Hypersensitivity pneumonitis differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

The differential diagnosis of hypersensitivity pneumonitis is, primarily, a group of diseases known as idiopathic interstitial pneumonias. This group of diseases includes idiopathic pulmonary fibrosis (IPF) (which manifests histologically as usual interstitial pneumonia (UIP)), idiopathic non-specific interstitial pneumonia (NSIP) and cryptogenic organizing pneumonia, among others. There are several important clinical syndromes that occur as a result of inhalation of organic agents but are not true forms of Hypersensitivity Pneumonitis.

Differential Diagnosis

Other diseases that are secondary to inhalation of organic agents but are not true forms of HP are as follows:

1. Inhalation fever: Patients present with fever, chills, headache, and myalgias however there are not pulmonary findings (although mild dyspnea may occur). Onset is 4-8 hours following exposure. There are no long-term sequelae occur.
2. Organic dust toxic syndrome: This syndrome is the result of exposure to bioaerosols contaminated with toxin-producing fungi (mycotoxins). Patients present with fever, chills, and myalgias 4-6 hours after exposure. In contrast to inhalation fever, the chest X ray may show diffuse opacities. Bronchiolitis or diffuse alveolar damage may be present on lung biopsy specimens. This is not a true form of HP because no prior sensitization is required.
3. Chronic bronchitis: This can result from chronic obstructive pulmonary disease, which is the most common respiratory syndrome among agricultural workers. The prevalence of chronic bronchitis is much higher at 10%, compared with 1.4% for HP.
4. Exposure to aerosolized Mycobacterium avium complex (MAC): A hypersensitivity pneumonitis like syndrome has been described in patients exposed to aerosolized Mycobacterium avium complex (MAC). Hot tub lung is a term used to describe these hypersensitivity pneumonitis-like cases because they have generally been associated with hot tub use. The syndrome has been linked to the high levels of infectious aerosols containing MAC organisms found in the water. Whether this syndrome represents a true MAC infection or classic HP remains controversial (Marras, 2005).

By frequency of Interstitial Lung Diseases (Xaubet, 2004):

1. Idiopathic pulmonary fibrosis (38.6%)
2. Sarcoidosis (14.9%)
3. Cryptogenic organizing pneumonia (10.4%)
4. Interstitial lung disease associated with collagen vascular diseases (9.9%)
5. Hypersensitivity Pneumonitis (HP) (6.6%)
6. Unclassified (5.1%)

In alphabetical order:

• Air-conditioner lung
• Aspergillus clavatus
• Bagassosis
• Bird breeder's lung
• Bird fancier's lung
• Cheese worker's lung
• Chemical worker's lung
• Cladosporium
• Farmer's lung
• Grain handler's lung
• Humidifier lung
• Malt worker's lung
• Micropolyspora faeni
• Mollusk shell hypersensitivity
• Mucor stolonifer
• Paprika splitter's lung
• Penicillium casei
• Saccharopolyspora rectivirgula
• Sitophilus granarius
• Thermoactinomyces sacchari
• Thermoactinomyces vulgaris

Restrictive lung disease must be differentiated from other diseases that cause dyspnea, cough, hemoptysis, and fever such as ARDS, bronchitis, hypersensitivity pneumonitis, pneumoconiosis.

Pulmonary Function Test	Obstructive Lung Disease	Restrictive Lung Disease
TLC	↑	↓
RV	↑	↓
FVC	↓	↓
FEV1	↓↓	↓
FEV1/FVC	↓	N to ↑
MVV	↓	↓

Approach to Lung Disorders

Spirometry

Low FEV1/FVC ratio

Normal to high FEV1/FVC ratio

Obstructive Lung Disease

Restrictive Lung Disease

Bronchodilator therapy

DLCO

Increased FEV1

No change in FEV1

Normal DLCO

Decreased DLCO

Asthma

COPD

Chest wall disorders

Interstitial Lung Disease

Spirometry Findings in Various Lung Conditions

Spirometry showing Obstructive and Restrictive Lung Disease ([Source:By CNX OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons])

