Chapter 21 Pathology
Infection occurs when microorganisms multiply in tissues where they are normally absent or present in only small numbers. Those that cause disease are described as pathogenic (Table 21.1). The types of microorganism involved in infection include bacteria, viruses, yeast, fungi and protozoa.
|Disease||Organism and type||Effect|
|Tuberculosis (TB)||Mycobacterium tuberculosis (bacterial)||Pulmonary or other organ infection resulting in chronic inflammation and tissue destruction|
|Cholera||Vibrio cholera (bacterial)||Severe diarrhoea|
|Pneumonia||Streptococcus pneumoniae and others (bacterial)||Pulmonary inflammation with fever|
|Osteomyelitis||Staphylococcus aureus and others (bacterial)||Often chronic bone infection resulting in pain and swelling|
|Gangrene||Clostridium perfringens (bacterial)||Severe tissue destruction resulting in tissue death and putrefaction|
|Acquired immune deficiency syndrome (AIDS)||Human immunodeficiency virus (HIV)||Destruction of T lymphocytes resulting in increased susceptibility to infection and development of rare tumours; e.g. Kaposi’s sarcoma|
|Influenza||Influenza virus||Severe respiratory tract infection. Potentially fatal depending on the strain|
|Squamous epithelial tumours||Human papilloma virus||Malignant transformation of infected squamous epithelium, especially in the cervix|
|Viral hepatitis||Hepatitis B virus and others||Liver inflammation and potentially fatal|
|Pneumonia||Pneumocystis carinii (fungus)||Another cause of pneumonia, particularly in the immunocompromised; e.g. AIDS|
|Aspergillosis||Aspergillus fumigatus (fungus)||Chronic lung infection|
|Thrush||Candida albicans (yeast)||Mucous membrane lesions|
|Malaria||Plasmodium (protozoa)||Fever, anaemia, liver, spleen and lymph node enlargement. Potentially fatal|
Inflammation is a physiological process that may occur in response to infection, damage orsome other disruption of the normal tissue structure. Inflammation is beneficial where it assists in the destruction or isolation of invading organisms; however, it is also possible for harmful effects to result from it. Inflamed tissue may be broken down by the release of digestive enzymes from cells of the immune system, such as neutrophils and macrophages. The swelling associated with inflammation may be harmful if it causes compression of the tissue around it. For example, inflammation in the respiratory tract may result in airway obstruction, or in the brain may result in increased intracranial pressure and impaired blood flow.
Chronic inflammation differs from acute inflammation in the cell types present (fewer neutrophils and more lymphocytes) and the increased amount of granulation tissue (new capillaries and fibrous tissue) formed.
The most favourable outcome of inflammation is repair of the damaged or infected tissue, with restitution of function and minimal scarring. Some tissues are much better able to repair themselves than others. For example, bone and other connective tissues can usually repair themselves completely, but the central nervous system cannot.
Infection by certain organisms produces large amounts of pus. Pus is a mixture of tissue debris, living and dead bacteria and neutrophils. A collection of pus may become ‘walled off’ by an outer fibrous coat. This is referred to as an abscess. Sometimes a channel called a sinus opens up between the abscess and the skin surface, or the between the abscess and a hollow organ – a fistula.
The surface tissue of an organ is lost and replaced by inflammatory tissue, forming a crater. If the ulcer is in the wall of a hollow organ it is susceptible to perforation; for example a duodenal ulcer.
Tissues that are growing or that have to replace cells lost or damaged as part of their normal function will show rapid cell division; in other tissues cell division will be very slow. In both cases the process is highly coordinated to ensure that the tissue is renewed in a way appropriate to itsfunction. Sometimes this careful coordination is lost and a cell may begin to divide more frequently than normal. As a result a mass of abnormal tissue may form which may be referred to as a neoplasm or tumour. The transformation of normal tissues or benign tumours into cancer is called carcinogenesis. Box 21.1 explains some of the terminology relating to tumours.
Box 21.1 Types of tumour
Cancer incidence increases with age (Fig. 21.1). The transformation of a normal cell into a cell that will form a tumour is primarily a genetic event, but this may be triggered by environmental factors. Cells from older people have had more time to experience the environmental factors that can lead to carcinogenesis and this may explain the increasing incidence with age. Table 21.2 gives details of some commonly seen cancers.
