Sandbox:Aditya

Pathophysiology

Pathogenesis

• The devlopment of primarylung abscess begins with aspiration of anerobic bacteria from the oro-pharnynx, due to altered level of consciousness, absent gag reflex or inability to swallow.
• Normally in a healthy individual, the defense mechanisms would cope up with the small amounts of aspirates with no ill effects, however, in conditions like alcoholism, DM, and immunocompromised state these defense mechanisms are compromised leading to decreased activity of alveolar macrophages and mobility of leucocytes .
• In secondary lung abscess, abscess formation depends on the underlying lung disease and predisposing factors.
• For example ,bronchial obstruction from benign or malignant intrabronchial lesions or extrinsic compression of bronchus as in middle lobe syndrome results in distal abscess formation due to decrease oropharyngeal clearance favoring the growth of organisms leading to infection.
• Once the aspirate is localized it results in pneumonitis
• Inflammatory mediators are released along with various bacterial toxins, leading to disrupture of small blood vessels and release of various proteolytic enzymes from the neutrophils resulting in the formation of colliquative necrosis
  

Location of abscess

• The right lung is more commonly affected than the left lung because the right lung is more angulated than the left bronchi.
• The most common location is the posterior segment of the right apical lobe or apical segments of lower lobes of both the lungs.
• Lateral part of the posterior segment of upper lobe of the right lung is most commonly involved in alcoholics and in patients in prone position.

Genetics

• Congenital diseases involving lungs, like cystic fibrosis, vasculitis, pulmonary sequestration and bronchial cysts are associated with increasing the risk of lung abscess in children.

Gross Morphology

• In acute lung abscess, the lesions are well circumscribed and do not demonstrate well-defined borders with the surrounding lung parenchyma, and is filled with necrotic debris.
• In chronic long standing abscess, the lesions are irregular and filled with grayish thick debris.

Microscopic Findings

• In acute lung abscess, neutrophilic granulocytes are demonstrated with dilated blood vessels and inflammatory edema.
• In chronic lung abscess, biopsy specimen demonstrates lymphocytes, plasma cells, and histiocytes around a layer of pyogenic membrane surounding the abscess cavity which is filled with pus.

Classification

• Lung abscess can be classified into 3 types: based on duration of symptoms, based on etiology and based on the mode of spread of the infection

Based on duration

• Acute : If symptoms are present for less than 6 weeks before presenting to medical care.
• Chronic : If symptoms persist for more than 6 weeks

Based on Etiology

• Primary: abscess is mainly due to aspiration of infected material (about 75% of the cases)with a polymicorbial infection
• Secondary: When abscess develops in patients due to complication of a co-existing lung disease such as post obstructive process(bronchial obstruction due to tumor , foriegn body or enlarged lymphnodes) and systemic process resulting in decreased immune response like HIV,amd patients on immunosuppresents and corticosteroids.

Based on mode of spread

Bronchiogenic
Abscess formation is due to aspiration and inhalation.

• Aspiration of oropharyngeal secretions
• Bronchial obstruction by tumor
• Foreign body, congenital malformations, and enlarged lymphNodes

Hematogenic
Abscess formation due to dissemination of causative agents from other infected sites

• Infective endocarditis
• Abdominal sepsis
• Septic thromboembolism

Risk Factors

• Numerous risk factors play a key role in the development of lung abscess, this includes the conditions, that result in altered level of consciousness and decreased immune response

Common Risk Factors

• Alcoholism
• Seizure disorder
• Artificial ventilation
• Coma
• Neuromuscular disorders with bulbar dysfunction
• Nocturnal aspiration
• Bronchial obstruction
• Gingivo-dental sepsis
• Diabetes mellitus
• Immunosuppression

Less Common Risk factors

• Drug abuse
• Malnutrition
• Mental retardation
• Gastroesophageal reflux disease

Causes

Lung abscess is caused by one or more of the following reasons.

• Necrotizing infection of lung parenchyma
• Necrosis of an infarcted lung tissue
• Cavitation in the necrotic tissue by malignant lesions

Microbiology

• About 90% of the lung abscess is caused by polymicrobial infection.
• Anaerobes are the predominant pathogens involved in primary lung abscess, followed by Streptococcus Pneumoniae.

