General and special pathology objectives. Basic terminology, purpose, tasks, methods

Pathology is the study of disease. The word comes from the Greek pathos (suffering) and logos (study). Its two main objectives are aetiology (the causes of disease) and pathogenesis (how the disease develops in the body). General pathology studies changes common to all tissues, such as inflammation, cancer and ageing. Special pathology studies particular diseases of particular organs, such as pneumonia or breast cancer. Pathology is the scientific foundation of medical practice, and it links clinical medicine with the basic sciences.

Pathology is the study of disease. It investigates the structural and functional changes that disease produces in cells, tissues and organs, and the causes and mechanisms of those changes. It bridges clinical practice and the basic sciences. The Greek roots are pathos, meaning suffering, and logos, meaning study, so pathology is literally the study of suffering. Robbins notes that Virchow was right to say disease originates at the cellular level, and that modern pathology now also explains cell changes in terms of altered genes and proteins.

Pathology is divided in two ways.

• General pathology studies the common changes that occur in all tissues, such as inflammation, cancer and ageing.
• Special pathology (also called systemic pathology) studies the particular responses of specialised organs, such as goitre or breast cancer.

A disease is a condition of an organism in which its dynamic equilibrium with the environment is impaired. It is an expression of discomfort due to a structural or functional abnormality. Health is the condition of dynamic equilibrium of an organism with its environment. It is more than the absence of disease: it is complete physical, mental and social well-being.

Diseases are classified in several overlapping ways.

The tasks of pathology are to establish the four aspects of a disease, which are its aetiology, its pathogenesis, its morphology and its clinical significance. Pathology provides the diagnosis, and so the basis for prognosis and treatment. The methods used to do this are autopsy, biopsy, and special laboratory techniques, described in the sections that follow.

Aetiology is the cause of disease, meaning what sets the disease process in motion. Causes are grouped as follows.

• Biological agents, such as bacteria and viruses.
• Physical forces, such as trauma and burns.
• Chemical agents, such as poisons and alcohol.

Most diseases are multifactorial, meaning that several causes act together. Cancer, heart disease and diabetes are examples.

Pathogenesis is the underlying mechanism of disease: the sequence of cellular and tissue events from the first contact with the causal agent until the disease is expressed. Aetiology describes what sets the disease process in motion. Pathogenesis describes how the process then evolves.

Morphology is the structural change that a disease causes. The changes are studied at several levels: macroscopic (with the naked eye), microscopic, biochemical in body fluids such as blood and urine, and molecular. Gross morphology is the study of the organ as seen without a microscope. It is studied at autopsy.

An autopsy is a post-mortem examination. There are two types.

• Hospital autopsy is carried out on patients who died in hospital of natural causes.
• Forensic autopsy investigates a death that was violent or in a legal case.

Autopsy has three functions: to identify the cause of death, to provide material for scientific research, and to improve the treatment of future patients. The examination is performed in three stages: an external evaluation of the body, a dissection of the organs (internal evaluation, also called necropsy), and the pathology protocol. The protocol contains the macroscopic diagnosis and the final pathological diagnosis.

Microscopic morphology is studied on tissue and cell samples taken during life, through biopsy, or after death, at autopsy.

A biopsy is the removal of tissue or cells from a living patient for diagnosis. There are two types.

• Histological biopsy examines a piece of tissue. It is taken at surgery (frozen sections can be cut during an operation), by endoscopy, or by large needle biopsy.
• Cytological biopsy examines isolated cells. It is used for body fluids, for fine needle biopsy of the thyroid, prostate and lymph node, and for vaginal smears (the Pap test).

Histological tissue is fixed in 10% formalin and embedded in paraffin wax to form a block. The block is cut on a microtome and stained, commonly with haematoxylin and eosin (H&E) and with Sudan III. Cytological material is dried in air, fixed in alcohol, and then stained.

Other methods used in pathology are:

• electron microscopy, which shows ultrastructure
• immunofluorescence
• immunohistochemistry
• molecular pathological methods, such as the polymerase chain reaction (PCR)

Clinical significance is the link between the disease and the patient. It has two parts.

• Signs are manifestations noted by the observer, for example an elevated temperature or a change in pupil size.
• Symptoms are subjective complaints from the person with the disorder, for example pain or dizziness.

The prognosis of a disease, and the treatment suitable for the patient, follow from the clinical picture and the pathology.

A disease has one of three outcomes.

• Recovery, when the disease resolves completely.
• Incomplete recovery, which is the usual course of a chronic disease.
• Death.

There are two types of death.

• Clinical death is the cessation of the blood circulation and breathing and of brain function. It may still be reversible with cardiopulmonary resuscitation (CPR).
• Biological death is the irreversible stopping of the circulatory and respiratory functions and of all brain function (areflexia).

The late signs of death are as follows.

• Algor mortisthe body cools after death.
• Livor mortisdiscolouration of the skin from settling of blood by gravity.
• Rigor mortisthe muscles stiffen, because the pH falls and ATP is depleted.
• Decompositionbegins at 2 to 3 days and produces an odour.