David J. Slutsky


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3475 Torrance Blvd., Ste F
Torrance, CA 90503


Appointments: 310.792.1809
Fax: 310.792.1811

Office hours: M-F, 9am – 5 pm

Worker’s compensation, Medicare and most insurance plans accepted.

In September 2008, we will be moving to a beautiful new 4,400 sq ft, state of the art dedicated hand center located at 2808 Columbia Ave in Torrance, CA., which will feature onsite nerve conduction studies, occupational hand therapy and digital x-ray.
 


PROCEDURES

NERVE CONDUCTION STUDIES

Electrodiagnostic studies of the upper limb are a useful method for testing peripheral nerve function. They are comprised of a nerve conduction study, that measures the actual speed of conduction of the nerves and electromyography, which directly assesses the nerve supply to and the integrity of the muscles. The following discussion will focus on the basic components of each test, the diagnostic relevance and the pitfalls.

Nerve Anatomy/Physiology:
An individual nerve cell consists of the cell body, which resides in the spinal cord, multiple short extensions or dendrites that connect with other nerve cells, and one long peripheral extension the axon. A peripheral nerve can contain thousands of axons which run in specialized tubes called fascicles. If the nerve cell body is unharmed, the peripheral nerve maintains the capacity to regenerate even 20 years later. Nerves that carry sensory information are called sensory nerves, and nerves that innervate muscle are called motor nerves. Sensory nerves can be repaired many years later and lead to some recovery of sensation to a part. This is because the sensory receptors located in the skin can regenerate following reinnervation even after many years. Motor nerves can also be repaired but the actual muscle itself loses it’s ability to contract after more than one year and can no longer function even if reinnervated. Hence there is a time limit of approximately one year from the time of repair to the time a muscle is reinnervated. This means that if the nerve injury is far removed from the muscle it supplies, the prognosis for recovery of motor function is poor. An example of this is the lack of intrinsic muscle recovery in the hand following an ulnar nerve laceration above the elbow. A peripheral nerve is a mixed nerve that consists of both sensory and motor fibers. These fibers separate and run in separate bundles from the mid forearm on down, but are intertwined above this. If a motor nerve is sutured to a sensory nerve, neither will work and vice versa. Despite this, repairs of mixed nerves can still lead to useful motor and sensory recovery that are independent upon the repair of the nerve.

Nerve Conduction:
A peripheral nerve consists of myelinated and unmyelinated nerves. The myelinated nerves are covered by specialized cells called Schwann cells, that manufacture a substance called sphingomyelin. The myelin wraps around the nerve to form a thick insulating sheath which functions in a similar fashion to the insulation around electrical wiring. A nerve axon has an electrical charge in the resting state. When the axon is stimulated, it will rapidly reverse it’s charge i.e. depolarize, going from a negative to a relatively more positive charge. This sudden change in the charge of the nerve is called an action potential. The action potential then spreads down the nerve causing the rest of the axon to reverse it’s charge. The myelin sheath prevents this charge from leaking out except at regular intervals where the sheath is missing (nodes of Ranvier). This allows the action potential to rapidly skip from one node to another, similar to the arc of an electrical charge. This skipping action, called saltatory conduction, rapidly speeds up the spread of the electrical charge down the length of the nerve. Nerves without the myelin sheath, or unmyelinated nerves conduct much slower, since the change in electrical charge must spread down the nerve sequentially, rather than skipping from node to node. Thus unmyelinated nerves conduct much slower than myelinated nerves. Typically larger nerves that convey touch are myelinated, as are nerves that supply muscle, whereas pain fibers are unmyelinated. Once the nerve is completely depolarized, it cannot be stimulated again for brief period of time, then gradually reverses back to it’s negative resting state, ready to be depolarized once again.

Nerve conduction studies:
During a nerve conduction study, a stimulator is used to discharge or depolarize a specific nerve by applying a small electrical stimulus to the nerve at the wrist, elbow or axilla. Recording electrodes placed on the fingers are used to record the change in the electrical charge of the nerve and to measure how long it takes for the electrical impulse to travel from the site of the stimulus to the recording electrode. Both the sensory nerve conduction and the motor nerve conduction is measured for each nerve. For example, in a sensory nerve conduction test, the median nerve is stimulated at the wrist and recording electrodes are placed over the index finger. The recording electrode measures how long it takes for the electrical stimulus at the wrist to reach the finger. This length of time it takes for the impulse to be picked up by the recording electrodes is called the latency of the nerve, and it is measured in milliseconds. The nerve is then stimulated at the elbow and the time that it takes to reach the finger is again recorded. It will obviously take longer for the action potential to reach the finger the further away from the finger that the nerve is stimulated, hence resulting in a longer latency. The distance between the elbow and the wrist is measured and subtracted, as are the latencies. This allows the calculation of the conduction velocity by dividing the speed of nerve conduction by the distance between the simulator and the recording electrodes. The size of the action potential or the amplitude is also measured. By comparing the latency, conduction velocity and the amplitude with standardized normal values, it is possible to determine if the nerve is working properly. Any given nerve is composed of faster and slower conducting axons. The measured action potential actually consists of a mixture of both. When the nerve is stimulated, progressively larger electrical impulses are applied to the nerve until there is no further increase in the action potential amplitude. The intensity of this stimulus is called supramax. This ensures that all of the nerve fibers are stimulated.

