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:
- Properly expose the area to be examined
- Explain to the patient what you are going to do and how they should respond.
- Apply the stimulus to a possible normal site so that the patient is aware of what the normal sensation feels like.
- When testing a sensation compare two sides and ask the patient whether the sensations are equal on both sides or not.
- To map the area of abnormal sensations, first examine the abnormal area and then move towards the normal area.
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, 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 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.
- Ask the patient to close their eyes. Place the patient’s limbs in a particular position, making sure that it doesn’t touch the body. Ask the patient to imitate it with the other limb. If the sense of position is intact, the patient will be able to imitate the position successfully.
- First show the patient up and down movements of the big toe. Then stabilize the proximal phalanx of the great toe by grasping it in between thumb and index finger of your left hand. Grasp the terminal phalanx of the great toe on its lateral sides between thumb and index finger of your right hand. It should not be in contact with other toes. Ask the patient to close their eyes. Move the terminal phalanx, gently and slowly, up and down in a random sequence so that patient cannot guess, and ask him to identify the direction of movement. Repeat the test on the contralateral side. In the upper limb, the terminal phalanx of the index finger is used for this purpose.
- Other indications that will alert you to loss of sense of position include the patient complaining of unsteadiness during darkness and deterioration during tests of coordination (described in motor system examination).
Sense of Vibration
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.
Sense of Localization
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.
Two Point Discrimination
A specially calibrated divider is used for this purpose. Open the divider and touch the patient with one or both points. Ask the patient at 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 able to recognize it.