eNewsletter

Benedict’s Maritime Bulletin
Vol. 5, No. 3/4  Third/Fourth Quarter 2007

Evaluation of Injuries of the Nervous System

April 16, 2008

By Ezekiel Fink, MD

Editor's Note:  Many thanks to Dr. Fink for informing us on a subject that increasingly is being dealt with by the Courts. See the LeJeune case reported in the Recent Cases section at p. 245. This is also timely because much medical attention is being directed to the combination of post traumatic stress disorder in conjunction with other neural injuries, all the way from concussion to significant traumatic brain injury — this latter mostly in Iraq veterans. This article should help you prepare for your next deposition that involves head and nerve injuries.

The nervous system is the most intricate system in the body.  A complex array of nerve cells make up communicating pathways in the central and peripheral nervous systems that impact the function of all organ systems in the body.  The nervous system directly controls multiple essential functions, including cognition, movement, and sensation, and modulates the interaction between an organism and its environment.  A variety of conditions affect the nervous system, some of which may result in impairment.  Properly diagnosing injuries to the nervous system is often difficult, as multiple disease processes can have similar features while a single diagnosis can carry a spectrum of symptoms that range from mild to debilitating.

The neurological examination is an essential component of evaluating an injury to the nervous system.  In certain instances, such as with the subjective complaint of pain, a clinical examination may not be adequate to quantitate the degree of injury.  It is therefore essential for the evaluating physician to have a comprehensive understanding of the qualitative methods of assessing neurological impairment.

Diagnostic tests can be an effective way of evaluating and diagnosing neurological injury. These tests are best utilized by the practitioner who understands the limitations of the technique while seeking to objectively identify a clinical diagnosis.  Diagnostic tests are also effective in documenting loss of function, confirming subjective symptoms, and excluding other causes of disease.  The predictive value of a diagnostic test is maximized when it is selectively chosen after a thorough history and physical examination identifies a patient with a high likelihood of having a specific disease process.  Conversely, routine and indiscriminate screening of individuals with multiple specialized tests can yield a high number of positive results in the absence of disease ("false positive"), regardless of the quality of the tests ordered.

Diagnostic testing of the nervous system has several challenges.  For many of the disease processes being evaluated, concrete values determining the presence or absence of disease are vague.  This is because a limited number of diagnostic tests are applied to multiple nervous system diseases and some of the disease processes have not been adequately studied to date.  Also, in developing standards for the tests evaluating a disease process, it is essential to identify who actually has the disease.  In many diseases of the nervous system, the diagnostic test utilized is the only test available with no secondary test to confirm whether the disease process actually is present(1).  There is also an unclear correlation between test results and degree of impairment with many of these diagnostic tests(2).  Another limitation is that multiple neurodiagnostic tests are able to determine the location of the deficit in the nervous system but not the cause of injury. The importance of a comprehensive neurological history and examination by a knowledgeable practitioner therefore is crucial because, in many instances, it is the secondary test identifying whether a disease process is present.  That being said, there are several studies demonstrating significant variability between the practitioner's history and neurological exarninations(3).

The ensuing discussion will focus on the tests ordered to identify disease among the anatomic divisions of the nervous system.

Central Nervous System Injury

Damage to the central nervous system can result in significant and widespread loss of function.  Depending on the area and extent of damage, injury may result in difficulties with mentation and speech, loss of strength and sensation, difficulty with gait and balance, chronic pain, loss of bowel and bladder control, and loss of sexual function, among other impairments.

Diseases of the central nervous system are diagnosed on the basis of clinical findings and the results of diagnostic tests.  There are several tests that are commonly utilized in this setting.  Magnetic resonance imaging (MRI), computed tomography (CT) as well as other radiologic studies are commonly used to identify anatomical lesions in the brain and spinal cord.  These tools, however, must be placed in clinical context. In some instances, the clinical spectrum of deficits can correlate with the location of the imaged lesion.  For instance, a patient with right sided weakness and difficulty producing speech would be displaying the classic symptoms of a stroke affecting the left frontal lobe of the brain and an imaging study that did not find this anatomical deficit would raise serious questions. Conversely, certain imaging findings are of unclear value in determining the scope of neurological deficit.  Advanced pathology of the spine, including significant disc herniations may be present in an asymptomatic individual.

Electroencephalography (EEG) is a study of the electrical current within the brain and is a useful test of brain function.  It is the test of choice for diagnosing epilepsy and has value in confirming the diagnosis of several neurological diseases including delirium and coma.  In the specific case of epilepsy, there are specifically defined diagnostic abnormal findings that are rarely present in healthy individuals.  An EEG can be useful in differentiating between cases of seizures associated with epilepsy and non-epileptic seizures.  A single EEG can diagnose epilepsy in approximately half of patients with a known history of seizures. There are a number of findings that can be easily misinterpreted as pathologic and others that are of unclear diagnostic value.  This can lead to misdiagnosis of epilepsy(4).  Recently, the use of computer analysis of EEG results has become more prevalent.  While the goal of these techniques was to increase the accuracy of results, there is controversy over whether these goals were realized(5) or whether this modality remains investigational(6).

