官能試験

A complete sensory examination consists of testing primary sensory modalities as well as cortical sensory function. Primary sensory modalities include pain, temperature, light touch, vibration, and joint position sense, or proprioception. While cortical sensory testing examines the higher order aspects of sensation, like identifying an object only with the help of touch. The pattern of sensory loss detected during this exam can help in the diagnosis of conditions like peripheral neuropathy, radiculopathy or cortical lesions.

Here, we will first briefly review the two major sensory pathways, and discuss the peripheral sensory nerve distribution. Then, we’ll demonstrate the steps involved in testing primary modalities and cortical sensory function assessment.

Let’s begin by revisiting the anatomy of the sensory tracts. The two major sensory pathways are the posterior column-medial lemniscus pathway and the spinothalamic tract. These paths involve first order, second order and third order neurons. The information relayed between these neurons ultimately reaches the postcentral gyrus, also known as the primary somatosensory cortex, which is a prominent structure in the parietal lobe.

The posterior column-medial lemniscus pathway is responsible for sensations like vibration, conscious proprioception, and discriminative, fine touch. The first order afferent neurons of this pathway carry information from the mechanoreceptors and proprioceptors all the way up to the medulla oblongata. Here they synapse with the second order neurons, which decussate, or crossover, and travel to the thalamus. From there, the third order neurons carry the information to the postcentral gyrus.

The spinothalamic tract works in a similar fashion, and relays information related to pain, temperature and crude touch. The first order neurons of this tract carry information from receptors like the nociceptors and thermoceptors. However, these neurons synapse at the spinal level. The second order neurons decussate in the spinal cord itself and relay the information all the way to the thalamus. And from there the third order neurons ultimately convey the message to the somatosensory cortex.

After discussing the tracts, let’s briefly review the peripheral sensory nerve distribution, an understanding of which is necessary for interpreting the physical findings of a sensory exam. The peripheral sensory nerves arise from single or multiple spinal nerve roots. Each of these nerve roots provides sensory innervation to a specific region on the skin known as the dermatome creating a pattern known as the dermatome map. Since most of the peripheral sensory tests are focused on the upper and lower extremities, it is helpful to know the dermatome pattern of these regions in a bit more detail.

The C5 through T1 spinal nerve roots form a network called the brachial plexus, which sub-divides into peripheral nerves namely the musculocutaneous, axillary, radial, median, ulnar, medial antebrachial and medial brachial nerve. Together, they innervate and carry sensory information from different dermatomes of the volar and dorsal arm and hand. Knowledge of this map may be helpful in localizing sensory dysfunction in this region.

Similarly, roots T12 to S4 form the lumbosacral plexus, which gives rise to the peripheral nerves: lateral cutaneous, posterior cutaneous, obturator, femoral, common fibular and tibial nerve. These projections innervate different leg and foot areas — anteriorly and posteriorly. A mental picture of this map while conducting a sensory test can aid in interpretation of the physical exam findings.

Now that we have an understanding of the sensory pathways and dermatomes, we can move onto the assessment of primary sensory modalities. During a screening sensory examination, light touch, pain, and vibration are tested in the feet. One should expand the examination to other regions if the patient has a complaint referable to the nervous system, or if other components of the neurological examination are abnormal.

Begin by asking the patient if they have been experiencing any change in sensation throughout their body. The patient can describe and demarcate the sensory changes to aid in the evaluation. Examine light touch sensation by asking the patient to close their eyes and instructing them to tell you when they feel your touch. Using the tip of your finger, lightly touch the patient’s skin in different dermatomes.

Next, for pain testing, inform the patient that you will be touching their body with either the sharp or the dull end of a safety pin. Assure them that it will not hurt. Ask the patient to close their eyes again. Using the sharp and dull ends, test the sensation in both feet. Each time you touch, ask the patient to determine if the stimulus is “dull” or “sharp”. Then, using just the sharp side, continue up the legs to make sure that the sensation does not get sharper proximally. At any point if the patient reports an area of numbness or no sensation, begin to work outwards from the numb point till the patient says, “yes”, they feel normal pinprick sensation. Using a grease pencil, you can outline the area of numbness to determine if there is a dermatomal pattern of sensory loss, which may be seen with peripheral neuropathy.

