine nerve conduction studies, which assess distal segments, are normal and the late responses are abnormal, a situation that implies a proximal lesion. F RESPONSE  The F response is a late motor response that occurs after the compound muscle action potential (CMAP, also known as the direct motor [M] potential)  FIGURE Normal F response. Stimulating the median nerve at the wrist, recording abductor pollicis brevis. F, F response; M, direct motor response (i.e., compound muscle action potential).  FIGURE Note that to accurately measure the F response, the gain must be increased to 200 µV and the sweep to either 5 or 10 ms. With these settings, the F response is well seen, but the M response saturates the amplifier and becomes distorted (malformed). The F response derives its name from the word “foot” because it was first recorded from the intrinsic foot muscles. In the upper extremity, when the median or ulnar nerves are stimulated at the wrist, the F response usually occurs at a latency of 25 to 32 ms. In the lower extremity, when the peroneal or tibial nerves are stimulated at the ankle, the F response usually occurs at a latency of 45 to 56 ms. If the stimulator is moved proximally, -the latency of the CMAP increases as expected, -but the latency of the F response actually decreases Normal F responses, distal and proximal stimulation. Median F responses recording abductor pollicis brevis, stimulating wrist (left trace) and elbow (right trace). DL, distal compound muscle action potential (CMAP) latency; PL, proximal CMAP latency. Note with proximal stimulation, the proximal CMAP latencies increase as expected, but the F response latencies decrease, due to the F response traveling a shorter distance antidromically to the spinal cord.  This is due to the circuitry of the F response, which is initially antidromic toward the spinal cord. Thus, with more proximal stimulation, the action potential has less distance to travel, hence the shorter latency.  During a routine motor nerve conduction study, one usually thinks of the action potential as traveling down the nerve across the neuromuscular junction (NMJ) to subsequently depolarize the muscle.  When stimulated, however, the nerve conducts well in both directions. The F response is derived by antidromic travel up the nerve to the anterior horn cell, with backfiring of a small population of anterior horn cells, resulting in orthodromic travel back down the nerve past the stimulation site to the muscle F response circuitry. When a nerve is stimulated distally (S), depolarization occurs both orthodromically and antidromically. The direct muscle response (M) occurs from orthodromic travel. The F response (F) is derived from antidromic travel to the anterior horn cell, backfiring of some anterior horn cells, and orthodromic travel back down the nerve past the stimulation site to the muscle. The F response is actually a small CMAP, representing 1 to 5% of the muscle fibers. The F response circuitry, both afferent and efferent, is therefore pure motor. There is no synapse, so it is not a true reflex. In conditions that selectively affect the sensory nerves or sensory nerve roots, the F responses are completely normal.  Each F response varies slightly in latency, configuration, and amplitude because a different population of anterior horn cells is activated with each stimulation. Presumably, the shortest latency represents the largest and fastest conducting motor fibers. Several measurements can be made on the F responses, with the most common being the minimal (or fastest) F response latency H REFLEX  The H reflex derives its name from Paul Hoffmann, who first evoked the response in 1918. The H response is distinctly different from the F response in that it is a true reflex with a sensory afferent, a synapse, and a motor efferent segment. Likewise, several other properties differentiate the H and F responses H reflexes are widely present in motor nerves, but beyond the age of two, they can only be routinely elicited by stimulating… -The tibial nerve in the popliteal fossa and recording the gastro–soleus muscle. Although there are techniques for obtaining an H reflex from the femoral nerve recording the quadriceps muscle and from the median nerve recording the flexor carpi radialis muscle, both of these have significant limitations. H reflex setup. To record the H reflex, G1 is placed over the soleus, two to three fingerbreadths distal to where it meets the two bellies of the gastrocnemius muscle, with G2 over the Achilles tendon. The tibial nerve is stimulated submaximally in the popliteal fossa, with the cathode placed proximal to the anode.  Optimal recording location for the H reflex. If one draws a line from the popliteal fossa posteriorly to the Achilles tendon where the medial malleolus flares out and then divides that line into eight equal parts, the optimal location for placing the active recording electrode (G1) is at the fifth or sixth segment distally. This location over the soleus is approximately two to three fingerbreadths distal to where the soleus meets the two bellies of the gastrocnemius. H reflex. Note at low stimulation intensities, an H reflex is present without a direct motor (M) response. With increasing stimulation, the H wave grows and the M response appears. At higher stimulation, the M potential continues to grow and the H reflex diminishes, due to collision between the H reflex and antidromic motor potentials.