NERVE CONDUCTION VELOCITY physiotherapy (2)-1.pptx

Physiotherapy

1. NERVE CONDUCTION VELOCITY Dr Bhawna Sharma MPT ( Neuro ) Asst Professor ITS IHAS

2. ANATOMY AND PHYSIOLOGY OF PERIPHERAL NERVES

3. Peripheral nerves are composed of a large number of fascicles, which are surrounded by connective tissue sheaths. Each nerve fascicles contains Schwann cells and fibrocytes , axons, myelin sheath, collagen fibrils of endoneurium , and blood vessels. Diameter varies from 1 micrometre to 20 micrometre The connective tissue forms three sheaths in the peripheral nerves, which are known as endoneurium , perineurium , and epineurium from inside out.

4. Endoneurium : present between the surface membranes of Schwann cells inn which axons are embedded Perineurium divides the nerve into fascicles, which run an interweaving course. Epineurium is the outermost sheath of peripehral nerve and continues with the dura mater of spinal root. The vasanervosa and lymphatics are present in the epineurium and give branches to perineurium to form capillary anastomosis .

5. These three connective tissue layers give strength and flexibility to the peripheral nerves. Nodes of Ranvier : junction between two Schwann cells ; uninsulated Internodal distance: distance between two nodes of ranvier Nerve conduction velocity depends upon fiber diameter and internodal distance.

6. CLASSIFICATION OF NERVE FIBERS NERVE FIBER SPECIFICATIONS GROUP A Contain both afferent and efferent myelinated somatic fibers of small, medium and large diameter (1-20 µm) Subdivided in α , β , gamma, delta according to conduction velocities GROUP B Consists of small preganglionic myelinated axons of ANS (1-3 µm) GROUP C Small unmyelinated fibers, which are present in visceral afferents, pain and temperature afferents and preganglionic autonomic efferent (2-2.2 µm)

7. AXONAL TRANSPORT

8. Depending upon the direction with refernece to soma, axonal transport is classified as Anterograde and Retrograde

9. NCV is done to determine the speed with which a peripheral motor or sensory nerve conducts an impulse EMG and NCV are two important diagnostic procedures that can provide complete information about the extent of nerve injury or muscle disease NCV can be tested for any superficial nerve that is superficial enough to be stimulated through skin at two different points Most common: ulnar , median, peroneal , posterior tibial

10. PRINCIPLES OF MOTOR NERVE CONDUCTION It is calculated by measuring the distance between two points of stimulation Expressed in m/sec Measurement of latency difference between the two points of stimulations eliminates any error. Conduction velocity = Distance/ ( PL- DL ) PL: proximal latency; DL : distal latency

11. With sensory and mixed NCSs, the recording electrodes are positioned over the nerve under study, whereas with motor NCSs, they are positioned over the muscle belly and tendon (the belly–tendon method) of the muscle innervated by the nerve under study To reduce shock artifact, a ground electrode is used and is best placed between the stimulating and recording electrodes.

12. The elicited responses are differentially amplified and displayed on the monitor. To record the CMAP, the stimulating current or voltage is gradually increased until a point is reached where an increase in stimulus produces no increment in CMAP amplitude. It is only at this ( supramaximal ) point that reproducible values for CMAP amplitude and the latency between the stimulus and the onset of the CMAP can be recorded accurately. Thus, a supramaximal stimulus generates a maximal response.

13. With motor NCSs, because the recording electrodes are placed over the muscle, the motor response actually is composed of muscle fiber APs (rather than motor nerve fiber APs) and, for this reason, is referred to as a compound “muscle” action potential (CMAP). The sensory response is composed of individual sensory nerve fiber APs and is referred to as a compound sensory nerve action potential (SNAP).

14. MOTOR NCS Motor  nerve conduction  studies assess the lower motor neurons from the level of the anterior horn to the muscle. The stimulating electrodes are applied over the nerve and the recording electrodes are applied over the muscle. Thus, motor NCSs are orthodromically recorded (i.e., the recorded APs traverse the nerve fibers in their physiological direction, from proximal to distal).