Next, test the temperature sensation using a tuning fork as the cold stimulus. Touch the patient’s skin with the fork over their extremities in the same manner as the pain sensation test, and ask them what sensation do they feel. Compare between the sides and between the proximal and distal areas of the same extremity.

Subsequently, test for vibration using a low-pitched tuning fork of 128 Hz frequency. Strike the tines against the heel of your hand to produce a vibration, and place the stem on the patient’s big toe. Instruct the patient to tell you when they can no longer feel the vibration. Allow the vibration to fade, or to dampen it faster run your finger along the tines. As soon as the patient notifies, place the fork against your own thumb to see if you still feel the vibration. If they cannot feel the vibration in their toes at all, repeat the test by placing the tuning fork over the medial malleolus and, if not felt there, move over to the patella. Note the most distal location where vibration is felt by the patient, and compare the two sides. If there was decreased vibration appreciation found in the lower extremities, test if it can be appreciated in the fingers.

Finally, assess the proprioception or joint sense. Hold the patient’s large toe on the sides and demonstrate the test by moving it upward and downward. Then instruct the patient to close their eyes and ask them to correctly identify the direction of toe movement. In no specific order, move the toe up and down. Repeat the test on the other side. If the patient cannot correctly identify your movements, attempt to move their foot up and down around the ankle joint. Normally, people are able to identify even a few degrees of movement. If any indication of abnormality is present, test the position sense in the fingers at the distal interphalangeal joints. This concludes the testing of the primary sensory modalities.

Now let’s discuss a few commonly performed tests that examine cortical sensory functioning. A clinician should perform these only when there is a reason to suspect a brain disorder, as the findings can assist with lesion localization. This is not routinely done during a screening neurological exam.

The first test is called tactile localization. Instruct the patient to close their eyes, and ask them to localize where you have touched them. Initially, touch the side that you are concerned about — to confirm that sensation to light touch is intact. Then, simultaneously touch both sides and ask the patient to identify the number and location of places they felt the sensation. Repeat the same test on the legs. Extinction of the stimulus on one side may be a sign of a lesion in the contralateral parietal cortex.

Next, conduct the stereognosis test, which assesses the patient’s ability to specifically identify a common object, like a nickel or a quarter, just using their touch sensation. Request the patient to close their eyes and then identify the object in their hand. The patient may move it around to feel it, but they may not transfer the object from one hand to the other. Test the other hand in the same way with a different object. Patients should be able to differentiate coins, so it is not an acceptable answer to say “coin.” The patient should be able to correctly determine whether it’s a “nickel” or a “quarter.”

Lastly, conduct the graphesthesia test, which is another method for assessing cortical functioning. Ask the patient to close their eyes and extend their arm with palm facing up. With the tip of a pen, draw a number from zero to nine on the patient’s palm. Make sure that the number is facing the patient and not you. Ask them to identify the number. Repeat the test on the opposite hand. Inability to correctly identify numbers may be indicative of a lesion in the contralateral parietal cortex.

“This concludes general sensory testing, which is the most subjective portion of the neurological exam, and requires patient’s cooperation and full effort. There are other ways to examine the sensory system, including two-point discrimination, which tests the ability of a patient to differentiate one stimulus from two. Overall, the sensory examination demands vigilance on the part of the examiner to make sure the patient is providing accurate and honest answers. Be suspicious of sensory findings that do not fit anatomical patterns, or those that may not correlate with the more objective findings seen during other sections of the neurological examination. Any abnormal results of sensory testing need to be correlated with the results from other parts of the neuro exam to ensure and determine the pattern of abnormality.”

You have just watched a JoVE video describing the sensory examination. In this presentation, we revisited the anatomy of the major sensory pathways, and the dermatomes. Then, we discussed the steps for examining primary modalities as well as a few tests for cortical functioning assessment. As always, thanks for watching!