Changes in sensation are some of the most common neurological symptoms in hospitalized patients. Patterns of weakness is covered in Chapter 28. In this chapter, we will survey an approach to evaluation of sensory changes. Most of the causes of isolated changes of somatosensory sensation are relatively benign or do not require hospital admission. In some situations, an etiology for such findings confined to sensation may not even be found. Conversely, sensory deficits that are associated with other neurological findings that point to specific neurological regions are clearly based on organic pathology, deserve attention, and help in localizing the lesion.
Clinical examination of the sensory system may be the most difficult part of the neurological examination because of the “psychophysical” nature of the tests employed; the stimuli applied are not standardized or quantified, and the findings are elicited by the nature of a judgmental response from the patient regarding the stimulus. This results in significant variability both within and among examiners in terms of the precise stimulation given, as also variability among even normal subjects provided the same type of stimulus. It is best to perform the sensory examination late in the neurological evaluation when one already has some idea as to the kind of sensory abnormality one may expect in the context of the patient’s presentation.
CASE 40-1
A 45-year-old obese woman presents with tingling and numbness of the left anterolateral thigh. The patient has researched her symptoms on the internet and is now worried that she may have multiple sclerosis. You are asked to evaluate the patient in the emergency department (ED). One of the ED residents asks for advice regarding how to perform the sensory examination.
Several factors determine the detection of a stimulus:
The strength of the stimulus: Stronger stimuli are easier to detect.
The size of the area where the stimulus is applied: Applying the stimulus to larger skin areas makes it easier to detect the stimulus due to spatial summation.
Duration of the stimulus: Applying the stimulation for a longer time makes it easier to detect the stimulus (temporal summation).
At the outset, the best strategy is to use the lightest stimulus strength, applied to the smallest possible area, and for the least possible time to be able to detect the mildest of sensory loss. With such a strategy, if several stimuli are applied to an area, subjects with lesions of the somatosensory system consistently miss a larger number of stimuli as compared to persons with normal sensation.
If a deficit is demonstrated then the stimulus parameters are strengthened (increased strength, increased area, and increased time). Most persons with organic pathology of the somatosensory system start perceiving the sensation unless there is complete discontinuity of the pathways. In contrast, persons with “psychogenic” sensory loss tend to report not feeling anything even when stimulation is strengthened.
If no loss of touch is expected, it probably suffices to compare the 2 sides of the body and face over the distribution of CN V and then to compare distal and proximal segments of limbs.
If a particular region of touch abnormality is expected, additional time should be spent in documenting the geographic distribution of such abnormality by concentrating on the region of expected abnormality.
For a diagnosis of motor only disease (eg, motor neuron disease) when one expects no sensory loss, a thorough sensory examination in the distribution of nerves already showing motor deficits to document the normality of sensation in that distribution is indicated. Thus, if a patient has very atrophic hand muscles, a normal sensory examination in the median and ulnar distribution raises the suspicion of such a disease.
There are 2 kinds of touch subserved by 2 separate sensory systems. Discriminating touch sensation is transmitted by large myelinated fibers in the nerves and travels primarily through the posterior columns. Crude touch is transmitted by small myelinated and unmyelinated fibers that travel through the anterolateral system.1
The authors use a wisp of cotton or a fine napkin/tissue for this. We touch a normal part of the face or body over a very small area and for a brief time with the patient’s eyes open to teach the patient the kind of stimulus we are applying; then we tell the patient to close eyes and to say “yes” every time he/she feels us touch any part of the body in a similar fashion. The time between stimuli should be varied. The objective is to determine if the patient misses the stimulus over a particular area of the body either all the time or more consistently than other regions.
Testing sensation using a cotton wisp tests for discriminating touch. To test for the integrity of the anterolateral system or small nerve fibers it is more efficacious to test for pain and temperature sensation.
Pain: We use a sharp (previously unused) pin. The patient is instructed on the feel of both the sharp and blunt ends of the pin applied briefly a single time with a modest pressure and asked to discriminate between the 2. Now with eyes closed, the patient’s ability to discriminate between the 2 ends in selected regions of the body is determined. The same principles used for touch can be used in selecting the regions of interest. The idea would be to detect regions in which the patient either completely or most of the time misses the stimuli in a consistent fashion.
