Two distinct types of gangrene are met with, which, from their most obvious point of difference, are known respectively as dry and moist, and there are several clinical varieties of each type.
Speaking generally, it may be said that dry gangrene is essentially due to a simple interference with the blood supply of a part; while the main factor in the production of moist gangrene is bacterial infection.
The cardinal signs of gangrene are: change in the colour of the part, coldness, loss of sensation and motor power, and, lastly, loss of pulsation in the arteries.
Dry Gangrene or Mummification is a comparatively slow form of local death due, as a rule, to a diminution in the arterial blood supply of the affected part, resulting from such causes as the gradual narrowing of the lumen of the arteries by disease of their coats, or the blocking of the main vessel by an embolus.
As the fluids in the tissues are lost by evaporation the part becomes dry and shrivelled, and as the skin is usually intact, infection does not take place, or if it does, the want of moisture renders the part an unsuitable soil, and the organisms do not readily find a footing. Any spread of the process that may take place is chiefly influenced by the anatomical distribution of the blocked arteries, and is arrested as soon as it reaches an area rich in anastomotic vessels. The dead portion is then cast off, the irritation resulting from the contact of the dead with the still living tissue inducing the formation of granulations on the proximal side of the junction, and these by slowly eating into the dead portion produce a furrow—the line of demarcation—which gradually deepens until complete separation is effected. As the muscles and bones have a richer blood supply than the integument, the death of skin and subcutaneous tissues extends higher than that of muscles and bone, with the result that the stump left after spontaneous separation is conical, the end of the bone projecting beyond the soft parts.
Clinical Features.—The part undergoing mortification becomes colder than normal, the temperature falling to that of the surrounding atmosphere. In many instances, but not in all, the onset of the process is accompanied by severe neuralgic pain in the part, probably due to anæmia of the nerves, to neuritis, or to the irritation of the exposed axis cylinders by the dead and dying tissues around them. This pain soon ceases and gives place to a complete loss of sensation. The dead part becomes dry, horny, shrivelled, and semi-transparent—at first of a dark brown, but finally of a black colour, from the dissemination of blood pigment throughout the tissues. There is no putrefaction, and therefore no putrid odour; and the condition being non-infective, there is not necessarily any constitutional disturbance. In itself, therefore, dry gangrene does not involve immediate risk to life; the danger lies in the fact that the breach of surface at the line of demarcation furnishes a possible means of entrance for bacteria, which may lead to infective complications.
Moist Gangrene is an acute process, the dead part retaining its fluids and so affording a favourable soil for the development of bacteria. The action of the organisms and their toxins on the adjacent tissues leads to a rapid and wide spread of the process. The skin becomes moist and macerated, and bullæ, containing dark-coloured fluid or gases, form under the epidermis. The putrefactive gases evolved cause the skin to become emphysematous and crepitant and produce an offensive odour. The tissues assume a greenish-black colour from the formation in them of a sulphide of iron resulting from decomposition of the blood pigment. Under certain conditions the dead part may undergo changes resembling more closely those of ordinary post-mortem decomposition. Owing to its nature the spread of the gangrene is seldom arrested by the natural protective processes, and it usually continues until the condition proves fatal from the absorption of toxins into the circulation.
The clinical features vary in the different varieties of moist gangrene, but the local results of bacterial action and the constitutional disturbance associated with toxin absorption are present in all; the prognosis therefore is grave in the extreme.
From what has been said, it will be gathered that in dry gangrene there is no urgent call for operation to save the patient's life, the primary indication being to prevent the access of bacteria to the dead part, and especially to the surface exposed at the line of demarcation. In moist gangrene, on the contrary, organisms having already obtained a footing, immediate removal of the dead and dying tissues, as a rule, offers the only hope of saving life.