Health, illness and disease3
Pathology is the study of the structural and functional changes which occur in cells and tissues as a result of injuries, abnormal stimuli or genetic abnormalities, and the consequences for the organism. It provides the link between basic biological sciences and the practice of medicine. In broad terms, the study of pathology encapsulates the way we think about diseases and about their causes, prevention and classification. Together with epidemiology (the study of disease at the level of populations), an understanding of pathology allows rational and practical classification of diseases.
• Pathology is the study of disease processes.
• Epidemiology is the study of diseases at the level of populations and provides a broad context for understanding pathology.
• Both provide a useful framework for classifying and understanding mechanisms of disease.
1.1. Health, illness and disease
Health and disease
Producing an accurate and complete definition of disease is not straightforward. Disease is often said to be a state of physiological or psychological dysfunction, or an abnormal variation in the structure or function of a part of the body. There is a wide range of normality and the human body can readily adapt to changes in the environment (e.g. by an increase in haemoglobin at an altitude where oxygen levels are low). Disease can be defined as a state in which these limits of normality are over-reached. Definitions like this one are ‘naturalistic’, and take the philosophical position that to have a disease is to fall outside the limits of normality, and that normality can be objectively defined. An alternative view is called ‘normativism’ and states that our ideas of what constitutes a disease are subjective and dependent on our personal and cultural values. For example, whether baldness is a disease depends on the cultural significance placed on hair loss. In some political contexts, political dissent has been considered a disease requiring psychiatric treatment. Whatever definition is used, the consequence of labelling someone as having a disease engages healthcare workers in a moral, ethical or professional obligation to intervene.
The World Health Organization defines health as a state of complete physical, mental and social wellbeing, and not merely the absence of disease or infirmity. Illness is the subjective state of not feeling well. Thus, it is possible to have a disease but not feel ill, because some diseases do not cause symptoms (a symptom is something noticed by the patient).
Classification of disease
Diagnosis uses concepts of disease classification to identify a disease in an individual patient (see Ch. 2). The aims of disease classification are to:
• determine the best treatment
• estimate the prognosis (expected future outcome)
• ascertain the cause, so the disease can be prevented in the future.
The most useful disease classifications are based on causes (aetiology) and underlying mechanisms (pathogenesis).
Aetiology
Diseases result from the interaction between individuals and their environment. The underlying cause is termed the aetiology. Some examples are:
• genetic: Down’s syndrome (extra chromosome 21)
• infective: bacteria, viruses, fungi
• chemical (drugs and toxins): cirrhosis of the liver caused by alcohol damage; respiratory failure as a result of paraquat poisoning affecting lungs; skin rash due to penicillin
• radiation: post-irradiation cancer (e.g. squamous cell carcinoma developing in the skin of a breast irradiated for mammary carcinoma)
• heat and cold: burns; frostbite
• metabolic: diabetes mellitus due to lack of insulin; gout due to deposition of uric acid crystals in joints
• nutritional: malnutrition; obesity; vitamin deficiency
• psychological: post-traumatic stress disorder due to witnessing a horrific event.
The causes of disease extend beyond pathological processes, however. Social and political factors may be of great importance, for example social stigmatisation of certain physical conditions, poverty, and government policy with respect to healthcare.
Idiopathic disease
In some instances, the underlying cause of a disease is obscure. Many euphemisms are used for this, including idiopathic, cryptogenic, essential and spontaneous. ‘Cause unknown’ is a simpler and more honest way of saying the same thing.
Pathogenesis (mechanisms of disease)
The pathogenesis of a disease is the mechanism by which the cause(s) interact with the target cells or tissues to produce pathological changes. There are a few fundamental processes that underlie most diseases:
• inflammation: response to injury in living vascularised tissue
• degeneration: deterioration of cell function resulting from metabolic disease or ageing
• neoplasia (oncogenesis): the process of transformation of cells from normal to the neoplastic, autonomous state in which the cells do not respond normally to factors controlling cell growth
• immune reactions: specific responses to foreign organisms or material.
Genetic, acquired and congenital disease
Broadly speaking, diseases can be classified into two categories: ‘genetic’ and ‘acquired’. Genetic diseases are due to abnormalities in the genome. Most are inherited, i.e. passed from parent to offspring, but about 15–20% occur due to new mutations in the affected individual. Some, such as trisomy 21 (Down’s syndrome), are evident at birth, whereas others, such as Huntington’s disease and familial adenomatous polyposis, produce symptoms only in later life. Table 1 gives a classification of the main types of genetic disease. Acquired diseases are caused by environmental factors, such as a road traffic accident resulting in a bone fracture.
| Type | Basis | Transmission pattern | Examples |
|---|---|---|---|
| Mendelian | Expression of mutation in a single gene | Autosomal dominant | Huntington’s disease: due to an abnormality in a protein called huntingtin; affected individuals develop movement disorders and dementia in adulthood |
| Marfan’s syndrome: defective matrix protein fibrillin-1 causes abnormalities in the skeleton, cardiovascular system and eye Familial adenomatous polyposis: mutation of APC results in large numbers of neoplasms in the colon and elsewhere | |||
| Autosomal recessive | Cystic fibrosis: abnormal transmembrane ion transport resulting from a defective transmembrane conductance regulator causes thick, sticky mucous secretions | ||
| Sickle cell anaemia: a point mutation in the beta-haemoglobin chain causes deoxygenated haemoglobin molecules to undergo abnormal polymerisation | |||
| X-linked | Haemophilia A: a defect in blood clotting factor VIII causes abnormal bleeding | ||
| Duchenne’s muscular dystrophy: abnormalities in the sarcolemma-associated protein dystrophin cause progressive weakness | |||
| Multifactorial | The combined action of two or more genes with additive effects | The severity of the disease is proportional to the number of deleterious genes | Hypertension Congenital heart disease Diabetes mellitus |
| Cytogenetic | An abnormal number of chromosomes, or major structural changes in chromosomes affecting many genes | Generally arise de novo | Trisomy 21 (an extra chromosome 21) causes Down’s syndrome: characteristic facial appearances, single palmar crease, learning disabilities, increased risk of heart defects and other diseases |
| Deletion of part of the long arm of chromosome 22 causes DiGeorge syndrome: congenital heart defects, T cell immunodeficiency, parathyroid hypoplasia, facial abnormalities, psychiatric problems | |||
| Lack of an X chromosome (X0 phenotype) causes Turner’s syndrome: infertility, short stature, neck webbing, etc. |
Although the classification of disease into genetic and acquired is conceptually useful, the distinction is blurred because genetic and environmental factors interact in the pathogenesis of most diseases. For example, although carcinoma of the lung is acquired due to environmental exposure to cigarette smoke, there are genetic factors that make some individuals more prone to developing the condition than others. Likewise, there are genetic components to the development of diabetes mellitus, but the individual’s diet determines the severity of the disease. Only a few diseases can be considered entirely genetic or acquired in nature.
The term congenital is used for a disease present at birth, even though it may not be recognised or recognisable at that time. Such diseases include not only genetic abnormalities but also diseases acquired due to environmental factors acting on the developing embryo and fetus. Examples of acquired congenital diseases are:
• cerebral palsy due to fetal hypoxia during delivery
• congenital rubella due to intrauterine infection with rubella virus (German measles)
• fetal alcohol syndrome due to intrauterine exposure to large amounts of ethanol.
1.2. Ways of thinking about diseases
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