Although we were especially concerned with pathological itching, we also became interested in physiological itching, especially in its relationship to sensorial sensations in general. Physiological itching can be regarded as a distinct sensation, not just a gradation of another sensation. Itching seems to have its own end organs. The propensity of certain regions of the body—such as the nasal mucous membranes, the skin near the nostrils, and the perioral and perianal skin—to itch in response to external stimuli can be correlated with the presence of such end organs. The proteopathic character of itching would also prompt us to consider less myelinized or even nonmyelinized nerves as its conductors. By analogy with pain, the existence of proper central centers could be conceived.

The most important characteristic of itching is that it can be induced by stimuli which, at other intensities, result in a different sensation such as touch, for instance. Although less manifest, other stimuli, such as heat and cold, also can induce itching. We have seen that stimuli which usually induce other sensations can produce physiological pain if they have an intensity above a threshold level. It is the intensity of the stimulus which determines whether it causes pain or a sensation of touch, heat or cold. Since pain appears if the stimulus is above the threshold level, it serves as a warning of a damaging incitation.

In studying itching under a similar aspect, it can be seen that it, too, is induced by nonspecific stimuli. But, for itching, the intensity of the stimuli is low. Everybody knows that an essential condition for the induction of itching is that the incitation be slight. This is easily seen for the skin, and especially the nasal mucous membrane, where a stronger stimulation will not induce itching but a touch sensation. Just as the intensity of a stimulus determines whether pain or touch is produced, so the intensity also determines whether itching or touch is felt. While the sensorial sensation of touch is induced by stimuli with intensities below those required for pain, itching is induced if intensities are below those required for touch.

The relationship of intensity of stimulus to itching, sensorial sensation of touch, and pain is shown in Figure 208 bis. This correlation explains why itching is present sometimes for a brief period when skin or mucous membrane sensorial sensation or even pain is induced. Immediately after an injury, for example, itching may be felt for a short time only to disappear just prior to the development of pain. The low intensity of the stimulus required explains a striking characteristic of itching: its disappearance when a stronger stimulus is applied. Thus scratching, which adds more intensive stimulation, makes itching disappear. The more violent the scratching, even to the point of inducing pain, the more effective it can be in eliminating itching.

The general reaction toward itching also appears related to the character of its induction. The individual responds to pain by fleeing or fighting in order to escape the intensive noxious incitation. As the incitation that produces itching is minimal by definition—the presence on the skin of a minor irritant such as a fly or mosquito, for instance—scratching is sufficient to eliminate it. With a fly on his skin, the individual need not flee or fight, but only scratch. With the concept that itching can result from exactly the same type of stimuli as pain and touch, we integrate it in the group of sensorial sensations. We can then establish a separate sensorial subgroup for itching and pain. While other sensorial sensations inform us of the nature of the excitation—heat, cold, sound, taste, etc.—itching and pain inform us only about the intensity of the stimulus, not its nature.

Pathological itching, like pathological pain, is related to the existence of abnormalities. In addition to the differences in stimulus intensity required to induce itching and pain, their different nervous formations help explain their clinical separation. No patient we have studied has ever indicated any confusion as to whether his discomfort was due to severe itching or pain. The two sensations are seldom concomitant; usually they succeed one another. The fact that proteopathic pain and itching both seem to be conducted through unmyelinized nerves indicates why they can appear under similar conditions, as in nerve regeneration. This seems to have led to confusion between itching and pain. However, itching and pain observed during nerve regeneration can be clearly differentiated by the patient. The fact that the itching sensation is produced by stimuli of low intensity also explains why itching is so often present on skin or mucous membranes without appreciable pathology. Minimal changes appear sufficient to induce the sensation.

Fig. 208 bis. Similar to pain, itching represents a special kind of sensation, with the aim to inform about the intensity of the excitation. If this is very slight, it induces itching. If above the threshold, at which the incitation acquires a noxious character, it induces pain.

Similar to pain, itching represents a special kind of sensationThe nasal pH measured with a glass electrode introduced deep in the nose

Fig. 209. The nasal pH measured with a glass electrode introduced deep in the nose shows the same dualism as the other analyses. With 6.5 as the average value, the curves of the nasal pH has more rapid and broader variations than other analyses. Curve of daily analyses shows values above the average line in a case of generalized melanoma.

The nasal pH shows persistent low values in a case of cancer of the liver

Fig. 210. The nasal pH shows persistent low values in a case of cancer of the liver.