NEET MDS Lessons
General Pathology
Pathology gives explanations of a disease by studying the following four aspects of the disease.
1. Etiology,
2. Pathogenesis,
3. Morphologic changes and
4. Functional derangements and clinical significance.
1. Etiology Etiology of a disease means the cause of the disease. If the cause of a disease is known it is called primary etiology. If the cause of the disease is unknown it is called idiopathic. Knowledge or discovery of the primary cause remains the backbone on which a diagnosis can be made, a disease understood, & a treatment developed. There are two major classes of etiologic factors: genetic and acquired (infectious, nutritional, chemical, physical, etc).
2. Pathogenesis Pathogenesis means the mechanism through which the cause operates to produce the pathological and clinical manifestations. The pathogenetic mechanisms could take place in the latent or incubation period. Pathogenesis leads to morphologic changes.
3. Morphologic changes The morphologic changes refer to the structural alterations in cells or tissues that occur following the pathogenetic mechanisms. The structural changes in the organ can be seen with the naked eye or they may only be seen under the microscope. Those changes that can be seen with the naked eye are called gross morphologic changes & those that are seen under the microscope are called microscopic changes. the morphologic changes will lead to functional alteration & to the clinical signs & symptoms of the disease.
4. Functional derangements and clinical significance The morphologic changes in the organ influence the normal function of the organ. By doing so, they determine the clinical features (symptoms and signs), course, and prognosis of the disease.
Aneurysm
An aneurysm is a localized abnormal dilation of a blood vessel or the heart
Types:
1. True aneurysm - it involves all three layers of the arterial wall (intima, media, and adventitia) or the attenuated wall of the heart.
e.g. Atherosclerotic, syphilitic, and congenital aneurysms, and ventricular aneurysms that follow transmural myocardial infarctions.
2 False aneurysm
(also called pseudo-aneurysm) is a breach in the vascular wall leading to an extravascular hematoma that freely communicates with the intravascular space ("pulsating hematoma").
E.g. ventricular ruptures after MI that are contained by a pericardial adhesion
E.g. a leak at the junction of a vascular graft with a natural artery.
Aneurysms are classified by macroscopic shape and size
Saccular aneurysms
spherical outpouchings (involving only a portion of the vessel wall, and often contain thrombi.
Fusiform aneurysms
diffuse, circumferential dilation of a long vascular segment;
they vary in diameter and length and can involve extensive portions of the aortic arch, abdominal aorta, or even the iliacs.
Aortic Aneurysm
The two most important causes are:
1- atherosclerosis : the most common cause
It causes thinning and weakening of the media. The intimal plaques compress the underlying media and also compromise nutrient and waste diffusion from the vascular lumen into the arterial wall. The media consequently undergoes degeneration and necrosis, thus allowing the dilation of the vessel
2- cystic medial degeneration of the arterial media. E.g. Marfan syndrome.
3- Other causes include: trauma, congenital defects (e.g., berry aneurysms), infections (mycotic aneurysms), systemic diseases, such as vasculitis.
Mycotic Aneurysm :
Infection of a major artery that weakens its wall is called a mycotic aneurysm
possible complications: thrombosis and rupture.
It can originate from:
(1) embolization of a septic thrombus, usually as a complication of infective endocarditis
(2) extension of an adjacent suppurative process;
(3) circulating organisms directly infecting the arterial wall
Mycotic AAAs are atherosclerotic lesions infected by lodging of circulating microorganisms in the wall
- e.g. bacteremia from a primary Salmonella gastroenteritis.
Abdominal Aortic Aneurysm
Atherosclerotic aneurysms occur most frequently in the abdominal aorta ,the common iliac arteries, the arch, and descending parts of the thoracic aorta can also be involved
Pathogenesis
AAA occurs more frequently in men and rarely develops before age 50.
Atherosclerosis is a major cause of AAA
hereditary defects in structural components of the aorta (e.g., defective fibrillin production in Marfan disease affects elastic tissue synthesis)
Morphology :
Usually positioned below the renal arteries and above the bifurcation of the aorta
AAA can be saccular or fusiform
as large as 15 cm in diameter, and as long as 25 cm.
