Page:WHR Rivers - Studies in Neurology - Vol 1.djvu/32

18 For the determination of the threshold for pressure-touch we have employed a simple form of pressure-æsthesiometer (fig. 1). This consists of a vulcanite cylinder (A), pierced in its length to allow a thin steel rod (B) to move freely in it. Each end of this rod projects some distance beyond the ends of the cylinder; one end is pointed and shod with a cork or vulcanite disc 3 mm. in diameter (F), which we have adopted as a standard area, while near the other end there is a small platform (C) on which weights, pierced in their centre, may rest. The weight of the steel rod with the contact disc is 2 grm., and this is consequently the pressure which falls on the skin when the unloaded instrument is brought vertically in contact with it; but by adding weights this pressure can be increased up to 50 grm. or more if necessary. The instrument is held by the vulcanite cylinder and the cork disc is brought gently in contact with the part to be tested; then by depressing the cylinder the desired weight falls on the surface. The instrument is simple, and suffers only from the disadvantage that it must be used vertically. The minimal pressure that can be applied by it is necessarily high, owing to the weight of the steel rod, and is about 2 grm. This pressure on a 3 mm. disc can be always and constantly appreciated on normal parts, and the instrument is consequently of use only after tests with von Frey's hair or cotton wool have shown that there is an alteration of tactile sensibility.

In attempting to determine a threshold with this æsthesiometer we adopt the procedure described for von Frey's hairs, beginning with a low pressure and increasing after each series of contacts until a weight is reached with which the sixteen successive contacts in one minute can be appreciated.

(a) Superficial pain.—Sensibility to pain may be tested first by pricking with a sharp steel pin or needle; the reaction to the prick should be observed, and the patient asked to compare the sensations he experiences when normal and affected parts are pricked in close succession. It must be remembered that even in this simple test there is a danger of confusion, as the contact of a point, in addition to evoking pain, gives an idea of "sharpness" due to the appreciation of the relative smallness of the stimulating object. Consequently, if the powder of recognising relative size is disturbed, the prick of a pin may be described as "less sharp" on the abnormal parts, although the pain evoked may be as great, or even greater, than on the normal side. It is therefore necessary to ensure that the patient distinguishes between the sharpness of the stimulus and the pain of soreness it produces. Unhappily,