Pathology

Chapter 21 Pathology

CHAPTER CONTENTS

Introduction 289

Inflammation 290

Carcinogenesis 291

Skeletal system 293

Cardiovascular system 296

Respiratory system 300

Digestive system 304

Urinary system 307

Nervous system 308

Pathology is the study of disease.

The purpose of medical imaging is to demonstrate the presence or absence of disease, so it is important for imaging practitioners to have some understanding of the types they might encounter.

Many diseases share common pathological processes relating to the causation and progression of the illness.

Understanding these general concepts in pathology can aid in the understanding of diseases specific to particular regions of the body.

INFECTION

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.

Table 21.1 Examples of pathogenic microorganism

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

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.

Inflammation may be of rapid (acute) or slow (chronic) onset.

ACUTE INFLAMMATION

Acute inflammation displays typical features:

• Warmth and redness – results from increased blood flow to the area, known as hyperaemia.

• Swelling – occurs because of infiltration of proteins and fluid from the blood into the extracellular spaces as a result of increased permeability of the blood vessel walls. The accumulated protein rich fluid is called exudate. Cells of the immune system, mainly neutrophils and macrophages, also migrate into the tissues from the blood to destroy infecting organisms or digest damaged tissue.

• Pain – occurs as a result of stretching of the tissues and the release of chemical mediators of acute inflammation.

Causes of acute inflammation include:

• physical – trauma, radiation, burns or frostbite

• immunological – hypersensitivity to a substance

• chemical – corrosive or poisonous

• tissue death (necrosis).

CONSEQUENCES OF INFLAMMATION

There are a variety of potential outcomes for inflamed tissue.

Resolution

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.

Suppuration

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.

Ulceration

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.

Fibrosis

Scar tissue may be formed from inflamed tissue, particularly where there has been a large amount of tissue destruction, or the exudate, and debris cannot be removed. This may lead to distortion of the organ shape, or adhesions between one organ and another.

CARCINOGENESIS

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 its function. 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.

CANCER INCIDENCE

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.

Figure 21.1 Incidence of cancer in different age groups in the UK.

Table 21.2 Description of some commonly seen cancers

TUMOUR BEHAVIOUR

Malignant tumours

• Have poorly defined margins.

• Invade adjacent organs and structures.

• Are typified by their tendency to spread throughout the body (metastasis), which they do either by the lymphatic channels or through the blood.

The grading and staging of malignant tumours

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.

Table 21.3 The main staging systems used to assess the extent of spread of breast carcinomas

Stage	Extent of spread
International classification
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
IV	Distant metastases
TNM
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
N₀	Node-negative
N₁	Axillary nodes mobile
N₂	Axillary nodes fixed
N₃	Supraclavicular nodes or oedema of arm
M₀	No distant metastases
M₁	Distant metastases

CANCER TREATMENT

Cancer treatment is usually simpler when the tumour is confined to a discrete area, and may be removable by surgery. Infiltration of adjacent tissues or metastasis may require radiotherapy or systemic treatments such as chemotherapy.

SKELETAL SYSTEM

The skeleton consists of 206 bones connected by mobile and fixed joints. Bone is living tissue and is susceptible to disease like any other.

OSTEOPOROSIS

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.

The causes of osteoporosis are:

• menopause (may be preventable with hormone replacement therapy)

• endocrine disease

• inadequate nutrition.

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

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.

Osteoarthritis affects mainly the elderly and is primarily a condition of the weight-bearing joints, though other joints are also affected. Common presentation includes:

• loss of cartilage thickness

• bony outgrowths called osteophytes around the cartilage margins

• thickening of the subchondral bone.

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:

• The synovium lining the joint capsule becomes inflamed and begins to encroach onto the articular surface, causing erosion of the cartilage.

• Eventually the cartilage is completely destroyed and there is a loss of any normal joint space.

• The peripheral joints are commonly affected.

• Loss of normal joint architecture can result in severe deformities, and this is especially evident in the fingers (Fig. 21.3).

• Severe rheumatoid arthritis in the upper cervical spine can cause spinal instability and potential spinal cord damage.

Figure 21.3 Radiograph of the hands showing severe erosions of the metacarpophalangeal and interphalangeal joints, due to rheumatoid arthritis.

OSTEOMYELITIS

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.

Osteomyelitis can be difficult to eradicate. The treatment is by antibiotic therapy and, sometimes, surgical removal of the infected bone.

PAGET’S DISEASE

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.

Figure 21.4 Deformity of the weight-bearing long bones occurs due to weakening of the bone structure through Paget’s disease.

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