DIAGNOSIS, or the process of identifying the disease by consideration of the history (anamnesis), symptoms, physical signs and examination in every way of the patient, includes differential diagnosis or its distinction from other diseases.
is essential for ra tional treatment, for without a knowledge of its cause treatment of a morbid symptom is merely empirical or, as it is often called, symptomatic, and, if beneficial, is so for the time being only and is not curative. The ideal of diagnosis is the discovery, as early as possible, of the disturbing factor at work, so that it may be removed or neutralized before structural damage results. In most diseases symptoms precede the signs or evidence of organic change in the body ; but correlation of the earlier symptoms with the later physical signs indicating the presence of structural change has made it possible to predict the nature, course and proper treatment of the disease. This knowledge is more advanced in acute than in chronic diseases, in which the processes are less obvious and may be more complicated. It is therefore important to study the earliest manifestations of disordered function and by keeping the patient under observation to collect data for the recognition of the beginnings of chronic disease. This was one of the objects of the clinic at St. Andrews, inspired and opened by the late Sir James Mackenzie in
Diagnosis is a logical process based on all the available data and is the outcome of their analysis in the light of knowledge gained by experience; diagnosis therefore leads to prognosis.
Diagnostic methods include in vestigation of the patient's past history and of his environmental conditions, of the symptoms of which he complains, and methodi cal examination of the patient and of his organs and of the manner in which they are performing their functions. This exami nation is physical and physiological; an example of the physical signs is a tumour which can be felt ; the physiological method is seen in the estimation of the functional efficiency of the heart or nervous system by means of exercises or special tests. Phys ical examination detects gross organic changes; functional tests reveal the disordered action of the organs, which is present in the earlier stage of symptoms without physical signs, mainly before structural damage has been produced. But it must be admitted that the sequence of symptoms first and bodily changes with physical signs later is not invariable; for a slow change in the organs or tissues of the body may be accompanied by com pensatory processes so complete as to prevent any functional impairment ; thus an innocent tumour may grow to considerable size before it produces any symptoms, which may then be mechan ical. A patient's past history is important not only as regards bodily illnesses, but with reference to mental experiences, which having become subconscious memories may be responsible for neurosis; Freud and his followers, by psycho-analysis (q.v.) , have brought the trauma buried in the subconscious into the field of consciousness when other psychotherapeutic means can be employed.
Signs.—Physical signs, in contradistinction to subjective symptoms of which the patient complains, are observed by the medical man. First, inspection in a good light may give a shrewd idea of what is the matter—for example severe anaemia, the pigmentation of Addison's disease, the cachexia of advanced malignant disease, the bony changes in acromegaly, the swollen features of myxoedema and cretinism and the shriv elled, sharp-featured countenance of acute peritonitis (facies Hippocratica). Facial diagnosis without further examination is not without its fallacies, but an experienced practitioner may pos sess "clinical instinct" which, without a conscious logical process, at once tells him what is the matter, and is probably due to recog nition by the subconscious mind of a similar condition seen years before but forgotten. The use of vision in diagnosis has been greatly extended in recent years. The ophthalmoscope, in vented by Charles Babbage (1792-1871) and modified by Helm holtz, enables the retina at the back of the eye, which is an expansion of the brain, to be seen in all its details, similarly the laryngoscope, and more recently the bronchoscope, oesopha goscope, sigmoidoscope, endoscope for lighting up and bringing before the observer's eye the interior of the bronchial tubes, the gullet, the lower part of the bowel, and of the urinary bladder, have greatly facilitated the detection of disease in these situa tions. Endoscopes have also been introduced into the chest (thoracoscopy) and into the abdomen (laparoscopy or coelio scopy).
Skiagraphy or examination of the body by X-rays (see ANATOMY; RONTGEN RAYS) has revolutionized diag nosis by rendering the greater part of the body translucent while the bony skeleton and certain abnormal formations remain opaque; X-rays provide a photograph or skiagram of great use in diag nosis; thus fractures can be shown, and similarly tumours, foreign bodies, certain morbid conditions, such as abscesses and collections of fluid, stones in the kidney and bismuth when intro duced into the alimentary canal appear dark, while the surround ing parts are light. In this way malignant disease, aneurysms, tuberculosis and other diseases of the lungs, and the condition of the alimentary canal can be recognized. Further by intro ducing air into the abdomen the picture produced is rendered much clearer ; recently the ingestion of dyes (tetraiodophenolph thalein or tetrabromphenolphthalein) which are excreted into the bile have been found to show up the gall bladder (cholecys tography) .
The well-established and obvious method of palpation or examination of the body by the hand has been greatly supplemented by the introduction of instru ments of precision which record the movements of the heart, arteries and muscles ; graphic records of the pulse by the poly graph, of blood pressure by the sphygmomanometer and of the heart by the electrocardiograph have added accuracy to the un aided but trained senses. The various forms of cardiac irregu larity have thus become clearly unravelled, and fresh information about the condition of the heart muscle and its beats, such as the pulses alternans and auricular fibrillation, has been secured. The electrocardiograph requires special technical skill (see HEART, DISEASES OF THE) ; but when once the existence of the under lying condition has been definitely established in diseases also accompanied by more obvious manifestations, the presence of the latter has now become sufficient to justify a diagnosis of the primary cause, and so to indicate its prognosis and appropriate treatment. This simplification of diagnosis by associated phe nomena will, it is to be hoped, follow in many diseases, in which the full explanation can originally be provided only by laboratory methods too elaborate for ordinary practice.