Microscopically: atherosclerosis with destruction and thinning of the underlying aortic media
the aneurysm frequently contains a laminated mural thrombus
Syphilitic Aneurysm
Caused by The spirochetes T. pallidum
Tertiary stage of syphilis can cause obliterative endarteritis of the involve small vessels in any part of the body, including the vasa vasorum of the aorta
This results in ischemic medial injury, leading to aneurysmal dilation of the aorta and aortic annulus, and eventually valvular insufficiency.
valvular insufficiency and massive volume overload lead to hypertrophy of the left ventricle. The greatly enlarged hearts are sometimes called "cor bovinum" (cow's heart).
CLINICAL CONSEQUENCES
1. Rupture → massive potentially fatal hemorrhage
2. Obstruction of downstream vessel → tissue ischemic injury
3. Embolism → from atheroma or mural thrombus
4. Impingement and compression on an adjacent structure
5. Presentation as an abdominal mass
Str. Pneumoniae
Probably the most important streptococci. Primary cause of pneumonia. Usually are diplococci. Ste. pneumoniae are α-hemolytic and nutritionally fastidious. Often are normal flora.
Key virulence factor is the capsule polysaccharide which prevents phagocytosis. Other virulence factors include pneumococcal surface protein and α-hemolysin.
Major disease is pneumonia, usually following a viral respiratory infection. Characterized by fever, cough, purulent sputum. Bacteria infiltrates alveoli. PMN’s fill alveoli, but don’t cause necrosis. Also can cause meningitis, otitis, sinusitis.
There are vaccines against the capsule polysaccharide. Resistance to penicillin, cephalosporins, erythromycins, and fluoroquinalones is increasing.
Chronic hepatitis
Chronic hepatitis occurs in 5%-10% of HBV infections and in well over 50% of HCV; it does not occur in HAV. Most chronic disease is due to chronic persistent hepatitis. The chronic form is more likely to occur in the very old or very young, in males, in immunocompromised hosts, in Down's syndrome, and in dialysis patients.
a. Chronic persistent hepatitis is a benign, self-limited disease with a prolonged recovery. Patients are asymptomatic except for elevated transaminases.
b. Chronic active hepatitis features chronic inflammation with hepatocyte destruction, resulting in cirrhosis and liver failure.
(1) Etiology. HBV, HCV, HDV, drug toxicity, Wilson's disease, alcohol, a,-antitrypsin deficiency, and autoimmune hepatitis are common etiologies.
(2) Clinical features may include fatigue, fever, malaise, anorexia, and elevated liver function tests.
(3) Diagnosis is made by liver biopsy.
8. Carrier state for HBV and HCV may be either asymptomatic or with liver disease; in the latter case, the patient has elevate transaminases.
a. Incidence is most common in immunodeficient, drug addicted, Down's syndrome, and dialysis patients.
b. Pathology of asymptomatic carriers shows "ground-glass"" hepatocytes with finely granular eosinophilic cytoplasm.
Bacillus anthrax
- large Gram (+) rods that produce heat resistant spores; Clostridia and Bacillus species are the two bacterial spore formers; they do not form spores in tissue; produces a powerful exotoxin.
- contracted by direct contact with animal skins or products
- four forms of anthrax are recognized → cutaneous (MC), pulmonary, oraloropharyngeal, and gastrointestinal.
- cutaneous anthrax (90 to 95% of cases) occurs through direct contact with infected or contaminated animal products.
- lesions resemble insect bites but eventually swell to form a black scab, or eschar, with a central area of necrosis ("malignant pustule").
Histopathological techniques
Histopathological examination studies tissues under the microscope. During this study, the pathologist looks for abnormal structures in the tissue. Tissues for histopathological examination are obtained by biopsy. Biopsy is a tissue sample from a living person to identify the disease. Biopsy can be either incisional or excisional.
Once the tissue is removed from the patient, it has to be immediately fixed by putting it into adequate amount of 10% Formaldehyde (10% formalin) before sending it to the pathologist.
The purpose of fixation is:
1. to prevent autolysis and bacterial decomposition and putrefaction
2. to coagulate the tissue to prevent loss of easily diffusible substances
3. to fortify the tissue against the deleterious effects of the various stages in the preparation of sections and tissue processing.
4. to leave the tissues in a condition which facilitates differential staining with dyes and other reagents.
PRIMARY LYMPHEDEMA
can occur as:
1- A congenital defect, resulting from lymphatic agenesis or hypoplasia.
2- Secondary or obstructive lymphedema
- blockage of a previously normal lymphatic; e.g. Malignant tumors
- Surgical procedures that remove lymph nodes
- Postirradiation
- Fibrosis
- Filariasis
- Postinflammatory thrombosis and scarring