Temperature: We use a tuning fork. We warm one of the prongs under warm water and leave the other cool. It is best to achieve a minimal difference in temperature between the 2 prongs. With eyes closed, the patient is asked to tell if stimulus one or two is “warmer” after application of the sides of both prongs to the selected area of the skin. One can continue to be certain that the two sides can be discriminated by self-application of the prongs to the examiner skin (assuming the examiner is normal). Over a short period of time, in fact, the temperature difference becomes smaller and smaller, making the stimulation even more “sensitive,” but at some point in time even the normal subject is not able to tell the difference. Since pain and thermal sense use almost the same pathways, some authors recommend not doing both. Individual judgment has to be used in this regard.
What are some other sensory modalities mediated by large myelinated fibers, and how does one test for them?
Vibration: Use a 128-Hz tuning fork. One technique the authors employ is to teach the patient the difference between a very light vibration and no vibration in an area of the body not expected to be abnormal with eyes open. Then the patient is asked to close eyes and the examiner randomly applies either a vibrating or nonvibrating tuning fork to selected regions of the body such as the toes, tibial tuberosity, ilial tuberosity, finger tips, and elbows to examine the extent of vibration sense impairment. Again, missing the right stimulus state (yes or no vibration) consistently or more times than can be expected would constitute an abnormality. When the tuning fork is struck, the examiner needs to employ methods to mask any auditory clue. The extent of regions examined depends on the expected type of abnormality. If a cord lesion is suspected, one can “go up” the body even over the spine to determine if there is a “level.” If a hemispheric lesion is of concern, then comparison of the 2 sides needs to be the focus.
Proprioception: This is usually done over the terminal digits of the toes and fingers but can be employed over larger joints such as ankle and wrist. The patient can be taught that the examiner will move the distal joints minimally either up or down by grasping the sides of the digits and minimizing clues such as brushing the skin of the other digits or applying pressure over the nails. Then with eyes closed, the digits are moved by the smallest excursion possible either up or down and the ability of the patient to detect the motion is examined; a consistent inability do this will be an abnormality. It can be difficult in some persons, and for each stimulus, the patient has a 50% chance of being right with random answers. One can ask if the patient can actually feel the right direction of movement. The 4th digit of the foot is thought to be especially sensitive to this test. Both vibration and kinesthetic sense are served by large myelinated fibers in peripheral nerves and by posterior columns.
CASE 40-1 (continued)
You examine the patient. The examination shows an area of decreased touch and pain sensation on the anterolateral aspect of the left thigh. Muscle strength in lower limbs is normal as are muscle stretch reflexes.
The presentation is typical for meralgia paresthetica. This results from compression of the lateral femoral cutaneous nerve of the thigh usually under the ilio-inguinal ligament. This is purely a cutaneous nerve and does not cause associated motor or reflex changes. There are unpleasant paresthesia and dysesthesia on the anterolateral thigh.
It is often associated with weight gain, pregnancy, tight belts, and metabolic disorders such as diabetes mellitus. Occasionally, a similar syndrome can occur with injury to the nerve during pelvic surgery.
Investigations such as spine imaging and electromyography (EMG) exclude other types of nerve lesions such as radiculopathy or femoral neuropathy, but with typical cases, they are not needed. Recording a nerve potential from the lateral cutaneous nerve of the thigh may be useful but is technically difficult.
CASE 40-2
A young woman complains of numbness on the lateral aspect of the foot for 3 weeks. She had gone skiing before this happened. She complains of numbness on the lateral aspect of the right leg and dorsum of the right foot. The examination shows no obvious change in strength.
L5 and S1 radiculopathy—Usually caused by arthritic or disc disease at the corresponding level.
Saphenous neuropathy—This is much rarer and can be seen postsurgically.
On the surface, this case seems similar to the previous vignette but differs in that unlike lesions of lateral femoral cutaneous nerve, most other isolated sensory changes of the lower limbs are rare. Saphenous neuropathy can be seen after surgical procedures in the distal leg. It is also classically seen in the context of wearing tight ski boots. The more common cause of numbness in that distribution would be S1 radiculopathy. Radicular lesions usually cause pain and may result in numbness in the appropriate distribution, but motor deficits are usually mild or not present. As such in this case spine imaging or electrical studies may be warranted.
Figure 40-1 illustrates the sensory nerve supply of the skin.
CASE 40-3
A 68-year-old man seeks help for progressive nearly symmetrical dysesthesias, paresthesia, and numbness in both feet extending to the mid shin. You are asked to assess this patient.
What are the different kinds of positive and negative sensory symptoms seen in peripheral nerve lesions?
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