The prognosis of a person’s cancer is determined by both its grade and its stage. Grade refers to how well differentiated the tumour cells are; that is, how much like the tissue of origin they are. Poorly differentiated cells that have lost the particular characteristics of the organ from which they arise tend to form more aggressive tumours. Stage is determined by the size of the primary tumour and the degree of spread to local or remote organs and lymph nodes. Table 21.3 explains the staging scheme used for breast cancer.
|Stage||Extent of spread|
|I||Lump with slight tethering to skin, but node negative|
|II||Lump with lymph node metastasis or skin tethering|
|III||Tumour which is extensively adherent to skin and/or underlying muscles, or ulcerating or lymph nodes are fixed|
|T1||Tumour 20 mm or less; no fixation or nipple retraction. Includes Paget’s disease|
|T2||Tumour 20–50 mm, or less than 20 mm but with tethering|
|T3||Tumour greater than 50 mm but less than 100 mm; or less than 50 mm but with infiltration, ulceration or fixation|
|T4||Any tumour with ulceration or infiltration wide of it, or chest wall fixation, or greater than 100 mm in diameter|
|N1||Axillary nodes mobile|
|N2||Axillary nodes fixed|
|N3||Supraclavicular nodes or oedema of arm|
|M0||No distant metastases|
The speed of fracture healing is highly variable, depending on their complexity and the age of patient, but an uncomplicated fracture in a child may show complete healing within 8 weeks. On occasions the normal process of reunification of bone fragments may be delayed or not occur at all. The following factors may be responsible:
Osteoporosis is primarily a condition of old age caused by loss of the bone connective tissue matrix. Mineralisation of the matrix is normal. This distinguishes it from the condition, osteomalacia, where the matrix is normal but undermineralised.
The loss of bone mass in osteoporosis weakens the bones so that fractures become more likely – wrist and hip fractures are common. Weakening of the vertebral bodies causes gradual height reduction and often wedge-shaped compression fractures.
Osteoporosis is seen radiographically as a loss of bone density (osteopaenia) when greater than 30% of the bone mass is lost. Vertebral body height reduction and wedge fractures are typical appearances in the spine.
Arthritis is a disease of the synovial joints. Pain and loss of mobility results from damage to the hyaline cartilage articular surfaces. In the healthy joint, the articular cartilage is smooth, facilitating the sliding of one bone against the other. In arthritis, the cartilage is thinned and loses its smooth surfaces, increasing friction between the articulating bones. Two important forms of arthritis are osteoarthritis and rheumatoid.
As a result of these changes, those with osteoarthritis may experience pain on movement, stiffness and joint instability. If this becomes severe it may be necessary to perform a total joint replacement (Fig. 21.2).
Figure 21.2 Radiograph of the pelvis showing advanced osteoarthritis in the right hip joint, with sclerotic bone growth in the femoral head and acetabulum and loss of the joint space. The left side has a total hip replacement.
Rheumatoid arthritis may affect the young as well as the old, though it commonly begins between the ages of 30 and 50. It differs from osteoarthritis in that it is fundamentally an inflammatory condition. Common changes are as follows:
Osteomyelitis is an infection of the bone by bacteria, most often Staphylococcus aureus. The infecting organisms gain access to the bone through a wound, such as an open fracture, or via the blood stream. Infection via the blood stream is referred to as haematogenous osteomyelitis, the bacteria entering the bone marrow through the nutrient artery.
The bone is painful and the overlying soft tissue swollen. There may also be a generalised fever, as is typical of many infections. If the infection becomes chronic some of the bone may die. There is also the possibility of sinus formation, which drains pus to the skin surface.
Paget’s is a fairly common disease affecting the elderly, arising from a disordering of normal bone turnover. Affected bones are characterised by an increase in bone synthesis, resulting in a thickened and sclerotic ‘cotton wool’ appearance radiographically. Paget’s usually affects the pelvis, skull and long bones, often progressing from one end towards the centre. Weight-bearing bones affected by Paget’s, such as the femur and tibia, can become bowed (Fig. 21.4). When the skull is affected there may be encroachment on the cranial nerve foramina, causing various neurological symptoms including vertigo, blindness and deafness.