15. Stimulation is applied at two sites along the nerve, yielding two separate motor responses. Each individually evoked motor nerve fiber AP propagates distally and elicits a much larger number of muscle fiber APs

16. Motor nerve is stimulated at two points along its course Electrode is bipolar electrode with the cathode and anode Small surface electrodes or needle electrodes may be used A biphasic action potential with the initial negativity is recorded

17. Surface stimulation of healthy nerve requires a square wave pulse of 0.1 ms duration with an intensity of 5-40 mA . In diseased nerve, the nerve excitability is reduced and the current requirement may be much higher than normal.

18. Measurement includes: Onset latency ( time in milliseconds from the stimulus artifact to the first negative deflection) Duration Amplitude of CMAP ( base to peak and peak to peak). It correlates with the number of fibers Nerve conduction velocity

19. For accurate, motor nerve conduction velocity measurement, the distance between two pints of stimulation should be 10 cm to reduce the error due to faulty distance measurements.

20. The distal motor latency is more than simply a conduction time along the motor axon; it includes not only (A) the conduction time along the distal motor axon to the NMJ, but also (B) the NMJ transmission time and (C) the muscle depolarization time. Therefore, to calculate a true motor conduction velocity, without including NMJ transmission and muscle depolarization times, two stimulation sites must be used, one distal and one proximal.

21. The proximal motor latency reflects four separate times, as opposed to the three components reflected in the distal motor latency measurement. In addition to (A) the  nerve conduction  time between the distal site and the NMJ, (B) the NMJ transmission time, and (C) the muscle depolarization time, the proximal motor latency also includes (D) the nerve conduction time between the proximal and distal stimulation sites

22. Therefore, if the distal motor latency (containing components A + B + C) is subtracted from the proximal motor latency (containing components A + B + C + D), the first three components will cancel out. This leaves only component D, the nerve conduction time between the proximal and distal stimulation sites, without the distal nerve conduction, NMJ transmission, and muscle depolarization times.

23. A drop in amplitude indicates a conduction block, whereas an increase in duration indicates a lack of uniform conduction along the axons.

24. In axonal loss The most striking abnormality is a reduction in CMAP amplitude as fewer functioning motor axons are connected to muscle fibres . In demyelination With loss of myelin thickness nerve conduction is slowed and, if severe enough, saltatory conduction fails (conduction block). NCS shows severely prolonged motor latencies and notably slowed conduction velocities.

25. PRINCIPLES OF SENSORY NERVE CONDUCTION Can be mesaured orthodromically or antidromically ORTHODROMIC CONDUCTION: distal portion of nerve is stimulated and sensory nerve action potential is recorded at a proximal point ANTIDROMIC CONDUCTION: nerve is stimulated at proximal point and action potential is recorded distally

26. In antidromic nerve conduction, AP may get hampered due to motor action as simultaneous stimulation of motor nerve takes place For orthodromic conduction, ring electrodes are preferred to stimulate the digital nerve. surface electrodes are commonly used for antidromic stimulation.

27. Measurements include: onset latency , amplitude (base to negative peak or positive to negative peak; suggest the dnesity of nerve fibers) , duration of SNAP (suggests the number of slow conducting fibers) and nerve conduction velocity

28. Latency of orthodromic potential is measured from the stimulus artifact to the initial positive or subsequent negative peak. The initial positive peak in SNAP gives it a triphasic appearance, which is a feature of orthodromic potential In antidromic potential, the initial positivity in SNAP is lacking.

29. The three phases are a small positive approaching phase, a large negative peak and a long positive tail. In fact, though, the initial positive phase, which is well defined with orthodromic recordings, is missing with antidromic recordings. The absence of this phase can be seen along the various recordings shown in the articles publishing recorded action potentials to antidromic stimulation 

30. Stimulation at single site is done. Why ? Sensory NCV calculated by dividing distance (mm) between stimulating and recording sites by the latency (ms)

31. Traces reproduced from the articles published by  Bannister and Sears (1962)  and  Murai and Sanderson (1975)  showing antidromic action potentials. See the absence of the approaching phase. The sensitivity and specificity were both 100% with the orthodromic technique and 45% and 85% respectively with the antidromic technique. Therefore, this author recommended only the orthodromic technique for confirming the diagnosis of mild CTS in the few cases in which the inching technique is required, which  Seror (2000)  considered helpful in 6%–8% of all CTS cases.

