Neurological examination of the sensory system

Inspection

A general inspection should commence as soon as the patient enters the room. Observe the patient’s gaitas individuals with sensory ataxia may have an abnormal walking pattern. During the general exam, skin changes such as acanthosis nigricans can suggest non-communicable diseases such as diabetes mellitus or malignancies that may be associated with sensory deficits. Another example of a disease that can be detected on inspection is Charcot-Marie-Tooth disease, where patients may have obvious elements of pes cavus, hammer toes, and claw hands. Long-standing vitamin B12 deficiencies are notorious for causing sensory neuropathies. One can develop a high degree of suspicion for the presence of this disorder if the patient presents with a beefy red tongue with the neuropathy.

Assess the patient for obvious scars as traumatic injuries can damage neurovascular structures and result in altered sensation. Patients with spinal cord injuries with sensory deficits may also display loss of muscle mass (wasting) as a result of a concomitant decrease in motor function. Involuntary movements, fasciculations, and tremors are disorders that are commonly associated with motor lesions, but may also be present on the background of a mixed disorder (i.e. both sensory and motor deficits). The acronym SWIFT is helpful in remembering the major things to identify on inspection.  

Light touch

To assess light touch, first obtain a wisp of cotton. Show the patient the item that will be used to conduct the test to help relieve any anxiety they may be experiencing. Gently tap or dab the skin over the manubrium or the vertex of the head and use that as a reference point. Ask the patient how the stimulus felt and use that as the normal. While moving to the foot of the bed, ask the patient to close their eyes as the stimulus is being applied. This eliminates the possibility that the patient will see when the stimulus is being applied and assume that it is felt. Apply the stimulus to the dermatomal areas and ask the patient:

• Whether or not they felt the stimulus.
• If it felt the same as the reference stimulus or is it different. Some practitioners take this a step further to characterize the difference (i.e. is it greater or lesser than the reference stimulus).

Also, avoid stroking or tickling the patient with the cotton as this tests another sensory pathway. The test should be done in a caudocranial manner, all while comparing the left with the right (i.e. after testing L5 on the left, test L5 on the right). Alter the rate at which the stimulus is being applied so that the patient will not be able to predict the stimulus and answer incorrectly. Also, alternate applying and withholding the stimulus to make sure that the patient isn’t just routinely answering “yes” even when no stimulus is being applied.

Pain

Neurological pins such a Neurotip are used to assess superficial pain. Do not use hypodermic needles for this test; and be sure to use a fresh Neurotip pin with each patient and discard them after use. Alternatively, break a tongue depressor (orange stick) in half to create a pointy end and a dull end. Establish a reference stimulus as was explained for the light touch stimulus using both the sharp and dull end of the test instrument.

At the foot of the bed, proceed to assess the dermatomes in an ascending manner (as described above) while the patient’s eyes are closed. Do not stick the patient; instead, allow the pin to slide between the fingers and fall on the skin. Sticking the patient would cause the patient to feel pressure as well as superficial pain; which would be considered a confounding variable in the test. Alternate between the sharp end of the neurological pin and the pointy end. Patients with impaired superficial pain sensation will report the stimulus as being dull or not feeling it at all. 

Temperature

A cold test tube or tuning fork is commonly used to assess temperature sensation. As described earlier, the cold item is shown to the patient, then a reference stimulus is established. Proceed to the foot of the bed to assess each dermatome with the cold tube. In some settings both hot and cold test tubes are used alternatively.

Vibration sense

Vibration is the impression given off by a rapidly alternating object. The ability to detect this stimulus – known as vibration sense – can be assessed with the aid of a 128 Hz tuning fork. Tuning forks are slender, metallic objects with high elastic recoil. It is made up by two prongs that are U-shaped, connected to a stem, which terminates at a base. 

To initiate the test, strike the prongs of the tuning fork against the thenar eminence of the hand to initiate vibration. Be careful to hold the tuning fork at the stem, and not by the vibrating prongs as the latter will reduce the vibration. Establish a reference stimulus by placing the foot piece of the vibrating tuning fork at the vertex of the head or the manubrium. Subsequently, strike the prongs against the hand to restart the vibrations while at the foot of the bed. Ask the patient to close their eyes, then place the foot piece of the vibrating tuning fork on the most distal bony prominence on the foot (i.e. the distal interphalangeal joint of the great toe). Repeat the process on the contralateral side. 

