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LibertyYou will find in this section hot information on when a Doctor of Chiropractic should refer to the consulting Neurologist which had eluded these professions for almost 100 years.


     Neurodiagnostic specificity will facilitate expediency in many diagnostic perplexities.  The procedures aid in the diagnoses of central and peripheral neuropathies.  Further  symptoms and manifestations, of epilepsy, stroke, diabetic radiculopathy, and chronic pain syndromes where state of the art imaging has failed to provide answers, Neurodiagnostic modalities can discover those elusive answers when the physician understands the science of findings discovery.   Even specificity on the precise nuclei within the brain and spinal column which may be diseased or dysfunctional can be located.  And finally utility for the purposes of biofeedback and the rehabilitation of previously non-mobile or non-ambulatory patients through reintegration bio-feedback reeducation can be achieved.  The purpose of this review is to familiarize the graduate medical physician with post graduate neuroscience taxonomy and practical understanding of it's application.


    It is essential to understand a few basic concepts and in doing so this examiner will take you from the beginning of the neuroscience up to 21’s Century cutting edge technology.  The first concept is latency.  Think of a stimulus S, a period of time L, and a response R.  The period of time between the stimulus and the response is the latency.

    During an examination when one elect’s a deep tendon reflex, the time between your stimulus of the stretch and vibratory receptors and the elicited response is the latency.  Thus when you elicit a deep tendon reflex, you have reproduced a “late latency response.”

    In 1918, Dr. Hoffman conceived that if he could electrically reproduce a deep tendon-like reflex, he could develop tremendous insight into the afferent sensory receptors, the afferent sensory nerve into the posterior horn of the associated spinal level of the central nervous system, the integrity of the internuncial neurons connecting the posterior horn and the anterior horn, the integrity of the anterior horn motor cells and the lower motor neuron integrity, as well as associated somatic structures.  Dr. Hoffman then presented a submaximal stimulus delivered to the tibial nerve in the poplitial fossa and evoked a late response in the soleus muscle, with a latency similar to that of the ankle jerk.  Dr. Hoffman correctly concluded that on the basis of his observations, both of these responses represent activity of the same stretch reflex arc.  Thus, Dr. Hoffman was able to test the functional integrity of the ankle jerk reflex arc and its associated neurosomatic counterparts.

    Because of the excitement Dr. Hoffman’s discovery made, Magladery and McDougal reproduced Dr. Hoffman’s test, and elicited a later latency response similar to the ankle jerk.  They named the test the “H-reflex” after Dr. Hoffman’s conclusions.  This is the only response test that will have the word “reflex” in the name  to honor Dr. Hoffman’s complicated conclusions, correlation’s and electrically induced neurosomatic ankle response simulating the neurophysiological ankle-jerk deep tendon reflex.  However, these researchers discovered that even though the large amplitude H-reflex in the soleus easily can be evoked by submaximal stimulation of the tibial nerve, no such response can normally be evoked by stimulation of other skeletal muscles.  These neuroscientists believed that if they could stimulate other spinal levels of the cord they could then test the integrity of other deep tendon reflexes.  They went on to demonstrate that a late response evoked by stronger stimulation and with different physiological features, but with similar latency to the H-reflex, was recordable from the muscles of the hands and feet and subsequently named this test the “F-response”.2   

    Originally the response was thought to be due to a polysynaptic reflex.  However, it is now generally believed that centrifugal discharge from individual motoneurons arise from antidromic volleys in their axons.  Thus the F-response may be generated by stimulation of skeletal muscles in the hands and feet, and assists in testing the integrity of the other deep tendon reflex pathways, as well as peripheral nerves such as the median nerve or ulnar nerve.  The test is indicated to check for peripheral neuropathies and root irritations, as well as thoracic outlet syndromes.  When considering thoracic outlet syndromes, it would be prudent to use hyperabduction tests as well as other orthopedic tests; however, never rely solely on the orthopedic tests as positive proof of thoracic outlet syndrome.  For example, if a female patient suffers from signs and symptoms that would normally be associated with a thoracic outlet syndrome, you may ask her, “When you place objects on an overhead cupboard or upon a shelf, do you feel a pain in your arms and neck that diminishes as you return your arms to the neutral position?” or “when you have to reach up to place objects on the shelf above the closet, or in over head bins on airplanes do you feel these symptoms?”  This will form an excellent history for ordering an F-response.  If the F-response report of findings indicates a neurovascular compression syndrome, then you have validating evidence to support your physical, orthopedic and neurological examination findings, ranging from a possible decrease in the radial pulse in the position that elicits the symptoms or a bruit in the supraclavicular area as well as a cervical rib or osseous abnormality of the clavicle or first rib as observed during your radiological examination.  Obviously, often the patient may not present with the classic signs and symptoms of thoracic outlet syndrome and your Neurodiagnostic test findings will be of great assistance in these circumstances.