These methods of physical diagnosis are clinical and have been increasingly supplemented by tests adopted from those carried out in laboratories. The distinction between the clinical and laboratory methods of diagnosis is one of degree, for some, such as the counting of blood corpuscles and the detec tion of various animal parasites, do not require the help of a specialized laboratory; hence clinical laboratories have now be come an essential part of the equipment of a modern hospital or clinic. But other valuable tests are not possible in less favour able circumstances. The chemical examination of the blood for its content of sugar and non-protein nitrogen has an invaluable bearing on diabetes and renal disease ; the examination of the blood for antibodies has enabled the diagnosis of infectious dis eases to be made when ordinary clinical methods fail, for example the Wassermann reaction in the detection of syphilis, and the Widal (also a complement-fixation) test in the diagnosis of enteric fever. Bacteriological examination of discharges and excretions, such as swabs from the throat in diphtheria, of the expectoration in pulmonary consumption, of the urine and of the blood, of the cerebrospinal fluid and of the faecal excreta, has enormously advanced the exact diagnosis of infectious diseases. Thus to take an example, diseases which formerly were sometimes with diffi culty distinguished by clinical examination (enteric fever, gen eralized tuberculosis, malaria and malignant endocarditis) can now be identified readily by examination of the blood. Chemical investigation of, and examination of the cells in, the blood, the fluids already mentioned and especially of the gastric juice by the method of fractional test meals have also rendered eminent service in the diagnosis of many diseases.
Tests.—Immunology (see IMMUNITY) or the sci ence of the reactions given by persons who are immune or sus ceptible to certain infections or poisons has made it possible to say whether a person is liable to contract certain infections; thus by the Widal complement-fixation test to determine whether or not an individual is protected against enteric fever, by the Schick test, against diphtheria and by the Dick test against scarlet fever. In the two latter the diphtheria toxin or the scarlatina) haemolytic streptococcus toxin is injected between the layers of the skin ; a local reaction shows that the individual is susceptible to the disease, against which he can then be protected by injections into his muscles of the corresponding toxin. This protective diag nosis will no doubt become more general in application.
The exact cause of idiosyncra sies to certain foods, for example eggs, or to animal or vegetable substances, such as hair, feathers and pollen, can be determined by rubbing on the abraded skin extracts of substances likely to be the exciting cause of one of the "toxic idiopathies," such as asthma or nettlerash. If an individual suspected to be hyper sensitive to, say, the dandruff of horses, is thus tested with such an extract, the appearance at the point inoculated of a red area or wheal, much larger and more prominent than that pro duced on a control area of skin similarly treated, except that saline solution is applied, strongly suggests that the disorder, such as asthma, is due to hypersensitiveness to this particular foreign protein.
General Disease.—General ill-health (dis-ease, want of ease), often of a vague character as shown by undue fatigue, but some times severe and prominent, such as pernicious anaemia or ulcera tive endocarditis, and also local affections, such as arthritis, are commonly due to some primary condition often in a distant part of the body, such as disease of the tonsils, in connection with the teeth, the gall bladder or the vermiform appendix. The symptoms are frequently, especially in the examples first men tioned, due to some local infection providing micro-organisms or their poisons only, which when carried to other parts of the body, e.g., the central nervous system, the blood-forming organs, the heart or the joints, set up changes and attract attention. while the f ons et origo remains latent. In such cases, and they are numerous, of secondary disease, search for the primary infective focus is the scientific method of diagnosis, and may require expert advice from the laryngologist, dentist, the radiologist or other specialists. In other instances the primary cause sets up, through the nervous system, symptoms often in a comparatively remote part, which are described as due to disordered reflex action ; just as a decayed tooth may cause headache, a blow on the ulnar nerve at the elbow (the "funny bone") tingling and pain in the little finger, so may a stone in the kidney show itself by pain in the testicle on the same side, or a chronically inflamed appendix or gall blad der be responsible for dyspepsia.
Conclusions.—The foregoing examples show that with the advance and necessary specialization of medicine diagnosis may make demands which no one man can effi ciently meet, and that in certain cases a satisfactory solution can be obtained only after examination and report by specialists.
He is a wise man who knows enough to recognize his limitations and to call for such assistance, whereas the converse course leads to the empirical treatment of symptoms, the continued discomfort and dependence, if nothing worse, of the pa tient and eventually perhaps to the doctor's discredit. Recognizing this, hospitals have long carried out the combined diagnostic method, and more recently, especially in America, groups of doctors, each proficient in some branch of the healing art, have amalgamated to form "diagnostic clinics" and to practise "group medicine." (H. R.)
