The problem of itching is interesting from other than the therapeutic point of view, since little is known regarding the nature and cause of this condition. Various hypotheses have been offered but have failed to explain its pathogenesis. Our interest in itching originated during the study of pain and led to an hypothesis which allows us to approach the problem from a new angle. By analogy with pain, we separated itching into two types: physiological, as the response of normal tissues to external stimuli; and pathological, as a sensation arising within diseased or damaged tissues. Tickling the normal skin or even stimulating it through heat, cold, etc., may cause a sensation of itching and lead to scratching. Certain mucous membranes, such as those of the nose, and the skin around natural orifices, are especially sensitive to such stimulation. This type of itching, as a response of normal tissues is physiological or sensorial.

Under pathological conditions however, the skin, mucous membranes and other formations may itch without external stimulation or in response to stimuli which ordinarily do not produce this sensation. The itching then can be considered as a manifestation of diseased or damaged tissues and, as such can be described as pathological. Just as does pathological pain, pathological itching represents a symptom related to abnormal changes already present. (Note 4)

In spite of their relative independence, the fact that similar fundamental mechanisms are involved in the production of itching and pain explains certain characteristics they have in common. Like pathological pain, pathological itching varies in intensity with the time of day. Patients with chronic pruritus are aware of this. Some have more itching in the morning, others experience exacerbations at night. The same dualism is seen with intake of food. For these reasons, the relationship between changes in acid base balance of the body, as reflected in the urinary pH, and variations in intensity of itching was investigated.

Patients with long standing pruritus associated with a variety of chronic skin conditions were studied. They were asked to note over periods of six to twelve hours the changes in itching intensity. Evaluation of the changes was made by the patients themselves, using a series of qualifications such as none, slight, moderate, severe, very severe and unbearable, or a scale ranging from 0 to 10. They were instructed to consider the average intensity of their itching for each hour, rather than to indicate the maximum intensity at the exact time of recording. Voided urine specimens were obtained at the end of each hour. The pH of the urine specimens was determined electrometrically. A graph was plotted to compare the variations in the subjective data furnished by the patient with the concomitant hourly changes in the pH of the urine. It was usually necessary to repeat the test several times before the patient appeared able to satisfactorily evaluate the changes in the intensity of the symptom for hour long periods rather than for just the moment of recording. (We also tried to judge the intensity of itching through the frequency, intensity and duration of scratching, as noted by an observer, but without success.) Fifteen patients were studied and, because of the limited number, the results are presented as merely preliminary.

The alkaline pattern of itching

Fig. 20. The alkaline pattern of itching is recognized in a case of senile vulvar pruritus, through parallel variations of the curves of the intensity of the itching and of the urinary pH.

Fig. 21. An acid pattern of itching is recognized in a case of pruritus ani through the divergence in the concomitant variations of the curves of itching intensity and urinary pH.

An acid pattern of itching

In four cases, the curves of itching and urinary pH did not show any definite correlation even after repeated tests. Of the remaining 11 cases, 7 showed a distinct parallelism of the two curves, and in the other 4, an inverse relationship between curves was apparent. The graphs obtained in two characteristic cases are presented here. (Figs. 20 and 21)

Administration of phosphoric acid

Fig. 22. Administration of phosphoric acid—1.5 cc phosphoric acid, sol. 50%—induces together with an acidification of the urine, an increase in the intensity of the itching with an acid pattern.

There is a distinct parallelism between the two curves in Figure 20. indicating that itching was more intense when the urine was relatively more alkaline, and slight or absent when the urine was more acid. We have considered this as an "alkaline pattern" of itching in accordance with the designation for pain. An inverse relationship between the two curves is seen in Figure 21. In this case, itching was more intense when the urine was more acid, and less severe when the urine was more alkaline. This represents an acid pattern.

The effect of a strong acidifying agent, phosphoric acid, in cases with acid and alkaline itching, is illustrated in Figures 22 and 23. The intensity of the alkaline itching in the first case was reduced by the acidifying action of phosphoric acid, while the acid itching of the second case was intensified. In Figure 24, the response of a patient with alkaline itching to the administration of sodium bicarbonate is shown. The intensity of itching was greatly increased after the alkalizing agent was given.

Itching with an alkaline pattern

Fig. 23. Itching with an alkaline pattern is reduced in intensity by the administration of an acidifying agent. In the above example, two doses, each of l.S cc phosphoric acid (50%), were necessary in order to obtain this effect.

The fact that both pathological pain and pathological itching undergo the same changes in intensity related to the general acid base balance would indicate that a similar mechanism may be involved in the pathogenesis of both symptoms. It can be conceived that a slight local pH change confined to the skin or mucous membrane could act on the itching end organs and evoke the sensation of itching. More intense pH would result in pain. The fact that itching, one of the principal symptoms of dermatological conditions, can be related to local acid and alkaline changes within the skin would permit the integration of skin pathology in a more general physiopathological mechanism. The concept of the intervention of two different abnormal processes, one resulting in acid substances and the other in alkaline compounds, represents a new approach to the study of many skin conditions. The therapeutic application of this concept has produced interesting results. (See Chapter XIV)

exacerbation of the symptom

Fig. 24. The administration of 4 grams of sodium bicarbonate to a subject with an alkaline pattern of itching induces together with the alkalinization of the urine, exacerbation of the symptom.