Measurement of mechanical properties of the skin in lower limb chronic venous disease compared to established non-invasive methods of assessment
Chronic venous disease (CVD) of the lower limbs is a major problem in the western world with 1% of the adult population estimated to be affected at any one time. The clinical sequelae of CVD of the lower limbs range from oedema, haemosiderosis and pigmentation, to gross lipodermatosclerosis (LDS) and venous ulceration. The site most commonly affected is the gaiter area of the lower limb. The extent and severity of venous disease can be assessed by clinical and physiological methods which include duplex ultrasonography and plethysmography. Tissue oedema can be assessed by volumetric or circumferential measurements and venous ulcers may be quantified by area measurements and response to treatment in ulcer healing studies. In the vast majority of patients a spectrum of skin changes precedes venous ulceration. At present, there is no standardised objective method of assessing the degree of skin change in these patients, so that the response to treatment can be objectively monitored. I have developed a tissue tonometer and standardised the methodology for the objective assessment and quantification of the skin changes seen in patients. The tissue tonometer is a simple non-invasive instrument which uses a sensing device that detects the movements of a loaded plunger placed on the skin. The movement of the plunger is dependent on the mechanical properties of the skin and subcutaneous tissue. The instrument is positioned on the gaiter region of the leg with the subject in the supine position. The movement of the plunger into the tissues is recorded and analysed by a computer. The data obtained from the tonometer were analysed as distance and rate constant parameters. A simple mathematical model using spring and dashpot constants was also applied to see if it fitted the data. Skin compliance was investigated in normal control subjects and patients with varying severity of skin changes due to CVD, clinically classified according to the CEAP (Clinical, (A)Etiological, Anatomical and Pathophysiological) method. There was a significant reduction in skin compliance in patients with clinically severe LDS as compared to normal controls and patients with pigmentation alone or oedema without any clinical evidence of skin change. I further investigated the correlation between the recently introduced CEAP method of classification and scoring of chronic venous disease of the lower limbs with the tissue tonometry findings and parameters obtained with duplex ultrasonography, air plethysmography and photoplethysmography. Tissue tonometry provides a standardised objective means of assessing the severity of skin change in CVD which may prove to be useful in evaluating response to a particular treatment and comparing data from different centres. The deterioration of the venous physiology shown by blood flow measuring techniques correlates poorly with the clinical sequelae of venous disease, whether assessed by a trained observer or measured by the tonometer. Patients show a wide range of sensitivity to venous valvular incompetence, suggesting that factors related to the tissue response to venous hypertension are crucial in determining which patients develop venous ulceration.