30 Patho

Respiratory Tract Infections (RTIs)

Overview of Respiratory Tract Infections

Respiratory tract infections (RTIs) are among the most common illnesses affecting humans, involving both the upper and lower parts of the respiratory system. The clinical presentation and severity depend on the specific area involved, the causative agent, and the host's health status.

• Upper respiratory tract: Includes the nose, sinuses, throat, and larynx. Common pathogens: Rhinovirus (common cold), Influenza virus, Respiratory syncytial virus (RSV), and Parainfluenza virus.

• Lower respiratory tract: Includes the trachea, bronchial tubes, and lungs. Common conditions: pneumonia, lung abscess, acute bronchitis.

• Both upper and lower airways: Some infections, such as influenza and COVID-19 (SARS-CoV-2), can start in the upper airway and progress to the lower respiratory tract, causing severe lung infections.

Factors Affecting Signs/Symptoms of RTIs:

• The function of the structure involved

• The severity of the infectious process

• The person's age and general health status

Transmission of the Common Cold

The common cold is a viral infection of the upper respiratory tract, caused by several viruses including rhinoviruses, parainfluenza viruses, RSV, coronaviruses, and adenoviruses.

• Mode of transmission: Fingers are the greatest source of spread; also via cough and sneeze droplets.

• Portals of entry: Nasal mucosa and conjunctival surface of the eyes are the most common entry points for the virus.

Rhinitis and Rhinosinusitis

Definitions and Classifications

• Rhinitis: Inflammation of the nasal mucosa.

• Rhinosinusitis (Sinusitis): Inflammation of the paranasal sinuses, often occurring with allergic rhinitis.

Classifications of Rhinosinusitis:

• Acute rhinosinusitis: May be viral, bacterial, or mixed; symptoms last from 5 to 7 days up to 4 weeks.

• Subacute rhinosinusitis: Lasts from 4 weeks to less than 12 weeks.

• Chronic rhinosinusitis: Lasts beyond 12 weeks.

Allergic Rhinosinusitis:

• Occurrence: Often occurs with allergic rhinitis; mucosal changes are similar.

• Symptoms: Nasal stuffiness, itching, burning, frequent bouts of sneezing, recurrent frontal headache, watery nasal discharge.

• Treatment: Oral antihistamines, decongestants, and intranasal cromolyn.

Influenza Viruses

Types and Antiviral Drugs

• Type A: Most common and can infect multiple species; causes the most severe disease. Subtyped based on two surface antigens: hemagglutinin (H) and neuraminidase (N).

• Type B: Not categorized into subtypes.

Antiviral Drugs for Influenza:

• Amantadine

• Rimantadine

• Zanamivir

• Oseltamivir

Types of Influenza Vaccines:

• Trivalent inactivated influenza vaccine (TIIV): Developed in the 1940s; administered by injection.

• Live, attenuated influenza vaccine (LAIV): Approved for use in 2003; administered intranasally.

Pneumonia

Definition and Risk Factors

Pneumonia is a group of respiratory disorders involving inflammation of the lung structures (alveoli and bronchioles). It can be caused by infectious agents (bacteria, viruses) or noninfectious agents (aspirated substances).

• Risk factors: Highly virulent organisms, large inoculum, weakened host defenses (e.g., illness, age, chronic disease).

Classification of Pneumonias

• By source of infection:

  • Community-acquired: Most common cause is Streptococcus pneumoniae.

  • Hospital-acquired (nosocomial): Most common causes are gram-negative bacteria (Pseudomonas, Klebsiella, E. coli, Acinetobacter) and MRSA.

• By host immune status: Pneumonia in immunocompromised individuals.

Tuberculosis

Etiology and Pathogenesis

• Cause: Mycobacterium tuberculosis

• Protective shell: Waxy capsule makes it difficult for the body to destroy the bacteria.

• Organs affected: Can spread anywhere, but lungs are most commonly involved.

• Immune response: Macrophages attack, causing local tissue damage. Cell-mediated immunity (T-cells) is crucial for host defense.

• Pathological effect: Tissue hypersensitivity leads to delayed T-cell reaction, resulting in inflammation and tissue damage.

Types and Transmission

• Tuberculosis hominis (human tuberculosis): Airborne infection spread by droplet nuclei from persons with active disease. Crowded and confined conditions increase risk.

• Bovine tuberculosis: Acquired by drinking milk from infected cows; initially affects the gastrointestinal tract. Virtually eradicated in developed countries.

Diagnosis

• Tuberculin skin test: Indicates cell-mediated immune response to M. tuberculosis infection, but does not confirm active disease.

Fungal Respiratory Infections

Histoplasmosis

• Cause: Histoplasma capsulatum (fungus)

• Source: Soil with bird or bat droppings; spores are inhaled.

• Geographic prevalence: Ohio & Mississippi River valleys (U.S.)

• At risk: Farmers, construction workers, cave explorers, immunocompromised individuals.

• Symptoms: Mild (flu-like: fever, cough, tiredness) to severe (lung infection, possible dissemination in immunocompromised).

• Diagnosis: Antigen detection (blood/urine), serology (antibodies), culture, biopsy with special stain.

Classification and Spread of Fungi

• Yeasts: Round, grow by budding.

• Molds: Form hyphae (tubular structures), grow by branching and forming spores.

• Dimorphic fungi: Grow as yeasts at body temperature and as molds at room temperature.

• Mechanism of spread: Inhalation of spores.

Lung Cancer (Neoplasms)

Causative Factors

• Smoking: Primary risk factor for lung cancer.

• Asbestos exposure

• Familial predisposition

• Primary vs. secondary tumors: Primary lung tumors (95%) vs. secondary (metastatic) tumors from other sites.

Categories of Bronchogenic Carcinomas

Type	Prevalence	Features
Squamous cell carcinoma	25%–40%	Closely related to smoking
Adenocarcinoma	20%–40%	Most common in North America
Small cell carcinoma	20%–25%	Small, round to oval cells; highly malignant
Large cell carcinoma	10%–15%	Large polygonal cells; early spread

Manifestations of Lung Cancer

• Due to involvement of lung and adjacent structures

• Local spread and metastasis

• Paraneoplastic manifestations (endocrine, neurologic, connective tissue)

• Nonspecific symptoms: anorexia, weight loss

Respiratory Disorders in Neonates and Children

Respiratory Distress Syndrome (RDS) in Neonates

RDS is a condition primarily affecting premature infants due to surfactant deficiency.

• Pathophysiology: Immature type II alveolar cells fail to produce enough pulmonary surfactant, leading to high surface tension and alveolar collapse (atelectasis) after exhalation.

• Consequences: Impaired gas exchange, low blood oxygen, reduced lung compliance, and development of respiratory distress.

Bronchopulmonary Dysplasia

• Definition: Chronic lung disease in newborns, mostly premature infants who required oxygen or mechanical ventilation.

• Causes: Immature lungs exposed to high oxygen, mechanical ventilation, inflammation, or infection.

• At risk: Premature infants (especially <28 weeks), infants with severe RDS.

• Pathology: Lung tissue becomes scarred and stiff, alveoli do not develop normally, and breathing problems persist beyond the newborn period.

Respiratory Disorders in Children

• Upper airway infections: Viral croup, spasmodic croup, epiglottitis

• Lower airway infections: Acute bronchiolitis

Signs of Impending Respiratory Failure in Infants and Children

• Rapid breathing

• Use of accessory muscles

• Nasal flaring

• Grunting during expiration