Evoked potentials comprise a set of neurodiagnostic tests that evaluate the sensory pathways in the central nervous system.  Testing of visual (VEP) and auditory pathways (BAEP) can be helpful in determining whether a deficit is present in the brain but the utility is limited; abnormal tests have not been shown to correlate with level of impairment and can be present in clinically asymptomatic individuals.  Somatosensory evoked potentials (SEP) are commonly used to evaluate injury at the level of the spinal cord but it only tests a limited aspect of the sensory system.  It therefore may be normal in instances when damage to other areas of the central nervous system is present.  SEP is best used to support diagnoses suggested by anatomical derangements seen on imaging.  Motor evoked potentials test the central nervous systems ability to transmit a message from the brain to muscle groups in the periphery.  There are studies that suggest that SEP and motor evoked potentials are more effective when used together to evaluate injury to the central nervous system(7).

Neuropsychoiogical and cognitive testing can identify psychological, cognitive, and psychiatric disturbances in patients without abnormalities on neurological exam or diagnostic testing.  There are also instances when significant injuries may result in no identifiable deficits in cognition or behavior.  The limitation of this testing modality is that, in many instances, it is dependent on the subject's mood, concentration, and alertness(8).  It is therefore not surprising that these tests often result in significant differences in medical opinions when determining the extent of cognitive impairment(9).  To add to the confusion, there is significant overlap between psychiatric disorders and cognitive dysfunction, so abnormal results may be due to different factors. Nonetheless, these tests remain essential in identifying long term morbidity in some central nervous system injuries, such as traumatic brain injury(10).

Peripheral Nervous System Injury

Injuries to peripheral nerves have diverse causes including industrial chemical exposure and prolonged alcohol use.  Nerves may also be injured as a result of repetitive minor trauma as often occurs in the workplace such as in the case of carpal tunnel syndrome.  Nerve injury can also occur with direct penetrating trauma or compression.  In the back, nerves can be compressed as they exit the spinal column in a disease process termed radiculopathy.  The comprehensive neurological examination remains a key diagnostic tool for suspected peripheral nerve injuries.

Radiologic studies are especially important in diagnosing radiculopathy.  It is important to note that evidence of a compressed nerve on imaging often exists in asymptomatic individuals(11) and conversely, symptoms of radiculopathy may be present in the absence of radiologic findings.  Imaging may also be effective for identifying the cause or confirming the diagnosis of compressive neuropathies(12).

Electrophysiologic testing (nerve conduction studies, reflex studies, and needle electromyography) remains the most important test for suspected injuries in the peripheral nervous system.  Changes occur in nerve's ability to conduct an electrical impulse even after subtle injuries.  These changes are reflected in motor and sensory nerves and even muscle function.  Of all the tests that comprise electrophysiologic testing, the most sensitive measure of focal nerve compression is often changes in the sensory nerve conduction values.  Needle electromyography can assess nerve dysfunction through testing of muscle function.  This is particularly important in the evaluation of radiculopathy (when nerve conduction studies are often normal, even in the presence of injury).

There are several tests that have, as of yet, not entered widespread use despite having diagnostic utility.  For example, neurodiagnostic skin biopsies are widely used in research settings for the diagnosis of a subset of painful nerve diseases (i.e. small fiber neuropathies). This technology has limitations in that it is largely an anatomical and not functional test of the peripheral nervous system.

Selected Disease Processes

While there are a number of diagnostic tests for injury of the peripheral and central nervous system, the specific tests ordered must be tailored to each specific injury.  Below are two examples of common injuries and the recommended evaluations.

Complex Regional Pain Syndrome:

Previously termed causalgia and reflex sympathetic dystrophy, this perplexing disease entity is a painful nerve injury syndrome in which the area of injury is out of proportion to the initiating noxious event.  This disease primarily affects the extremities and is characterized by severe pain, swelling, and changes in blood flow to the affected area.  The diagnosis of this disease is based on the clinical examination, although diagnostic testing can support the diagnosis.  Radiologic testing and electrophysiologic studies may play a role in supporting the diagnosis.  Other specialized tests that may be useful in this clinical context include bone scintigraphy, thermography, autonomic testing, and diagnostic sympathetic blockade.

Low Back Pain:

Low back pain is one of the most common causes of pain and disability in the industrialized world.  The cause of the pain is thought to often be due to several mechanisms including nerve injury, degenerating/herniated discs, arthritis, muscle spasm and narrowing of the spinal canal.  Diagnostic studies to evaluate low back pain include radiologic studies, electrophysiologic studies, and rarely, serological studies to exclude systemic causes of disease (e.g. infection, inflammation, malignancy).  Neuropsychological assessment may be indicated in some cases.

Conclusion

The evaluation of a patient with an injury to the nervous system is complex.  The importance of a comprehensive neurological history and examination by a knowledgeable practitioner is essential.  Diagnostic tests can be an effective way of evaluating and diagnosing neurological injury.  These tests are best utilized by the practitioner who understands the limitations of the technique while seeking to objectively identify a clinical diagnosis.
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Ezekiel Fink, M.D. is a Harvard-Trained Neurologist and Pain Medicine Specialist.  He is currently a Clinical Instructor in the Departments of Neurology and Physical Medicine and Rehabilitation at UCLA.  He is also director of the Neuro-Pain Service at UCLA.