Neurological examination of the sensory system
Like the motor system, the sensory pathway plays an important role in the transmission and interpretation of environmental stimuli. Without adequate sensory input, appropriate motor responses could not be generated. The two systems work synergistically to provide optimum perception and response to the ever-changing external environment. As a result, it is equally important for clinicians to understand the inner workings of the sensory pathway, how it can be assessed, and what are the signs of sensory dysfunction.
This article will first review the dermatome maps and ascending spinal tracts; as a solid understanding of both is creates an invaluable foundation. Subsequently, the article will go on to discuss the examination of the common sensory modalities and what signs are considered abnormal.
|Dermatome||Area of skin supplied by a single nerve|
|Ascending tracts of the spinal cord||Spinothalamic, dorsal column, spinocerebellar, cuneocerebellar, spinotectal, spino-olivary|
|Sensory exam||Introduction & informed consent, adequate exposure, inspect for SWIFT, pain, light touch, temperature, vibration sense, proprioception, graphesthesia & stereognosis, Romberg's test|
|SWIFT||Scars, wasting, involuntary movements, fasciculations, tremors|
- Review of the dermatomes
- Review of the ascending spinal tracts
- Examination of the sensory system
Review of the dermatomes
The dermatomes are not anatomically distinct areas found on the body. It is more of a theoretical outline used to facilitate the interpretation of the physiological underpinning of the sensory supply of the skin. On the trunk of the body, dermatomes travel in a relatively horizontal plane. However, this regularity changes with respect to the limbs owing to the fact that there is significant lengthening and rotation of the limb buds in the intrauterine period. Therefore, the dermatome maps of the limbs will be arranged along the long axis of the limbs (albeit in similarly slender strips). There is some degree of discord among different dermatomal maps. Therefore the clinician should use their discretion and experience when applying the dermatome concept.
There are two caveats to the dermatome principle. The first is that C1 nerve roots do not contribute to a dermatome segment. The second is that the dermatomes of the face are provided by the trigeminal nerve (CN V). The ophthalmic division accounts for the upper third, maxillary division supplies the middle third, and the mandibular division covers the inferior third of the face. A list of the dermatomal segments and the area they supply is provided below. Please note that although the table only highlights common testing points, the dermatomes may extend over a long, slender area. Therefore it is useful to know the full extent of each dermatome so that the dermatome may be tested at multiple levels.
|C2||Covers the occiput and ears|
|C3||Below the base of the skull posteriorly
Anterior part of the neck
Base of neck anteriorly and posteriorly
Anterolateral surface of the arm
Ventrolateral surface of the forearm
|C6||Over the posterior part of the deltoids
The thenar eminence and thumb
|C7||Dorsal midline of the arm and forearm
Second and third fingers
|C8||Medial aspect of arm
Ulnar aspect of forearm
Fourth and fifth digits
|T1||Anteromedial surface of the arm
Ventromedial surface of the forearm
|T2||Apex of the axilla|
|T3||Commences at the level of the midclavicular line and the third intercostal space|
|T4||At the nipple line|
|T5||Between the nipples and the xiphoid process|
|T6||Junction of a horizontal line at the xiphoid process and the midclavicular line|
|T7||Horizontal line just below the xiphoid process intersects with the midclavicular line|
|T8||Horizontal line halfway between the xiphoid process and the umbilicus meets the midclavicular line|
Horizontal line three-fourths between the xiphoid process and the umbilicus meets the midclavicular line
|T10||Horizontal line at the umbilicus meets the midclavicular line|
|T11||Horizontal line halfway between the inguinal ligament and the umbilicus intersects the midclavicular line|
|T12||Midpoint of the inguinal ligament meets the midclavicular line|
|L1||Midway between L2 & T12|
|L2||Halfway point of an imaginary strip extend from the medial femoral epicondyle to the midpoint of inguinal canal|
|L3||Medial epicondyle of the femur|
|L4||Lateral epicondyle of the femur
Medial surface of the leg, foot, and entire great toe
Continues inferiorly along the posterior thigh
|S1||The level of the neck of the fibula
Along the lateral edge of the leg and foot
|S2||Mons pubis, scrotum, labia
Posterior to medial malleolus
|S4 & S5||Perineum|
There are quite a lot of them, right? Tackle the following study unit to reinforce your knowledge about dermatomes and master them:
Review of the ascending spinal tracts
The skin, muscles, and tendons are filled with nociceptors and mechanoreceptors that aid in detecting stimuli from the environment. These receptors communicate with the ascending spinal tracts via the dorsal nerve root. Once within the spinal cord, the nerve endings synapse at different points within the dorsal horn of the gray matter, before entering the white matter on the ipsilateral or contralateral side (depending on the tract).
There are roughly nine named ascending spinal cord tracts that are responsible for transmitting general somatic afferent stimuli. The ascending spinal tracts and the sensory modality that they carry are listed below:
|Dorsal column||Vibration sense
|Anterior & posterior spinocerebellar||Unconscious proprioception|
|Spino-olivary||Additional information to the cerebellum|
Examination of the sensory system
Ironically, the sensory examination is the easiest one to conceptualize, but the most time consuming one to execute. The test is completely depended on the participation of the patient and their understanding of the examiner’ instructions. The test requires instruments that can be readily found on the ward (cotton wisp, Neurotip pins, cold test tubes) and others that the practitioner should have in their arsenal of instruments (128 Hz tuning fork). The next step is to explain the examination and end goals with the patient in order to obtain informed consent. It is also important to expose the patient adequately in order to assess the dermatomes adequately. Adequate exposure includes removing covering from the entire lower limbs (sparing the perineal region), the thorax, and upper limbs. The dermatomes of the perineum and genitals are not commonly tested.
The practitioner should also be familiar with the jargon associated with the sensory examination as well. The suffix -algesia is a derivative of the Greek word algesis, which refers to an individual's sensitivity to pain. Another suffix commonly encountered while discussing disorders of the sensory system is -esthesia, which refers to the ability to perceive tactile sensations. Occasionally the suffix -dynia may also be encountered in the literature; it is synonymous with algesia. Patients may experience one or more of the following variations of sensation:
|Allodynia||Painful response to a non-painful stimulus|
|Analgesia||Numbness, loss of sensation|
|Dysesthesia||Painful or unpleasant pins and needles|
|Hyperesthesia||Increased sensation in response to stimuli|
|Hyperalgesia||Increased detection of painful stimulus|
|Hypoesthesia||Decreased detection of normal stimulus|
|Paresthesia||Pins and needles; Not Painful or unpleasant|
A general inspection should commence as soon as the patient enters the room. Observe the patient’s gait as 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.
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.
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.
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 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).
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.
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.
The sensory examination is a neurological assessment of the afferent pathway of the periphery and spinal cord. It involves assessment of:
- Overall appearance of the patient and signs of potential impairment identified on inspection,
- Superficial pain
- Light touch
- Vibration sense
- Graphesthesia and stereognosis
- and gait.
The test requires firm understanding of the dermatomal map; which is an imaginary outline describing cutaneous bands innervated by a single nerve root. The examiner should also be familiar with the ascending tracts of the spinal cord and the modalities they transmit. Finally, there are several abnormalities of sensation that a patient can experience that should be elicited from the history.