   There are some important “key” differences both electrically and physiologically between the H-reflex and the F-response.  The most important differences follow:1,2,3 

1.       The H-reflex is primarily a monosynaptic reflex evoked by a direct electrical stimulation of large primary muscle afferent (1a) fibers in the mixed tibial nerve.  

          However, it differs from the tendon jerk, in which       stretching or vibration of muscle spindles results in       activation of group 1a afferent fibers, as opposed       to the H-reflex where the afferent sensory              receptors are electrical bypasses.

2.       The F-response is not a reflex. The afferent and efferent arcs of this response consist of the same alpha motor axon.

3.       F-responses appear following a stronger stimulus than that which would be required for an H-reflex.

4.       Fluctuations in the latency of a single motor unit (SMU) activated in an F-response are less than in an H-reflex.         

5.       Contrasting with the H-reflex, which is routinely only recorded in soleus-gastrocnemius muscles in adults, F-responses can be recorded from almost every skeletal muscle in the adult.

   Primarily the H-reflex and the F-response studies are used to evaluate function in the peripheral nervous system,.  It has been suggested that these two tests be reserved for evaluation of condition only in large diameter nerve fibers.  The H-reflex in the adult is generally recorded only from a few muscles in the resting state; although usually only the soleus.  However, evaluation of the F-responses perhaps will represent a more versatile method of studying motor nerve conduction because they can be recorded from most skeletal muscles.  Moreover, F-responses are evoked whenever tests for motor conduction velocity are performed.2,3 

    H-reflexes and F-response abnormalities have been noted in-patients with neuropathies of the primary “axonal” type, as well as of the segmental demyelinating type.  Whenever there are disorders that specifically affect large sensory fibbers, such as pansensory neuropathy or Friedreich’s ataxia, H-reflexes and tendon jerks are absent but F-response latencies remain normal.  For example, a patient’s history reveals that the patient had slipped down some stairs while at work, indicating a worker’s compensation injury.  During your neurological examination, you note bilaterally silent ankle jerks.  First you could request an H-reflex study.  If the H-reflex study comes back as bilaterally silent or diminished, then you would request that an F-response study be performed.  If the F-response come back as a normal late response, then you would know that this patient has some pansensory neuropathy such as a uremic neuropathy, an alcoholic neuropathy, a diabetic neuropathy (which is a metabolic neuropathy) or perhaps a Friedreich’s ataxia.  Further, if you were performing the tests, you would be strapped into waiting and would have performed them as addressed above.  In any event, perhaps this patient’s leg gave out due to their underlying pansensory neuropathy, which in turn caused the patient to fall down the steps, which may preclude this person from coverage under the worker’s compensation amalgam or cause apportionment to be a factor dependent upon additional situational criteria.

    Patients with neuropathies in which the primary pathology appears to be segmental demyelination such as Guillain-Barr’e syndrome, prolongation of late response latencies may be the first quantifiable objective sign.2,3  Unlike conventional nerve conduction studies, which do not correlate will with clinical findings, presence or absence of conduction block documented by a later response study does correlate well with the patient’s clinical status, such as weakness in appropriate limbs.1,2,3  

   F-response studies or late response studies have been demonstrated not only  in the evaluation of patients with peripheral neuropathies but also in the assessment of those with entrapment neuropathies and root compression syndromes.

   Carpal Tunnel syndrome and Ulnar Entrapment Neuropathy as well as Thoracic Outlet Syndrome have demonstrated the appropriate abnormalities of late responses.1,2,3   Earlier we discussed using the F-response against the H-reflex to locate pansensory neuropathies.  The exception to this is in patients with peripheral neuropathies affecting small diameter fibers such as an amyloid neuropathy, where the ankle jerk may be absent because of the involvement of the fusimotor fibers, whereas H-reflexes will be preserved because they bypass muscle spindles.

   Most neuroscientists emphasize that patients with root compression syndromes are candidates for H-reflexes and F-responses, as these tests are extremely useful in localizing the lesions in the patients’ lumbosacral and cervical roots.

    Graphically, the H-reflex excitability curves following double stimulation of the tibial nerve in the poplitial fossa have been examined in various neurological disorders.  These curves are often altered in-patients with CNS lesions, producing spasticity, rigidity or both.  Finally, F-response studies can also provide a measure of the central excitatory state of spinal motoneurons. 1   Patients with acute CNS lesions, F-responses occur with decreased persistence or amplitude on the clinically involved side.  These F-response abnormalities correlate clinically with the severity of the weakness, decreased tone and decreased deep tendon reflexes.  In-patients with chronic spasticity, amplitude and persistence of F-responses are increased.

    Of course there are many more substantiated and experimental uses for the F-responses and the H-reflexes, however the intent of this article is to familiarize Agents, Investigators, Psychologists, and Medical Examiners who are not neuroscientists on when an H-reflex study or F-response study was medically necessary or should be requested for an expedient diagnosis. 

    In Part Two this examiner will present Needle-electromyography, Nerve Conduction Velocity studies or “SNAP’ studies. 


© & TM 1998 American Academy for Justice Through Science. All rights reserved.

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