The patient should be able to tell when they first note the stimulus and when it stops. If the patient is able to identify and characterize the stimulus at the most distal point, then there need to move to the next bony prominence. However, if they are unable to identify the stimulus, then move to the next bony prominence (i.e. the metatarsophalangeal joint of the great toe, medial malleolus, tibial tuberosity, anterior iliac spine). Avoid placing the tuning fork on the pulp of the great toe or over any soft tissue area; as vibration is better felt over a bony area.

In the upper limbs, vibration is assessed by placing the foot piece of the vibrating tuning fork at the distal interphalangeal joint of the index finger (forefinger). As was the case in the lower limbs, if the sensation is impaired, then proceed proximally to the next bony prominence (i.e. the distal head of the radius, then the olecranon process, then to the clavicle).

Proprioception

The ability to recognize the location of a body part with respect to the environment and the rest of the body is known as proprioception. This sensory modality is assessed by testing the joint position sense of the patient. This test begins at the foot of the bed, where the examiner uses the non-dominant hand to grasp the great toe at the sides of the interphalangeal joint. The dominant hand then holds the same digit at the distal phalanx and then move the distal phalanx dorsally and plantarly (i.e. up and down). Now ask the patient to close their eyes and move the phalanx as described earlier. Be random with the movements to avoid the patient guessing what direction the joint is in.

The distal interphalangeal joint and phalanx of the middle finger is the joint of choice for the upper limbs. If proprioception is intact, then there is no need to move to the next joint. However, if the modality is impaired, then repeat the process at the next joint. Also, holding the phalanx as described before not only isolates that part of the digit, but it also removes (to some extent) the up or down impression if it was held on the dorsal and plantar aspect. 

Graphesthesia and stereognosis

Graphesthesia refers to the ability to detect the tracing of letters or numbers on the skin just by feeling it (i.e. without visual input). Stereognosis on the other hand, is the ability to identify a three-dimensional object by touching it and without visual support. Both tests are most commonly done in the upper limb. Graphesthesia is assessed by using a blunt object to make tracings of letters or numbers in the palms of the patient while their eyes are closed. The patient should be able to tell the examiner what letter or number was outlined if this modality is intact. 

Stereognosis is tested by, again, asking the patient to close their eyes, and placing a relatively common object like a pen, coin or a key in their palm. With stereognosis intact, the patient should be able to identify the object without trouble. Be sure to test both sides during this procedure as well.

Romberg’s test

If proprioception is impaired, then voluntary movements – including gait – will also be significantly affected. However, this is also a problem if there is a lesion (vascular or space-occupying) in the cerebellum; which is responsible for balance, movement, and motor coordination. The former disorder is referred to as sensory ataxia, while the latter is called cerebellar ataxia.

The Romberg’s test was designed to differentiate the two abnormalities. The test requires that the patient stands with a narrow stance base and close their eyes. The physician should be close by to catch the patient should they topple over. Ask the patient to close their eyes and attempt to walk; provide reassurance that someone is close by to prevent them from falling. If the patient has a sensory ataxia, then the examiner will note that the patient’s gait is relatively normal as long as their eyes are open. However, once their eyes are closed, their stance and gait becomes uncertain, insecure, and teetering. This is because having visual input compensates for not being able to determine the position of the body parts with respect to the environment. However, in cerebellar ataxia, there is no change with the removal of visual input as the problem is at the source of proprioceptive integration. 

Scoring of the sensory system

Now the maneuvers of the sensory examination are easy to perform; and are far less taxing than those of the motor examination. However, the hard part for the sensory exam is getting the patient to report the sensation accurately. There have been numerous scoring systems suggested that includes asking the patient to grade the sensation on a scale of 1 to 10 or 1 to 100. All those variables add an additional confounding variable to the test. If the patient is unable to communication the sensation, then how can the test be interpreted?

Thankfully, the International Spinal Cord Society and the American Spinal Injury Association devised an International Standard for Neurological Classification of Spinal Cord Injury devised a scoring system to report the sensory deficits of a patient. The scoring system takes into consideration if the sensation is absent (0/2) or if it is present. If the sensation is present, then determine if it is the same as the reference point (2/2) or if it is altered (1/2). Some will take this a step further and try to determine if the alteration is such that the sensation is greater (hyperesthesia) or less (hypoesthesia) than the contralateral side or the reference stimulus.