The Exteroceptive Sensations

There are many methods for testing superficial pain
sensation. A simple and commonly used method, as reliable as any, is to
use a common safety pin bent at right angles so its clasp may serve as
a handle. The instrument should be sharp enough to create a mildly
painful sensation, but not so sharp as to draw blood. A hypodermic
needle is far too sharp unless its point has been well blunted against
some hard surface. A broken wooden applicator stick is often used, and
is usually satisfactory provided the shards are sharp. Adequately sharp
ends can be obtained by holding the stick at the very ends while
breaking it. Disposable sterile devices, sharp on one end and dull on
the other, are commercially available. While it is not necessary for
the stimulating instrument to be sterile, whatever is used must be
discarded after use on a single patient to avoid the risk of
transmitting disease from accidental skin puncture. A helpful trick is
to hold the pin or shaft of the applicator stick lightly between thumb
and fingertip, and let the shaft slide between fingertip and thumb tip
with each stimulation. This helps insure more consistent stimulus
intensity than putting a fingertip on the end of the instrument and
trying to control the force with the hand or wrist. Experience teaches
how to gauge the intensity of the applied stimulus and the expected
reaction to it.

It is best to do the examination with the patient’s eyes
closed. The patient should be asked to judge whether the stimulus feels
as sharp on one side as on the other. Always suggest the stimuli should
be the same, as by language such as, “Does this feel about the same as
that?” Avoid such language as “Does this feel any different?” or “Which
feels sharper?” Suggesting there should be a difference encourages some
patients to overanalyze and predisposes them to spurious findings and a
tedious, often unreliable examination. A commonly used technique is
asking the patient to compare one side to the other in monetary or
percentage terms, for example, “If this (stimulating the apparently
normal side) side is a dollar’s worth (or 100%), how much is this
(stimulating the apparently abnormal side) worth?” The overanalytical
but neurologically normal patient often responds with an estimate on
the order of “95 cents,” while the patient with real, clinically
significant sensory loss is more apt to respond with “5 cents” or “25
cents.” Delivering alternately sharp and dull stimuli, as with the
sharp and blunt ends of a safety pin, and instructing the patient to

reply
“sharp” or “dull” is frequently useful but may not detect subtle
sensory loss only detectable in comparison with an uninvolved area.
Slight changes can sometimes be demonstrated in a cooperative patient
by asking her to indicate the alterations in sensation when a pinpoint
is drawn lightly over the skin. A cooperative patient with a discrete
distribution of sensory loss may be able to map out the involved area
quite nicely if instructed how to proceed and left alone for a short
time with tools and a marking instrument. The affected area can then be
compared with a figure showing sensory distributions.

The latent time in the response to stimulation is
eliminated and the delineation more accurate if the examination
proceeds from areas of lesser sensitivity to those of greater
sensitivity, rather than the reverse. If there is hypalgesia, move from
areas of decreased sensation to those of normal sensation; if there is
hyperalgesia, proceed from the normal to the hyperalgesic area. There
may be a definite line of demarcation between the areas of normal and
abnormal sensation, a gradual change, or at times a zone of
hyperesthesia between them. It is occasionally useful to move from the
normal to the numb area. In myelopathy, a spinal sensory level that is
the same going from rostral to caudal as from caudal to rostral
suggests a very focal and destructive lesion; when the two levels are
far apart the lesion is usually less severe. If testing is done too
rapidly, the area of sensory change may be misjudged. Applying the
stimuli too close together may produce spatial summation; stimulating
too rapidly may produce temporal summation. Either of these may lead to
spurious findings. If stimulation is too rapid, or if conduction is
delayed, a given response may refer to a previous stimulation. Stimuli
should be applied at irregular intervals to avoid patient anticipation.
If the patient knows when to expect a stimulus, a seemingly normal
response can occur even from an anesthetic area. Include control
stimuli from time to time, especially if the patient is comparing sharp
and dull (e.g., using the dull end of the pin while asking if it is
sharp), to be sure the patient has understood the instructions and is
paying attention.

Temperature sensation may be tested with test tubes
containing warm and cool water, or by using various objects with
different thermal conductivity. Ideally, for testing cold the stimuli
should be 5°C to 10°C (41°F to 50°F), and for warmth, 40°C to 45°C
(104°F to 113°F). The extremes of free-flowing tap water are usually
about 10°C and 40°C. Temperatures much lower or higher than these
elicit pain rather than temperature sensations. Normally, it is
possible to detect a difference of about 1°C in the range around 30°C.
The tubes must be dry, as dampness may be interpreted as cold. The
tines of a tuning fork are naturally cool and work well for giving a
quick impression of the ability to appreciate coolness. The tines
quickly warm with repeated skin contact; applying the tines alternately
and waving the fork in the air between stimuli helps prevent this
warming. Holding the tines under cold running tap water may also be
helpful. Some examiners warm one tine deliberately by rubbing, and then
test the ability to discriminate between the warm side and the cool
side of the fork. This technique has limited practicality because the
cool side warms so rapidly with skin contact. The latency for detecting
temperature is longer than for other sensory modalities and the
application of the stimulus may need to be extended.

In the general examination, it is sufficient to
determine whether the patient can distinguish hot and cold stimuli. It
may be useful in some circumstances, such as the detection of mild
peripheral neuropathy, to determine whether the patient is able to
differentiate between slight variations in temperature. This is best
done with special devices for testing temperature sensation
quantitatively. In most instances, heat and cold sensibility are
equally impaired. Rarely, one modality may be involved more than the
other; the area of impaired heat sensibility is usually the larger.
Pain and temperature sensibility are usually involved equally with
lesions of the sensory system, and it is rarely necessary to test both.
Testing temperature may be useful when the patient does not tolerate
pinprick stimuli, has confusing or inconsistent responses to pain
testing, or to help map an area of sensory loss. In some instances the
deficit is more consistent with temperature testing than with pinprick.
Temperature testing may not be very reliable in patients with
circulatory insufficiency or vasoconstriction causing acral coolness.

There are many methods available for evaluating tactile
sensation. Light touch can be tested with a wisp of cotton, tissue
paper, a feather, a soft brush, light stroking of the hairs, or even
using a very light touch of the fingertip. Some appreciation of light
touch may be obtained by noting the responses to the blunt end of the
stimulus used to test pinprick.

More detailed and quantitative evaluation can be
accomplished using Semmes-Weinstein filaments, an aesthesiometer, or
von Frey hairs. These methods employ filaments of different thicknesses
to deliver stimuli of varying, graded intensity. For routine testing,
simple methods suffice. It is enough to determine whether the patient
recognizes and roughly localizes light touch stimuli and differentiates
intensities. The stimulus should not be heavy enough to produce
pressure on subcutaneous tissues. Ask the patient to say “now” or “yes”
on feeling the stimulus, or to name or point to the area stimulated.
Allowance must be made for the thicker skin on the palms and soles and
the especially sensitive skin in the fossae. Similar stimuli are used
for evaluating discriminatory sensory functions such as tactile
localization and two-point discrimination. It is best to avoid hairy
skin because the sensory stimulation due to hair motion may be confused
with the test stimulus; hairy skin is exceptionally sensitive to touch.
Two-point discrimination is considered both a delicate tactile modality
and a more complex sensation requiring cortical interpretation.