Intro to Pathology

Pathology (definition)

The scientific study of disease; the body of knowledge, ideas, and investigative methods essential for understanding and practicing modern medicine, including structural and functional changes at molecular, cellular, tissue, and organ levels and their effects on the individual.

Period of supernatural (disease causation)

Earliest phase of disease-causation belief; dominance of animism and magic, with disease attributed to adverse supernatural forces.

Period of humors (300BC-1500AD)

Phase attributing disease to accumulation of body humors (e.g., phlegm, bile).

Period of abiogenesis (before 1800AD)

Phase believing disease changes arose spontaneously (e.g., by metamorphosis), independent of external cause.

Period of Modern Pathology (started ~1850)

Phase when environmental causation of disease became clear via autopsy, cellular pathology, toxicology, microbiology, and epidemiology; 20th-century molecular pathology later revealed genetic bases of disease and gene-environment interaction.

Louis Pasteur (contribution to pathology)

Demonstrated that microorganisms in the environment cause changes in substances; debunked abiogenesis theory.

Rudolf Virchow (contribution to pathology)

Used the light microscope to show that changes at the cellular level (cellular pathology) are the basis of disease.

Clinical pathology

Study of disease based on observations of patients to find cause and mechanisms of disease and its effects on organs/systems.

Experimental pathology

Observation of effects of manipulations on experimental systems (e.g., animal models, cell cultures); limited by difficulty reproducing the intact body's physiological milieu in cell culture.

Histopathology

Investigation of disease by examination of tissues.

Cytopathology

Investigation of disease by examination of isolated cells.

Haematology

Study of disorders of cellular and coagulable components of blood.

Microbiology (as a pathology subdivision)

Study of infectious diseases and the organisms responsible for them.

Immunology (as a pathology subdivision)

Study of the body's specific defense mechanisms.

Chemical pathology

Study and diagnosis of disease from chemical changes in tissues and fluids.

Genetics (as a pathology subdivision)

Study of abnormal chromosomes and genes.

Toxicology (as a pathology subdivision)

Study of the effects of known and suspected poisons.

Forensic pathology

Application of pathology to legal purposes.

Gross pathology

Naked-eye (macroscopic) appearance of disease; often allows confident diagnosis before further study by light microscopy.

Autopsy

"See for oneself" (also necropsy/postmortem exam); direct inspection and analysis of organs, done to determine cause of death, audit clinical diagnosis accuracy, educate learners, research disease mechanisms, and gather statistics.

Fixation

Preserving tissue in a state that fixes cells in the condition present at sampling; commonest fixative is 10% buffered formalin at ≥10x tissue volume.

Tissue processing (paraffin embedding)

After fixation, tissue is impregnated with a space-filling substance (e.g., paraffin wax), then thinly sectioned onto a glass slide for light microscopy.

Frozen sections

Sections made after freezing tissue, used when rapid diagnosis is needed.

H&E stain

Routine tissue staining method used to distinguish components such as nuclei, cytoplasm, and collagen fibers.

Histochemistry

Study of tissues, usually by light microscopy, after treating sections with specific chemical reagents.

Immunohistochemistry / immunofluorescence

Techniques using poly- or monoclonal antibodies linked to enzymes or fluorescent dyes to detect specific tissue components.

Two challenges facing the pathology student

Language (learning new terms and their accurate meanings) and process (learning disease mechanisms and their effects on organs/patients).

Hyperplasia

Increase in the size of an organ due to increase in number (by proliferation) of its constituent cells.

Disease (definition)

A condition in which the presence of a bodily abnormality causes loss of normal health (not mere presence of abnormality); also defined as the clinical manifestation, via signs and symptoms, of an underlying functional and/or structural abnormality.

Characteristics of disease (headings used to study a disease)

Name/definition, aetiology (causes), pathogenesis, manifestations, complications, diagnosis, epidemiology, prognosis, treatment.

Aetiology

The cause of disease; the initiator of the subsequent events resulting in illness, arising from interplay between host (genetic) and environmental factors.

General categories of aetiological agents

1. Genetic abnormalities; 2. Infective agents (viruses, bacteria, fungi, parasites); 3. Chemicals; 4. Physical agents (e.g., radiation, mechanical trauma).

Terms for disease of unknown cause

Primary, idiopathic, essential, spontaneous, or cryptogenic.

Risk factors

Group characteristics or traits (e.g., occupation, life habits, geographic location, family history) associated with increased risk of developing a disease; may provide clues to the causative agent.

Predisposition

An increased risk of having a disease or condition; can be genetic (a defect influencing phenotype, modifiable by environment) or environmental (e.g., malnutrition increasing risk of other conditions).

Causal association

A statistical marker for the risk of developing a disease, not necessarily its cause; the stronger the association, the more likely it is causal (strongest in infectious diseases, e.g., M. tuberculosis causing TB).

Pathogenesis

The process(es) and mechanism(s)/pathways through which the aetiology (cause) acts to produce disease effects — i.e., how the cause brings about disease.

Mechanism (in disease)

The means by which an effect or result is achieved (e.g., the vascular and cellular phases within the process of acute inflammation).

Inflammation (as a common pathogenetic pathway)

A response of vascularized living tissue to injury, which may be caused by microorganisms or harmful agents (e.g., bacteria, heat, trauma).

