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Sensations are of two types: primary and cortical. Primary sensations are touch, pain, temperature, position, passive movements, and vibrations. Cortical sensations are localization, two-point discrimination, stereognosis, and graphesthesia.
Sensations are carried by two afferent pathways: the spinothalamic tracts and the dorsal column tracts. Spinothalamic tracts detect pain, temperature, and crude touch. They travel from the periphery, enter the spinal cord and then cross to the other side of the cord within one or two vertebral levels of their entry point.
They then continue up that side to the brain, terminating in the cerebral hemisphere on the opposite side of the body from where they began.
Dorsal Column tracts nerves detect proprioception, vibratory sensation, and the light touch. They travel from the periphery, entering the spinal cord and then moving up to the base of the brain on the same side of the cord as where they started.
Upon reaching the brain stem they cross to the opposite side, terminating in the cerebral hemisphere on the opposite side of the body from where they began. When testing the primary sensations, the following steps should be undertaken:
Complete loss of sensation is called anesthesia, impaired sensation perception is hypoesthesia, and heightened sensory perception is dysesthesia. The different kinds of sensations can be tested in the following ways.
Touch the skin with a small point of cotton wool or a piece of paper. Ask the patient to close their eyes, and raise their finger or say yes when they feel the object. This should be repeated on the other side. The patient should also mention if the sensations felt on both sides are equal or not.
Use a disposable pin and touch the patient with both ends of the pin at a presumably normal site to make them experience what you define as “sharp” and “blunt.”
Then ask them to close their eyes, and touch them with the sharp and blunt ends of the pin in a random sequence. If the patient can feel but cannot distinguish between the two sensations that means their sense of touch is intact, but the sense of pain is lost.
Take two test tubes containing cold and hot water and apply them to a healthy part so that the patient is aware of the sensation. Then apply these tubes in a random sequence to the skin of the part to be tested and ask the patient to indicate whether it is hot or cold.
Squeeze the patient’s muscles and tendons and ask them to indicate when the pressure becomes painful.
This will require the use of a tuning fork. Ask the patient to close their eyes. Strike the tuning fork against a rubber pad and place the tuning fork on the dorsum of the terminal phalanx of the toe.
Alternate between placing a vibrating and a non-vibrating tuning fork on the phalanx of the toe. If the sense of vibration is impaired, test proximal parts by placing the tuning fork on the lateral malleolus, shin, tibial tuberosity, iliac crest, and costal margins.
In the upper limbs, the tuning fork is placed on the terminal phalanx, wrist, and elbow. You can also compare the patient’s response to your own by checking if you can still feel the vibrations after the patient has said they have stopped feeling the vibrations.
Before checking for cortical sensations, ensure that the patient is mentally alert, has normal speech, and that their primary sensations are intact.
Touch a part of the patient’s body with their eyes closed. Then ask the patient to open their eyes and place their finger on the part you had touched.
A specially calibrated divider is used for this purpose. Open the divider and touch the patient with one or both points. Ask the patient how many sites they have been touched.
Change the width to determine the minimum distance at which the patient can identify the two points as separate stimuli. Normally it varies from 2 mm (on the pulp of fingers) to 100 mm (in the legs). If this distance is more than 5 mm on the pulp of fingers, it is abnormal.
Ask the patient to close their eyes. Give the patient a suitable object like a coin, key, or pencil and ask the patient to identify it after carefully palpating the object.
A normal person will be able to appreciate the size, shape, and texture of the object, which will help in the identification of the object.
Draw a number on the patient’s palm with a blunt object while their eyes are closed. A normal person will be able to recognize it.
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