Capillary resistance in rheumatoid arthritis.

For more than 50 years, tests of "capillary resistance" have been used to measure the facility with which petechiae may be produced experimentally, mainly with a view to detecting latent purpuric tendencies. Hare and Miller (1951) listed the many different methods which have been evolved, and assessed the merits and demerits of techniques in general use. "Pressure" tests, such as the snake venom test, have been used more extensively than any other method and can be defined in two broad categories. In "positive pressure" tests, the arm is constricted by a sphygmomanometer, the number of petechiae appearing in response to venous engorgement of the forearm serving as an index of capillary resistance. "Negative pressure" tests require apparatus consisting of a vacuum pump, a manometer, and a suction cup which can be applied to the skin. The pressure of air in the system is reduced to a predetermined level for a period, and the number of petechiae thus produced in the underlying skin, in relation to the magnitude and duration of the negative pressure applied, is used as a measure of capillary resistance. While it is unlikely that all the different modifications of these tests measure the same biological properties, suction or "negative pressure" techniques have an advantage over other procedures in that tests can be repeated in an individual at intervals of a few hours and can be used to measure, for example, the influence of substances the administration of which may produce rapid and transient changes in capillary resistance. It has been customary to regard the expressions "capillary resistance" and "capillary fragility" as interchangeable, but it should be noted that "increased resistance" is equivalent to "diminished fragility" and vice versa. Although these tests were originally introduced to facilitate the study of haemorrhagic diseases, the results of investigations in this field have been somewhat conflicting. In a review of the subject of capillary resistance tests in scurvy, Munro, Lazarus, and Bell (1947) concluded that most investigators had failed to demonstrate a significant relationship between capillary strength and ascorbic acid reserves assessed by chemical methods. Nevertheless, Scarborough (1953) observed reasonably good correlation between depletion of ascorbic acid and the level of capillary resistance in otherwise normal subjects. In other haemorrhagic disorders, capillary resistance appeared to be unrelated to the bleeding time, the coagulation time, or the number of thrombocytes (Hare and Miller, 1951). In thrombocytopenic purpura, however, Elliott (1938), Elliott and Whipple (1940), and Robson (1949) found evidence of a consistent relationship between capillary resistance and the "bleeding tendency". It is possible that tests of capillary resistance are a measure of haemostatic mechanisms other than those primarily involved in haemorrhagic disorders, and this conception might explain the frequently anomalous results of such tests in patients with purpura. The available evidence indicates that the maintenance of normal levels of resistance may depend upon the integrity of the perivascular connective tissue; histological studies of petechiae induced by suction applied to normal skin have revealed fragmentation of elastic fibres in the vicinity of haemorrhages without evidence of damage to capillary endothelium (Peck, Rosenthal, and Erf, 1937). The low capillary resistance found in premature, but otherwise normal, infants has been attributed to under-development of the connective tissues around cutaneous blood vessels (Hare and Miller, 1951). From experimental evidence, Zweifach (1955) concluded that increased capillary fragility may be a consequence of a disturbance in the structure of the connective tissue sheath of small cutaneous vessels. The foregoing therefore suggests that tests of capillary resistance