Degeneration

A deterioration of cells and tissues.

Carcinogenesis

The mechanism by which cancer-causing agents lead to tumors.

Neoplasia

The process that results in the formation of a tumor.

Hypersensitivity

Excessive, undesirable effects of the body's immune response to innocuous antigens.

Incubation period

The time lapse (days, weeks, or months) between exposure to a causative agent and the appearance of manifestations, during which pathogenetic processes are occurring; useful for quarantine (e.g., rabies).

Latent interval

A long time lapse (years) between cause and manifestation, typically used for neoplasia or certain infectious diseases (e.g., tertiary syphilis).

Lesion

The structural or functional abnormality responsible for ill health; may lead to further lesions, and may be purely biochemical with no visible structural change (e.g., depressive illness, schizophrenia).

Manifestations

The clinical signs and symptoms of disease: symptoms are patient complaints (e.g., weight loss, headache, fever); signs are findings on examination (e.g., high temperature, enlarged liver); abnormal features/lesions account for them.

Pathognomonic

Describes a feature of disease that, when present, makes the diagnosis of that disease certain.

Complications and sequelae

Prolonged, secondary, or distant effects of a disease, occurring during or as a consequence of it but not an essential part of it (e.g., intracerebral haemorrhage or LVF from hypertension).

Sequelae vs complications (distinction)

Sequelae arise only as a result of the disease itself; complications may result from the disease or from independent causes such as treatment.

Prognosis

A forecast/prediction of the known or likely outcome or course of a disease; may be influenced by medical or surgical intervention.

Prognostic factors

Factors that help predict likely disease outcome/course, e.g., stage (extent of spread), grade (severity), presence of complications, age.

Epidemiology

The study of disease in populations ("pathology of populations"); concerns identifying causes and modes of disease acquisition via population-level data rather than individual cases; used for aetiological clues, planning prevention (e.g., immunization), and population screening.

Incidence rate

Number of new cases of a disease occurring in a population of defined size during a defined period.

Prevalence rate

Number of cases of a disease found in a defined population at a stated time.

Remission rate

Proportion of cases of a disease that recover.

Morbidity

The sum of a disease's effects on the patient; may or may not lead to disability (loss of function and earning power).

Mortality (of a disease)

The probability that death will be the outcome; usually expressed as a rate (e.g., infant mortality rate, maternal mortality rate).

Crude death rate

Total number of deaths per year per 1000 people (currently ~8.23/1000/year worldwide).

Case fatality rate

Percentage of people presenting with/diagnosed with a disease who die from it; indicates how deadly a disease is (e.g., Ebola ~90% in DR Congo, 60-70% in Sierra Leone/Liberia/Guinea epidemics).

Nomenclature (of disease)

A systematic way of giving names to diseases and disease processes to provide unique identification.

Primary disease

A disease with no antecedent/known cause (also called essential, spontaneous, cryptogenic, or idiopathic); can also mean the initial stage of a disease (e.g., primary TB = first infection on contact with M. tuberculosis).

Secondary disease

A disease that is a complication or manifestation of an underlying lesion (e.g., secondary hypertension from renal artery stenosis); can also mean a later stage manifesting elsewhere (e.g., secondary syphilis) or following re-exposure to the causative agent (e.g., secondary TB).

Acute (disease dynamics)

Refers to rapid onset, often with rapid resolution; commonly used for inflammatory conditions.

Chronic (disease dynamics)

Refers to prolonged duration, often insidious onset; may follow an acute disease.

Benign

Likely outcome is good; e.g., a benign tumor often does not cause death (unless critically located, e.g., brain); benign hypertension is mildly raised BP with slow, gradual tissue damage.

Malignant

Likely outcome is bad; e.g., a malignant tumor will kill in time if not excised and often kills despite treatment; malignant hypertension shows rapid BP rise with severe tissue damage and symptoms.

Prefix: An-

Absence; e.g., anaemia = lack of blood.

Prefix: Dys-

Disordered; e.g., dysplasia = disordered proliferation.

Prefix: Hyper-

Excess, above normal; e.g., hypertension = high blood pressure.

Prefix: Hypo-

Deficiency, below normal; e.g., hypotension = low blood pressure.

Prefix: Meta-

Change from one state to another; e.g., metaplasia.

Suffix: -opathy

Abnormal state.

Suffix: -oma

Tumor/swelling.

Suffix: -emia

Pertaining to blood.

Suffix: -penia

Lack, reduced number.

Suffix: -cytosis

Increased number.

Suffix: -plasia

Disorder (of growth/development).

Suffix: -osis

State/condition.

Suffix: -itis

Inflammation.

Suffix: -oid

Resembles.

Suffix: -ectasis

Dilatation.

Classification of diseases (major schemes)

Genetic vs non-genetic; genetic vs environmental; congenital vs acquired.

Congenital disease

A condition present from birth; may be genetic or non-genetic in origin.

Acquired disease

A disease caused after birth by environmental agents.

Sub-classifications of acquired/environmental disease

Inflammatory (e.g., acute appendicitis); Vascular (e.g., myocardial infarction); Growth disorder (e.g., leukemia); Injury/disordered repair (e.g., fractures); Metabolic/degenerative (e.g., diabetes mellitus); Iatrogenic (due to medical personnel, e.g., injection abscess, drug reaction).