Welcome back to the continuing series exploring functional neurology. In the next several articles I will be introducing some examination techniques and how they can be used to evaluate the level of function in various areas of the neuraxis. Let’s get started!
Steven Rose, the award winning neuroscientist from Cambridge, has stated that all scientific knowledge about the world comes from two types of study: the search for underlying regularities in seemingly dissimilar phenomena; and the analysis of the causes of variation or, in other words, looking for the small differences in seemingly similar phenomenon. We apply these axioms in the functional neurological examination.
The neurological examination is traditionally taught using a disease or ablative lesion orientated model. While this approach may help to detect the presence of both serious and benign disorders, it is less helpful for the practitioner who wishes to investigate and estimate the physiological functional integrity of the nervous system. A more functional approach to the neurological examination heightens the examiner’s sensitivity to physiological aberrations that are responsible for the vast majority of neurological symptoms. At the same time, a practitioner using this approach is more likely to detect subtle signs of pathology.
The practitioner who intends to utilize the functional approach of examination must be concerned with the identification of ablative lesions and the presence of disease processes, but must also attempt to identify any physiological lesions that are manifesting themselves as physical symptoms. For example, a common presentation in my office is an athlete with a shoulder or wrist dislocation or sprain for which they have been strapped or casted for a period of time. Now the cast or strapping has been removed and they have started a rehabilitation program. They present to me with headaches, lack of concentration, balance problems, incoordination and decreased drive to return to their previous activity level. The decreased movement in the joints involved during the casting period has resulted in a functional change in some of the neuronal circuits in their spinal cord, brainstem, cerebellum and cortex that receive input from the area. The rehabilitation program did not allow for this but focused on the joints and soft tissues, resulting in over stimulation to areas of the neuraxis that had down regulated their activity capability during the period of inactivity. This over stimulation has resulted in transneural degeneration and injury to certain neuronal circuits that have resulted in the symptoms that the patient has presented with. In these cases, a thorough neurological examination must be performed to both identify any physiological lesions and to rule out any ablative lesions that may also present with similar symptoms.
So how can we evaluate this patient to determine if he has a physiological dysfunction and, if so, what areas are involved? We need to have a system that will allow us to measure small changes in the state of function at various levels of his neuraxis.
The Five Parameters of Effector Response Are Important Clues in Gauging the Central Integrative State (CIS) of Upstream Neuron Systems
The response of an effector (e.g., muscle) to a stimulus or command is largely dependent on the central integrative state of the presynaptic neuronal pool projecting to the motor neuron of the effector. Therefore, the CIS of a neuronal pool can be predicted or estimated by observing the characteristics of the motor response of the downstream motor neuron to a unit stimulus. The parameters of the effector response observed can be summarized under the following observational findings:
1. Latency and velocity of the response,
2. Amplitude of the response,
3. Smoothness of movement of the response,
4. Fatigability of the response,
5. Direction of the response.
All of the responses observed during the functional examinations performed on a patient should be evaluated with the above parameters in mind. It is also important to visualize the pathways that are actively involved in producing the actions that you are examining. This allows the practitioner the advantage of performing additional or more detailed tests directed at the same pathways throughout the examination, should disparities in the patient’s responses become apparent.
Latency and Velocity of a Response
The latency refers to the time between the presentation of a stimulus and the motor, sensory, autonomic or behavioural response of the patient. This provides information concerning conduction time along nerve axons and spatial and temporal summation occurring in the neurons involved in the functional action chain of the response. The velocity of the response is another window of spatial and temporal summation and conduction time.
The time to summation (TTS) and time to peak summation (TTSp) are abbreviations that describe, respectively, the latency and average velocity of effector responses. The pupillary action observed in response to a light stimulus offers a good illustration of these concepts. Under normal conditions, the pupils will respond with a relatively equal TTS and TTSp in both eyes when stimulated with an equal light stimulus. However, in the situation were the central integrative state of the neurons in the right Edinger-Westphal nucleus or mesencephalic reticular formation is further away from threshold, the TTS of the right eye would be expected to be increased from that of the left. The same result may be expected when measuring the velocity of the response, or an increased time to maximal pupil constriction (increased TTSp). The same result, that is increased TTS and TTSp in the right eye, may be found with an afferent pupil defect such as would occur if the right eye end organ were impeded by a photoreceptor or axonal conduction deficit, such as in retinal or optic nerve dysfunction. Thus, the need for a complete fundoscopic and visual acuity exam when unequal pupil responses are present.
Amplitude of a Response
The amplitude of the response refers to the maximum change in the parameters being assessed. This can be a useful indicator of the relative frequency of firing in a neuronal pool—for example, the degree of excursion of the eye during the smooth phase of pursuit movement during opticokinetic testing of eye movements or, in keeping with our first example, the maximum change in pupil size when testing the pupil light reflex.
Smoothness of a Response
Smoothness of any movement is dependent on complex interactions between multiple neuronal pools. An example is the smoothness of visual tracking in the horizontal plane. This requires complex interactions between the cerebellum, vestibular system, neural integrator, and occipital, parietal and frontal lobes. A poor central integrative state in any of these areas may affect the quality of visual tracking in one or more directions. Specific features of the visual tracking deficit may alert to greater involvement of one area over another. Uncoordinated or jerky movements are referred to as dysmetric in nature.
Fatigability of a Response
This refers to the ability to maintain a response during continued or repeated presentation of a stimulus. A progressive reduction in the amplitude and speed of a tendon jerk reflex over several repeated taps with the reflex hammer is a good example of this response. For instance, most normal systems should be able to sustain the reflex response for five to six taps before fatigue sets in. Poor maintenance of a response reflects increased fatigability. The fatigability coefficient is an arbitrary descriptor of the fatigability of a neuronal pool.
Direction of a Response
The direction of response elicited is compared to the expected normal response, to provide further information about the integrity of a neuronal pool. For example, the direction of change of pupil size when shining a light in the eye, the direction of nystagmus during caloric irrigation of the ear, and the direction of change of skin temperature in response to a cognitive task or vestibular stimulation, all have an expected normal response direction. If the direction of response is different to the expected outcome, this may indicate the presence of pathology, fatigue or plastic alterations in neural circuitry.
Try evaluating these five response characteristics in the different tests you include in your physical examinations over the next month.
In the next article we will look at specific examination techniques for various levels of the neuraxis.
Randy Beck, B.Sc., D.C., Ph.D., is a graduate of Canadian Memorial Chiropractic College. He has completed postgraduate studies in Psychology, Immunology and Neurology. He is presently involved in a number of international research projects and is co-authoring a textbook on Functional Neurology. He was formerly the Dean of Chiropractic and Basic Sciences and Director of Research at the New Zealand College of Chiropractic. Presently, he practices Chiropractic Functional Neurology at the Papakura Neurology Center and The Maungakiekie Clinic located in Auckland, New Zealand.
References:
1. Beck, R. W. Functional Neurology for Manual Therapists. Elsevier, In press. 2007.
2. Rose, S. The making of memory: From molecules to mind. ISBN 0099449986, Random House, 2003.