Extrinsic Allergic alveolitis (Hypersensitivity Pneumonitis)

Related Subjects: | Idiopathic Pulmonary Fibrosis | Diffuse Parenchymal Lung disease | Asbestos Related Lung disease | Sarcoidosis | Coal Worker's Pneumoconiosis | Silicosis | Farmer's Lung | Cryptogenic Organising Pneumonia | Extrinsic Allergic alveolitis (Hypersensitivity Pneumonitis) | Byssinosis | Pneumoconiosis | Cor Pulmonale

EAA not typified typically with wheeze and bronchospasm. Due to a Type IV + III immune reaction

About

• Allergic response to antigens, often animal or fungal
• Leads to a lymphocytic inflammatory response with fibrosis
• Develops a restrictive lung defect and interstitial shadowing.
• Antigen avoidance is the treatment.
• Causes 2% of industrial diseases mostly affecting farmworkers
• Over 300 antigens identified

Main issues

• Extrinsic:caused by something originating outside the body
• Allergic:an abnormally increased (hypersensitive) body reaction to a common substance
• Alveolitis:inflammation in the small air sacs of the lungs (alveoli)

Aetiology

• Antigen exposure leads to an acute inflammatory reaction in bronchioles and alveoli
• It is not an allergic reaction as seen with Asthma
• Type IV Hypersensitivity reactions with the finding of granulomas
• Type III reactions: Antigen-specific Ig and complement activation
• Levels of interleukins 1 and 8 and tumour necrosis factor-alpha are increased

Clinical

• Acute form
  • Fevers, chills, chest tightness and breathlessness and malaise, cough and pyrexia
  • Starts several hours after exposure, e.g. afternoon time if work starts at 9 am
  • Absence of symptoms at the weekend is a valuable clue for those who work a five day week, unlike farmers who don't
• Chronic form
  • Over time, symptoms become more chronic and are insidious with increasing dyspnoea
  • Maybe weight loss, fatigue and reduced exercise tolerance.
  • Mid to late inspiratory crackles in upper lobes
  • Tachypnea, Tachycardia, Cyanosis if severe
• Historical clues
  • Symptoms follow new job or new home or new hot tub
  • Symptoms post sauna, swimming pool, water damage
  • Keeping birds as pets or involved in their care
  • Relief of symptoms when away from usual activities
  • A recurring atypical pneumonia

Differential

• Asthma - Wheeze and bronchospasm not typical of EAA. It tends to come on acutely with exposure to the allergen. Not delayed. Obstructive rather than restrictive, less systemic features with acute phase

Investigations

• FBC: ↑ neutrophils, ↑ Raised ESR
• CXR: Mid zonal opacities and nodules, Bilateral reticulonodular interstitial infiltration, Pulmonary fibrosis develops with continued exposure
• Respiratory function testing: Restrictive lung defect and &darr
• Transfer Factor (diffusion capacity): reduced
• Blood tests - precipitating "precipitins" are antibodies to the offending antigen in the serum
• CT chest - Ground glass opacities
• Bronchoalveolar lavage - ↑ CD8+ve cells
• Lung biopsy: Lymphocytic response, granulomatosis, interstitial fibrosis

Complications

• Ongoing exposure leads to restrictive lung disease
• Respiratory failure and cor pulmonale and pulmonary hypertension

Factors that Support Diagnosis

• Exposure to a known offending antigen
• Positive Precipitating antibodies to offending antigen
• Recurrent episodes of symptoms
• Inspiratory crackles on examination
• Symptoms 4-8 hours after exposure
• Weight loss

Management

• The main treatment is avoidance of the precipitant antigen. In some cases where this is not possible positive pressure masks are worn to avoid inhaling the precipitant.
• Steroids may be used but are not a substitute for antigen avoidance. In chronic disease, a reducing dose may be tried but stopped if no significant response.
• There may be compensation depending on the degree of lung disease