Hypersensitivity pneumonitis is a lung disease caused by inhalation of an antigen that the individual is sensitized to. It can present acutely following high dose exposure or chronically after long term low dose exposure. Common causes include moldy hay, grain, or bird droppings. Symptoms include breathlessness, cough, and fever occurring 4-8 hours after exposure. Chest imaging shows small nodules or infiltrates. Treatment involves avoiding the antigen and using corticosteroids. Prognosis depends on exposure type and duration, with acute cases often resolving but chronic exposure risking persistent symptoms.

1. Moustapha Mounib
Consultant Chest Diseases
Military Medical Academy

2. •Introduction
•Definition
•Aetiology
•Pathophysiology
•Clinical picture
•Smoking
•Investigations
•Diagnostic criteria
•Differential diagnosis
•Treatment
•Prognosis

3. Introduction

4. One of the first written description of
hypersensitivity pneumonitis or extrensic allergic
alveolitis was in 1713 by Ramazzini, who observed that ‘
minute worms ‘ contained in grain cause a syndrome of
dyspnea and cachexia associated with a shortened life
span.

5. Definition

6. It is a group of lung diseases caused by
inhalation of organic antigen to which the
individual has been previously sensitized to.
It is often divided into ‘ acute ‘ and ‘ chronic
‘ forms based on the time course of
presentation.

7. Acute form often follows a short
period of exposure to a high concentration
of antigen, and is usually reversible.

8. Chronic form typically follows a period of
chronic exposure to a low antigen dose and is
less reversible.

9. These two presentations may overlap
and ‘ subacute ‘ form of the disease is
recognized.

10. Aetiology

11. The disease is usually named colourfully
after the environment in which it occurs (e.g.
farmer’s lung and bird fancier’s lung ) and has
been reported in over 30 different occupations and
environment.

12. Some etiologic agents in Hypersensitivity Pneumonitis
Major antigen or
microbe
Source of exposureDisease
MicropolysporafaeniMoldy hayFarmer's lung
Micropolysporafaeni,
Thermoactinomyces vulgaris
Moldy grainGrain handler's lung
ThermoactinomycessacchariMoldy sugar cane fiberBagassosis
TrichosporoncutaneumHouse dust or bird
droppings
Summer type
hypersensitivity
M. faeni, Tvulgaris, occasionally
amoebae are implicated
Contaminated forced-air
system, heated water
reservoir
Humidifier or air-
conditioner lung
CryptostromacorticaleMoldy barkMaple bark stripper's lung
AspergillusclavatusMoldy maltMalt worker's lung
Aureobasidiumpullalans and
Graphium spp.
Moldy redwood dustSequoiosis
SitophilusgranariusWheat weevilWheat weevil disease
PenicilliumcaseiiCheese moldCheese worker's lung
PenicilliumfrequentansMoldy cork dustSuberosis
Avian or animal proteins (in
excreta)
Pigeons, parakeets, fowl,
rodents
Bird breeder's lung
Trimellitic anhydride,
diisocyanate, methylene
diisocyanate
Manufacture of plastics,
polyurethane foam, or
rubber
Chemical workers lung

13. Pathophysiology

14. The pathogenesis is not fully
understood, and may involve T-cell
mediated immunity and granuloma
formation ( type IV hypersensitivity ) and/or
antibody-antigen immune complex
formation ( type III hypersensitivity ).

15. It is not an atopic disease, and is not
characterized by a rise in tissue eosinophils
or Ig E ( type I hypersensitivity ); this may in
part be due to the small particle size of
offending antigens which tend to be
deposited more distally in the air spaces
than the larger particles associated with
asthma.

16. Clinical picture

17. •Breathlessness, dry cough, and systemic symptoms (
fever, chills, arthralgia, myalgia, headache ) occur 4-8
hours after exposure to antigen.
•Examination : crackles and squeaks on auscultation,
fever.
•In the absence of ongoing exposure, symptoms settle
spontaneously within 1-3 days.
•Episodes may be recurrent.

