This name is applied to a condition in which the constituents of the tissues are converted into a substance whose chemical characters are different from those of any normal principle in the body. The degeneration is also called waxy and lardaceous from the physical characters of the substance produced. This may be called for convenience amyloid substance, and, as the name suggests, was originally supposed to be allied to starch. It has really no chemical relation to starch, being a nitrogenous substance and a modified form of albumen. It resembles starch, however, in respect that it gives a colour reaction with iodine.
The presence of amyloid substance is determined by its physical characters and by certain colour tests. The earliest known of these latter is the reaction with iodine. The iodine reaction is useful for roughly testing macroscopically at the time of the post-mortem. For this purpose a watery solution, consisting of iodine 10 grains, iodide of potassium 20 grains, and water 4 ounces, is poured on the surface of the structure to be tested. A mahogany-red colour indicates the presence of amyloid matter. The further addition of dilute sulphuric acid sometimes produces a deeper red or a bluish colour.
For microscopic purposes watery solutions of methylviolet or gentianviolet as introduced by Cornil are most suitable. These dyes produce a rose-pink colour with the amyloid substance, while normal tissues are stained blue. In testing by iodine microscopically a solution half the strength of that mentioned above is to be used.
The substance itself has a peculiar bright translucent glancing appearance (see Fig. 50), and, as the structures in which it occurs are enlarged, they are often remarkably prominent under the microscope. It is a very dense heavy material, and, after death, at least, is some-what brittle, but the usual absence of haemorrhage in amyloid organs would seem to indicate that it is not so during life.
Looking to the history of the cases in which amyloid degeneration occurs it is clear that it is to be referred primarily to an alteration in the blood. The disease is not an independent one, but comes on in certain cachectic states due to chronic tuberculosis, syphilis, diseases of bone involving prolonged suppuration, chronic dysentry, etc.
As more unusual causes of amyloid degeneration, may be mentioned leukaemia, Hodgkin's disease (malignant lymphoma), very rarely cancer or sarcoma. It is frequently associated in the kidney with chronic inflammation of that organ, but it is doubtful whether the latter is to be regarded as its cause, for, on the one hand, amyloid disease may lead to nephritis, and, on the other, both conditions may be the result of syphilis.
It will be seen that in the forms of disease mentioned as leading to amyloid degeneration there are, for the most part, morbid poisons in the blood.. It is probable that the action of these agents is the direct cause of the condition rather than any drain on the system such as has been generally asserted as the cause. Whilst in tuberculosis there is usually a prolonged discharge, yet in syphilis amyloid degeneration is common without any such condition.
It is more difficult to determine the nature of the connection between the vice in the blood and the disease in the tissues. By some it is supposed that the amyloid substance arises in the blood by modification of the albumen and is then infiltrated into the structures. But this view cannot be accepted, for various reasons. In the first place, the substance is eminently insoluble, and it is difficult to understand how it can be carried by the blood; besides this it does not displace the normal structures simply, but replaces them, these structures being converted into the amyloid substance. It is more consistent to suppose that the tissues are reduced in vitality by the altered condition of the blood, and that the albumen of the blood enters into combination with the protoplasm in such a way as to produce this peculiar substance. The process may perhaps be compared to the coagulation of the tissues, which, as we have seen, sometimes occurs when they undergo necrosis, the tissues entering apparently into a chemical union with the fibrinogen in the fluid exuded from the blood-vessels, so as to form fibrine or some substance allied to it. Amyloid matter has frequently been compared to fibrine, and Dickinson has suggested its affinity with de-alkalized fibrine. The existence of localized amyloid disease is strongly confirmatory of this view. In this condition abnormal structures enter into this peculiar chemical combination with the albumen of the blood, while normal structures do not. In this connection also, the fact that amyloid disease affects the connective structures of the body is not to be forgotten. It is as if the chemical basis of these structures had a special relation to the amyloid substance. Amyloid disease is therefore essentially a degeneration, although, in order to the formation of the amyloid substance, it is necessary to have, added to the tissue, material from without, and this adds greatly to the bulk and weight of the structures.
Fig. 50. - Advanced amyloid disease of the liver. The arrangement of the transparent amyloid material suggests its formation in the capillaries. The round bodies at the peripheral parts of lobules are fat drops, there being slight fatty infiltration, x 70. (Thirrfelder).
According to Wichmann the amyloid matter is always interstitial, that is to say, between the cells. He holds that it occurs when the cells, owing to anaemia, are no longer able to assimilate the normal albumen exuded from the blood. The albumen lying in the spaces of the tissues undergoes a chemical transformation into the amyloid substance. This suggestion does not seem a likely one.