32. In generalised disorders In both axonal and demyelinating pathologies the SNAP amplitude is reduced for different reasons. Sensory axonal loss will result in a smaller SNAP. Demyelination also produces small SNAPs but with prolonged durations. Focal lesions Multiple sensory NCS allow the investigator to locate sensory neuropathies that involve single or multiple digital nerves distally (for example, vasculitis or hand arm vibration syndrome) right up to the major trunks, cords, and divisions of the brachial plexus proximally. In proximal nerve trauma, maintenance of the sensory potential depends on the intact cell bodies in the dorsal root ganglia. Thus sensory NCS are extremely useful in localising a PNS lesion as either pre- and/or post- ganglionic .

33. VARIABLES AFFECTING NCV

34. Physiological Variables

35. Age The NCV in a full term infant is nearly half of the adult value. As the myelinaiton progresses, the NCV attains the adult value by 3-5 years of age. Conduction velocity begins to decline after 30-40 years of age

36. Upper versus Lower Limb Median and ulnar NCV is faster as compared to Tibial and Peroneal . An inverse relationship between height and NCV suggests that the longer nerves conduct slower than shorter nerves Reasons being: Abrupt axonal distal tapering Shorter internodal distance Progressive reduction in axonal diameter Lower temperature of feet compared to hands

37. Temperature Influences the velocity and the amplitude of CMAP. Low temperature: Slow NCV, increased amplitude For each degree celsius fall in temperature, latency increased by 0.3 ms, due to effect of cooling on sodium channels On increasing the temperature, the velocity increases by 5% per degree celsius .

38. Technincal Variables

39. Stimulating System Failure of stimulaitng system may result in absent or unexpectedly small responses. If such fault is seen, the stimulator should be relocated close to nerve and pressed firmly. In case of obesity, oedema , needle electrodes may be tried,

40. Recording System Faulty connection in recording system may result in errors in spite of optimal stimulation. Common errors: Break in electrode wire Connection to a wrong preamplifier Incorrect oscilloscope setting of gain, sweep and filter.

41. Inadvertent Stimulaiton of U nintended Nerves Spread of stimulating current to an adjacent nerve or root not under study is frequent Needle electrodes are helpful.

42. LATE RESPONSES

43. Late responses are the potentials appearing after motor response ( M wave) following mixed nerve stimulation. There are three important late responses: H reflex F response Axon reflex

44. H REFLEX Described by Hoffman in 1918 Diagnostic measure for radiculopathy and peripheral neuropathy Monosynaptic stretch reflex Elicited by submaximal stimulation of tibial nerve and recorded from the calf muscle In normal adults, it can also be recorded in other muscles of limbs; but not from the small muscles of hand and feet.

45. The reflex arc of H reflex includes: Large fast conducting group 1a fibers Spinal cord Efferents motor fibers supplying nerves H reflex is facilitated by submaximal stimulaiton and inhibited by stronger stimulation Inhibition is attributed to collision of orthodromic impulses by antidromic conduction in motor axons.

46. H reflex has the advantage of evaluating the proximal sensory and motor pathways Evaluation of plexopathies and radiculopathies GBS , H reflex is absent or delayed S1 radiculopathy , soleus H reflex may be absent Flexor carpi radialis H reflex abnormal in C6-C7 radiculopathy

47. F waves F for foot where they were first described First described by Magladary and McDougal in 1950 in small muscles of foot F wave is a late response resulting from antodromic activation of motor neurons involving conduction to and from spinal cord and occurs at interface between the PNS and CNS Motor conduction velocity along the whole axon, including the proximal portions, can be studied by eliciting the  F-wave  response, a small, late muscle response that occurs as a result of backfiring of  anterior horn cells .

