Nosology


Nosology is the branch of medical science that deals with the classification of diseases.

Types of classification

Diseases may be classified by cause, pathogenesis, or by symptom.
Alternatively, diseases may be classified according to the organ system involved, though this is often complicated since many diseases affect more than one organ.
A chief difficulty in nosology is that diseases often cannot be defined and classified clearly, especially when cause or pathogenesis are unknown. Thus diagnostic terms often only reflect a symptom or set of symptoms.
Traditionally diseases were defined as syndromes by their symptoms. When more information is available, they are also defined by the damage they produce. When cause is known, they are better defined by their cause, though still important are their characteristics.
Probably the last described kind of diseases are molecular diseases, defined by their molecular characteristics. This was introduced in November 1949, with the seminal paper, "Sickle Cell Anemia, a Molecular Disease", in Science magazine, Linus Pauling, Harvey Itano and their collaborators laid the groundwork for establishing the field of molecular medicine.

Coding systems

Several classifications of diseases have been historically proposed, and normally all of them assign a code to every supported disease. Some of them codify diseases following the path of the classification tree, and others like SNOMED use a multifactor classification system.
The most known coding system is the World Health Organization ICD-Series, but there are other accepted classifications like DOCLE, NANDA or SNOMED. Historically there were others like the Berkson Coding System that are not maintained anymore.
There are also coding systems for symptoms present in the diseases and biological findings. They are normally included in medical dictionaries, also with a codification system. Some of them are MeSH, COSTART or MedDRA Other systems like Current Procedural Terminology do not deal directly with diseases but with the related procedures.

Extended nosology and general medical conditions

In a wide sense, nosology deals not only with diseases, but with any kind of medical condition, like injuries, lesions or disorders.
Medical conditions, like diseases, can be defined by cause, pathogenesis, or by a collection of symptoms, medical signs and biomarkers, particularly when the other two definitions are not available.
From a nosological point of view, medical conditions could be divided in disorders, diseases, syndromes, lesions and injuries, each one with some specific meaning:
;Disease
;Syndrome
;Injury and lesions
Some medical conditions cannot be classified in any of these groups, but they can still be important enough to be considered as medical conditions. For example, to be a carrier of a genetical disease, or a viral infection unable to progress to disease, normally is not considered inside any of the previous groups. Cases of infections able to progress, but with low possibilities, like latent tuberculosis, are also considered outside the category of diseases.
The term "medical condition" can also be applied to physiological states outside the context of disease, as for example when referring to "symptoms of pregnancy". It can also refer to the normal residual scars of a disease after it has resolved, for example lung fibrosis after a tuberculosis.

History

Ancient medical treatises had a variety of different ways of classifying and grouping illnesses. Chinese texts like the Huangdi Neijing categorized diseases by which of the atmospheric influences was believed to be responsible for them. Many ancient Greek, Mesopotamian, Roman, and Egyptian authors categorized diseases by the body parts they affected, while others divided diseases into acute or chronic illnesses. Mental disorders were classified into categories like mania and paranoia by Hippocrates, and this system was utilized by later authors like Najib ad-Din Samarqandi.
Many popular ancient disease classification systems largely relied upon humorism, which carried over into medieval times. Early attempts to develop more comprehensive approaches to the classification of diseases were made by Jean Fernel in the 16th century. Early modern nosological efforts grouped diseases by their symptoms, whereas modern systems focus on grouping diseases by the anatomy and cause involved.
In the 17th century, the English physician Thomas Sydenham was the first to propose a syndrome-based classification of diseases. For Sydenham a disease and a syndrome were equivalent concepts.
In the 18th century, the taxonomist Carl Linnaeus, Francois Boissier de Sauvages, and psychiatrist Philippe Pinel developed an early classification of physical illnesses. In the late 19th century, Emil Kraepelin and then Jacques Bertillon developed their own nosologies. Bertillon's work, classifying causes of death, was a precursor of the modern code system, the International Classification of Diseases.

Applications