• Klebsiella pneumoniae is the more common cause of lung abscess in alcoholics.

• Staphylococcus aureus is the most common pathogen responsible for lung abscess in children with cystic fibrosis.

The following table elaborates the Most common etiological pathogens responsible for lung abscess

Polymicrobial                                                                                                                                     Bacterial                     Fungal       Parasites                                                                                                               Anerobic                   Aerobic         Histoplasma Blastomyces Coccidoides Aspergillus Cryptococcus       Entamoeba histolytica Paragominus Westermani                                                                                         Gram Negative       Gram Positive       Gram Positive       Gram Negative                                                     Bacteroides fragilis Fusobacterum capsulatum Fusobacterum necrophorum       Peptostreptococcus Microerophilic streptococci Actinomyces       Staphyloccocus areus(including MRSA) Streptococcous Pneumonia Streptococcus Pyogens Nocardia       Klebsiella pneumoniae Heamophillus influenza type B Pseudomonas aeurongiosa Escherichia coli Legionella Pneumophilia Acinetobacter spp

Natural History ,Prognosis and Complications

Natural History

• Lung abscess is most commonly seen in the fourth decade of life in patients with risk factors or underlying other lung disorders.
• Clinical manifestations include fever, productive cough, pleuritic chest pain and occasional episodes of hemoptysis, typically develops 8-14 days after aspiration.
• The progression of the abscess is dependent on two important factors: immune status of the patient and antibiotic therapy.
• In favorable conditions, the abscess resolves by the host immune response forming a granulation tissue scar, in adverse situations the abscess progressively worsens and can result in septicemia, hemorrhage, and death.

Prognosis

The prognosis of lung abscess is good with appropriate antibiotic treatment with a high success rate. The outcomes depend on the other associated conditions underlying lung abscess. The mortality rate is around 75% in patients with an immunocompromised state and poor prognosis. The following factors are considered to be associated with poor prognosis among patients.

• Large size cavities(>6cms)
• Old age
• Necrotizing pneumonia
• Prolonged symptoms
• Abscess due to aerobic bacteria
• Bronchial obstruction due to tumors or foreign body (secondary abscess)
• Immunocompromised individuals

Complications

Without treatment, lung abscess can result in the following complications:

• Hemorrhage
• Pyopneumothorax
• Pleural empyema
• Fibrosis and calcification of lung tissue
• Mediastinal, pleural and cutaneous fistulas
• Sepsis

Diagnosis

History and symptoms

Early signs and symptoms of lung abscess include

• Fever with chills,
• Cough (at the beginning cough is non-productive, but when communication with bronchus appears, productive cough is produced which is a typical sign)
• Pleuritic chest pain

Chronic cases present with additional features like

• Dyspnea
• Weight Loss
• Anemia and fatigue is more commonly seen in anaerobic infections
• Clubbing of fingers

Putrid lung abscesses may report discolored phlegm and foul-tasting or foul-smelling sputum if the infection is a result of anaerobic organisms.The disease follows a fulminant course of high fevers with rapid progression if it is by aerobic organisms like Staph aureus.Fungi, Nocardia and Mycobacterium sps have an indolent and gradual progressive course

Physical examination

Physical examination findings of Lung abscess largely depends on the other underlying conditions,and organisms involved and severity of the disease and other comorbidities

General appearnce

Patient appears to be having Fever with chills and appear cachectic

HEENT

Poor oral hygiene with gingivitis, dental erosions or poor dentition

LUNGS

• Decreased breath sounds on the side of lung abscess
• Dullness to percussion
• Bronchial breath sounds on Auscultation
• Inspiratory Crackles

If it is associated with other conditions like empyema or effusion clinical signs like

• Contralateral shift of mediastinum
• Absent of breath sounds over the effusion can be appreciated

Extremities

Digital clubbing is seen in chronic cases of lung abscess

References

Sputum Analysis

Acid Fast Stain

Culture on Sabourad's medium

Direct Microscopic Examination

Gentain Voilet Stain

Aerobic Culture

Tuberculosis

Yeast and Fungi

Actinomyces and other mycelia of Fungi

Fusiform Bacteria and Spirochetes

Pyogenic organsims