48. Useful supplement to nerve conduction studies and EMG measures Most helpful in diagnosis of conditions where the most proximal portion of the axon is involved Direct activation of motor axon Both orthodromic and antidromic stimulation is recorded. Useful in GBS, brachial plexus injuries and radiculopathies

49.   F waves  may be obtained from almost any mixed nerve that can be stimulated, but the median, ulnar , peroneal , and  posterior tibial nerves  are the most commonly used. If the standard distal motor conduction velocities are normal but the F-wave value is prolonged, slowing must be occurring somewhere more proximal to the distal normal segment. Comparison with the opposite limb may be most helpful if that limb is asymptomatic.

50. Axon Reflex Late potential generally appearing between M and F waves Occurs due to collateral branching in the proximal portions of the nerves

51. Polyneuropathy  (poly- + neuro - + - pathy ) is damage or disease affecting peripheral nerves (peripheral neuropathy) in roughly the same areas on both sides of the body, featuring weakness, numbness, and burning pain. Plexopathy  is a disorder affecting a network of nerves, blood vessels, or lymph vessels. The region of nerves it affects are at the brachial or lumbosacral plexus. Symptoms include pain, loss of motor control, and sensory deficits. Radiculopathy , also commonly referred to as pinched nerve, refers to a set of conditions in which one or more nerves are affected and do not work properly (a neuropathy). This can result in pain ( radicular pain), weakness, numbness, or difficulty controlling specific muscles.

52. Most authors would agree in keeping a standardized distance between the stimulating cathode and the recording active electrode of 14 cm in normal sized hands. It is also generally accepted that the study of short segments of the nerve across the site of compression increases the sensitivity of the study ( Jablecki et al., 2002 ). In fact, one of the most sensitive tests recommended for the assessment of compression of the median nerve at the wrist is stimulation at the palm and recording over the wrist at a distance of 8 cm ( Jablecki et al., 2002 ,  Sandin et al., 2010 ).

53. Onset latency, usually measured at the beginning of the negative phase, depends on the fastest conducting fibers, while peak latency is an expression of the mean conduction velocity value among all fibers participating in the SNAP. For this, the stimulating electrode activates the median nerve at the wrist and the response is recorded over digital nerves of the index or middle fingers. The stimulating electrodes should be placed longitudinally over the median nerve, to avoid unintended concomitant activation of the ulnar or radial nerves in transversally oriented stimuli.

54. Supramaximal intensities used for the stimulation of sensory fibers at wrist level will unavoidably also activate motor fibers and, therefore, generate movements because of contraction of hand muscles A single stimulus is usually enough to obtain a sizeable action potential.

55. Stimulating electrodes are usually ring electrodes placed around the proximal and middle phalanxes of the 2nd or 3rd digits and the recording electrodes are placed on the ventral aspect of the wrist, over the median nerve, usually at about 1–2 cm proximal to the proximal wrist crease.

56. In any case, orthodromic responses are more reliably measured after averaging some 8–10 traces. While recording at the palm is rather problematic due to the interference of activity from deep interossei and lumbricalis muscles and the proximity of the stimulus artifact, recording at the wrist yields clean and reproducible action potentials, with little noise from surrounding structures.

57. Examples of representative recordings of antidromically and orthodromically generated SNAPs are shown in  Fig. 1 . Recordings of antidromic (A and B) and orthodromic (C and D) sensory nerve action potentials of the 3rd finger, at progressively increasing stimulus intensity. Antidromic testing with stimulation at the wrist over the median nerve and recording with ring electrodes on the 3rd finger. Orthodromic testing with stimulation at the finger with ring electrodes and recording at the wrist. Distance between stimulating cathode and active recording electrodes: 14 cm. Inter-electrode distance for stimulation and recording with both techniques: 3 cm. At each graph, the top traces are recorded at threshold intensity for eliciting a recognizable action potential and the bottom traces are those corresponding to a supramaximal stimulus intensity.

58. There is a progressive shortening of latency with increasing intensity with both types of stimulation when stimuli are of long duration.

59. Table 1 Physical and physiological differences between antidromic and orthodromic techniques to examine median sensory nerve conduction between finger and wrist. AntidromicOrthodromicRecordingNerve locationSuperficialDeepNerve sizeThinThickNerve lengthProximalDistalSize of the SNAPLargeSmall StimulationFiber typeMixedSensoryMovement artifactPresentAbsentRelevance of stimulus durationLittleGreat