Motor nerve conduction is measured in a similar fashion, except that the recording electrode is placed over a muscle belly. The nerve impulse reaches the muscle, and then slows down dramatically, since the spread of the electrical impulse down muscle fibers must spread sequentially, similar to unmyelinated nerves. There is also a small lag time for the nerve impulse to spread from the nerve, across the motor end plate into the muscle (approximately 0.1 msec). Thus the measured latency for muscle is slower. The sensory latencies across the wrist are called the sensory distal latency, and the motor latencies across the wrist are called the motor distal latencies. These latencies are used to tell if there is slowing of the nerve conduction at the wrist level, which occurs in carpal tunnel syndrome. Generally, sensory nerve conduction will be affected early on, whereas motor nerve conduction is affected after more severe degrees of compression. In the upper extremity, typically at least 3 nerves will be evaluated: the median, ulnar and radial nerves. Because the conduction velocity must be measured above and below any suspected site of nerve injury or entrapment, the nerve conduction test cannot directly evaluate nerve conduction in the neck. Hence, the diagnosis of cervical nerve root compression cannot be made with nerve conduction. Specialized tests such as F-wave conduction, which is a type of nerve reflex, can be used to infer that proximal nerve conduction may be slower, but this test can be fraught with error, and may be difficult to evaluate.

Nerve Compression:
When a nerve is compressed, it conducts much slower, similar to a sprinter running a race with someone sitting on her shoulders. With chronic compression, the nerve will actually lose it’s myelin sheath. This demyelination is responsible for the slowing of nerve conduction. If the compression continues, some of the axons will die. If there are less nerve fibers the amplitude or size of the electrical charge will be smaller, leading to smaller amplitudes. The nerve conduction is of value since a nerve is assessed above and below the wrist and elbow. If there is slowing of the median nerve conduction at the wrist, but not the elbow or forearm, the diagnosis of carpal tunnel syndrome would be appropriate. If the ulnar nerve conduction is slower across the elbow but normal at the wrist, the diagnosis of cubital tunnel syndrome would be suggested. If all parts of the nerve are affected, and this occurs for multiple nerves, a process that affects the peripheral nerves in general, or a polyneuropathy may be present. Unfortunately, in complex cases, both conditions can coexist, such as a diabetic polyneuropathy with superimposed carpal tunnel compression. In cases of severe nerve entrapment with clinical signs of sensory loss or muscle atrophy immediate surgery may be indicated. In these cases an electromyogram will also be performed. Pitfalls:
One of the confusing aspects of nerve conduction is that it is quite common for the tests to be within normal limits even with hard clinical signs and symptoms of carpal tunnel syndrome and cubital tunnel syndrome. This is because the nerve conduction test preferentially measures the faster conducting axons. If there are enough of the faster conducting axons left intact, the latency and amplitudes will be within normal limits. In this case, the clinical exam is more accurate.

The neurophysiology of electrical recording is such that the recording electrode will detect activity in the largest myelinated fibers first, since these fibers conduct at the fastest rates. Latency and conduction velocity, the commonly recorded parameters, depend on the time required from the electrical stimulus to the first recording. Electrically recordable events that occur thereafter, such as those related to activity in the slower conducting, smaller and more thinly myelinated fibers, are not reflected in routine electrodiagnostic reports. If only a fraction of the large, thickly myelinated fibers remain and transmit impulses i.e. even if _ of the fibers are not functioning, the recorded latency and conduction velocity remains normal because the pick up electrode detects the fastest fibers as still functioning. It is also of note that the periphery of a fascicle has a greater degree of injury than the center. The fascicles within a nerve are not uniformly affected by compression. Large myelinated and small unmyelinated fibers can be affected differently. This means that the patient’s symptoms i.e. middle finger with persistent numbness or clumsiness, may be due to a severe degree of compression, with its attendant demyelination and axonal degeneration, while the normal findings on the electrodiagnostic testing are due to an adjacent, less severely affected fascicle. This has been likened to a traffic helicopter reporting on freeway traffic. If 2 lanes are blocked but 5 lanes are moving normally, then the traffic flow is reported as being normal. The blocked lanes are effectively ignored. Even with neural regeneration following operative decompression, and despite restitution of neural blood flow and distal axonal regeneration, remyelination of the axon is often incomplete. Thus, although the patient may be without symptoms, follow up nerve conduction studies may suggest persistent or recurrent nerve compression.

Technical Errors:
There are a number of possible technical errors associated with nerve conduction testing. If the electrodes are placed improperly, or if the patient has a cold hand, the nerve conduction will be artificially slower leading one to the incorrect diagnosis of nerve entrapment. If the distance between the electrodes is erroneous, the calculated velocity will also be inaccurate, leading to false conclusions of nerve dysfunction. If the initial stimulating impulse is of insufficient amplitude, i.e. a less than supramaximal stimulation, not all of the nerve fibers will be stimulated, which can lead to a falsely lower nerve amplitude. All of these errors can be minimized by attention to detail.

Combined with a detailed medical history and a thorough upper extremity examination, the nerve conduction test can yield very useful information. It cannot be taken out of context however, since it is not uncommon for a patient to be totally asymptomatic yet a nerve conduction study is reported as showing mild slowing of the conduction velocities and latencies if the hand is cold, or if the electrode is making poor contact. Hence, the nerve conduction test is never used as the sole indication for surgical treatment. The patient must also have reproducible clinical signs of nerve entrapment or repeated examination.