Nosology

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nosology

[nō′säl·ə·jē]
(medicine)
The science of classification of diseases.

Nosology

 

[in this article the term is used as a cognate to the Russian nozologiia], traditionally a branch of pathology that comprised the following disciplines: (1) nosology in the broad sense, which is the general study of disease; (2) etiology, which is the study of the causes of disease; (3) the study of pathogenesis, or the development of disease; (4) clinical nosology, which is the study of the clinical features of individual diseases; and (5) the classification and nomenclature of diseases [in English usage, “nosology” is the term that exclusively encompasses this last category]. However, “nosology” in these senses is not distinct from the term “pathology.”

The term “nosologic approach” is commonly used in modern medical literature to describe the attempt of clinicians and medical theorists to identify an illness as a nosologic entity. To define the nosologic entity, the medical scientist must identify a definite cause, a clear-cut pathogenesis, typical external manifestations, and specific structural disturbances in organs and tissues. For example, whooping cough has a specific causative agent, a definite mechanism of development, and clinical signs, some of which are externally manifested and some of which are identifiable from laboratory data.

In 1761, G. Morgagni established nosology as a separate science by classifying diseases as fevers, external surgical diseases, and diseases of individual organs. The work of several scientists, notably R. Virchow in pathological anatomy and L. Pasteur in bacteriology, enabled scientists to develop a morphological and etiological approach to diagnosis and establish a classification of diseases based on the affected organs. The main nosologic entities were identified at the end of the 19th century by a synthesis of etiological, clinical, and morphological data. C. Bernard, I. P. Pavlov, A. D. Speranskii, and H. Selye described nonspecific responses of the organism that are similar in different diseases as important elements of the pathological process.

In modern medicine, a nosologic entity is determined chiefly by the primary factor that is responsible for the disease. Thus, all diseases that can be classified under the general heading “tuberculosis” are caused by Mycobacterium tuberculosis. However, a one-to-one correspondence does not always exist between a causative agent and a nosologic entity: a given disease, for example, diabetes mellitus, can be caused by a hereditary factor as well as by pancreatic trauma in response to an infection. On the other hand, a single factor—a streptococcus, for example—can be the causative agent of several conditions, including scarlet fever, abscesses, and sepsis. However, a gap in our knowledge usually can be found when several factors are considered to be primary in the development of a single disease.

A collection of symptoms makes up the clinical picture of a nosologic entity. A patient may exhibit variations of a single symptom that do not, however, change the basic nature of the disease. The degree of variation is determined by the particularities of the pathological process, for example, the characteristics of the causative agent and the effects of hereditary and acquired features on the body’s response to the causative agent. Nonclinical methods of examination are important; these comprise laboratory methods, morphological methods, and instrumental techniques—for example, electrocardiography in myocardial infarction. In some cases, a morphological criterion determines the nosologic entity. For example, diagnosis of a malignant tumor requires histological evidence of the occurrence of abnormal cellular structures; in addition, the primary origin of the malignant cells must be established. Various external signs that are associated with malignant growth have less diagnostic value.

Elucidation of the molecular mechanisms of many diseases has contributed a degree of precision to the science of nosology. For example, the A and B forms of hemophilia were differentiated when their causes could be traced to different gene disturbances; it follows that the treatments of these two forms differ as well. Biochemical and genetic analysis revealed many previously unknown forms of hemoglobinopathy. These are examples of advances in medicine that were made possible by analysis of a specific nosologic entity. The identification of new nosologic entities was also aided by advances in therapy. For example, acute lymphoblastic leukemia in children, which is sometimes curable, proved to be an independent form of acute leukemia.

More than 5,000 nosologic entities are known to modern medicine. The pathogenetic mechanisms have not been completely elucidated for those pathological conditions whose causes are unknown, although a common set of manifestations—including morphological and clinical ones—has been identified. Diseases with unknown causes continue to be classified according to the affected organ or to the syndrome. Functional disturbances constitute a group for which no morphological criteria for diagnosis are as yet available.

The identification of nosologic entities is important in that it focuses on the development of a single approach to each disease and especially on the development of therapies that are best suited to a particular disease.

A. I. VOROB’EV and A. N. SMIRNOV