18. •Progressive exertional breathlessness, dry cough,
sometimes systemic symptoms ( weight loss ) over
course of months-years.
•May be history of acute episodes.
•Examination: crackles and squeaks on
auscultation, clubbing rare, may be features of cor
pulmonale.

19. Smoking seems to protect towards
hypersensitivity pneumonitis, although the
disease has been described in a small number of
smokers. The reason behind this protection
might be the downregulation of the immune
system by tobacco smoke and nicotine.

20. Investigations

21. Imaging

22. Chest X ray
Diffuse small (1-3 mm) nodules or
infiltrates, sometimes ground glass change, apical
sparing.
Normal in up to 20% of cases.

23. High resolution CT
Patchy ground glass change and
poorly defined nodules.
Areas of increased lucency ( enhanced
in expiratory HRCT ) occur due to air
trapping from bronchiolar involvement.

24. Both chest X ray and HRCT
appearances may quickly normalize
following removal from antigen exposure.

25. Chest X ray
Typically upper and mid zone reticulation.
High resolution CT
Diffuse well defined centrilobular nodules,
ground glass change, increased lucency from air
trapping. May mimic UIP.

29. •Typically restrictive pattern with reduced gas
transfer and lung volumes.
•Mild obstruction is also sometimes observed.
•Hypoxia may occur.
•Inhalation antigen challenge may be unpleasant,
and it is not recommended routinely.

30. Acute form associated with neutrophilia but
not eosinophilia.
Inflammatory markers are often increased.

31. Are neither specific nor sensitive. In fact,
10% of asymptomatic farmers and 40% of pigeon
breaders have precipitating antibodies to causative
antigens, but no clinical evidence of disease.

32. A lymphocytic alveolitis characterizes the BAL
fluid of patients. In fact, a BAL lymphocytic count of
less than 30% makes the diagnosis unlikely, except in
smokers and more chronic forms in which
lymphocytosis is less prominent.
However, a BAL lymphocytosis is not specific
because it may be present in many other conditions,
including sarcoidosis, chronic beryllium disease, and
several autoimmune lung diseases.

33. Findings on transbronchial biopsy are non
specific and non diagnostic in 50% of patients.
Proceeding to surgical lung biopsy may be
necessary when faced with diagnostic uncertainty,
because features of the disease overlap with many
other inflammatory lung diseases.

35. •Exposure to a known offending antigen.
•Symptoms occurring 4-8 hours after exposure.
•Positive precipitating antibodies to the offending
antigen.
•Inspiratory crackles on physical examination.
•Recurrent episodes of symptoms.
•Weight loss.

36. •Atypical pneumonia.
•Idiopathic interstitial pneumonia ( particularly
UIP and COP )
•Sarcoidosis.
•Vasculitis.
•Occupational asthma.
•Drug induced lung disease ( including
pesticides).
•Organic Dust Toxic Syndrome ( follow very
high levels of exposure to agricultural dusts,
symptoms transient, benign course ).

37. Treatment

38. The only treatment for allergic diseases is to
avoid exposure to the offending allergen.
Respiratory protection can be used to
minimize the exposure as much as possible.

39. Systemic glucocorticosteroids are usually
required to treat severely symptomatic patients,
although there is no formal evidence that such
treatment is associated with long term
abatement of symptoms or radiologic or
pulmonary function tests abnormalities.

40. The usual treatment is prednisone
or prednisolone, 40 to 60 mg a day for 2
weeks, followed by a gradual decrease over
2 to 4 weeks.

41. Prognosis

42. The natural history of the disease is
variable and probably depends on the type
and duration of antigen exposure and the
host immune response.
Acute form generally resolves within
several weeks with corticosteroid therapy
and removal from antigen exposure.

43. Continued symptoms and progressive lung
impairment have been reported after recurrent
acute attacks and even after a single acute attack.
Additionally, progressive persistent airway
hyperresponsiveness and emphysema may impact
long term recovery.