Disease		Clinical manifestations											Diagnosis
		Symptoms				Physical exam							Lab findings	Imaging			Gold standard
		Cough	Dyspnea	Hemoptysis	Fever	History/Exposure	Cyanosis	Clubbing	JVD	Peripheral edema	Auscultation	Other prominent findings		CXR	CT	DLCco
Acute Respiratory Distress Syndrome (ARDS)		-	+	-	-	Trauma Sepsis Drug overdose Massive transfusion Acute pancreatitis Aspiration	+	-	-	-	Coarse breath sounds Rhonchi crackles Decreased breath sounds	Initially respiratory alkalosis transforming to respiratory acidosis	BNP level of less than 100 pg/mL PaO2 / FiO2 <300 Leukopenia Leukocytosis Thrombocytopenia	Bilateral pulmonary infiltrates Initially patchy peripheral Later diffuse bilateral Ground glass Frank alveolar infiltrate	Bronchial dilatation within areas of ground-glass opacification	↓	PaO2 / FiO2 <300
Bronchitis	Acute	+	-	+/-	+	-	-	-	-	-	Diffuse wheezes High-pitched continuous sounds The use of accessory muscles  Prolonged expiration Rhonchi Rales	Hoarseness	N/A	Normal	N/A	-	Clinical diagnosis
	Chronic	+	+	-	-	A positive history of chronic productive cough  Shortness of breath	+	-	+	+	Prolonged expiration; wheezing Diffusely decreased breath sound Coarse crackles with inspiration Coarse rhonchi	N/A	Chronic hypoxemia may lead to polycythemia  Increase in Neutrophils count Chronic respiratory acidosis.	Radiolucency Diaphragmatic flattening due to hyperinflation Increased retrosternal airspace on the lateral radiograph	N/A	↓	N/A
Hypersensitivity Pneumonitis		+	+	-	+	History of allergen exposure	-	+	-	-	Diffuse fine bibasilar crackles	Constitutional symptoms Weight loss Anorexia Muscle weakness	Neutrophilia Elevated ESR Elevated CRP Elevated immunoglobulin No peripheral blood eosinophilia	Poorly defined micronodular or diffuse interstitial pattern In chronic form Fibrosis Loss of lung volume Coarse linear opacities	Ground-glass opacities or Diffusely increased radiodensities Diffuse micronodules Focal air trapping Mosaic perfusion Occasionaly thin-walled cysts Mild fibrotic changes	↓	Clinical diagnosis
Pneumoconiosis	SIlicosis	+	+	+/-	-	Occupational history Sandblasting Bystanders Quartzite miller Tunnel workers Silica flour workers Workers in the scouring powder industry	+	+	+	-	Hyperresonant Fine crackles Rhonchi Bronchial breath sounds Expiratory wheezing Increased tactile fremitus. Loud P2	Increased susceptiblity to tuberculosis.	Respiratory acidosis Abnormal sputum Anemia Neutrophilia Elevated ESR Elevated CRP Elevated immunoglobulin	Small round opacities Symmetrically distributed Upper-zone predominance Diffuse interstitial pattern of fibrosis without the typical nodular opacities in chronic case	Nodular changes in lung parenchyma Progressive massive fibrosis Bullae, emphysema Pleural, mediastinal, and hilar changes	↓	Lung biopsy
	Asbestosis					Shipyard workers Pipe fitting Insulators						Lung cancer Mesothelioma		Predilection to lower lobes Fine and coarse linear, peripheral, reticular opacities	Subpleural linear opacities seen parallel to the pleura Basilar lung fibrosis Peribronchiolar, intralobular, and interlobular septal fibrosis; Honeycombing Pleural plaques.
	Berylliosis					Electronic manufactures						-		Hilar adenopathy Increased interstitial markings.	Ground glass opacification Parenchymal nodules Septal lines
	Byssinosis					Cotton wool workers						Increased susceptibility to Actinomyces and Aspergillus infection.		Diffuse air-space consolidation	Pulmonary fibrosis with honeycombing Peri bronchovascular distribution of nodules Ground-glass attenuations
Sarcoidosis		+	+	+	+	African Americans Autoimmune	-	-	-	-	Usually normal Occasional crackles	Dermatologic manifestations Ocular involvement Osseous involvement Heart failure from cardiomyopathy Lymphocytic meningitis Cranial nerve palsies	Hypercalcemia or hypercalciuria  Elevated 1, 25-dihydroxyvitamin D levels Elevated angiotensin-converting enzyme (ACE)	Bilateral hilar lymphadenopathy	High-resolution CT (HRCT) scanning of the chest may identify Active alveolitis Fibrosis	↓	Biopsy
Pleural Effusion		+	+	+/-	+/-	Transudate CHF Cirrhosis Exudate Parapneumonic causes  Malignancy	+/-	+/-	+/-	+/-	Decreased tactile fremitus  Diminished or inaudible breath sounds Pleural friction rub	Peripheral edema, distended neck veins, and S3 gallop suggest congestive heart failure. Edema may also be a manifestation of nephrotic syndrome, pericardial disease, or, when combined with yellow nailbeds, the yellow nail syndrome. Cutaneous changes and ascites suggest liver disease. Lymphadenopathy or a palpable mass suggests malignancy.	Thoracentesis  Exudate Transudate LDH, glucose, cytology Other specific labs of underlying etiology	Supine Blunting of the costophrenic angle Homogenous increase in density spread over the lower lung fields Lateral decubitus Free flowing effusion as layers	Thickened pleura Mild effusions can aslo be detected	↓	Thoracocentesis
Interstitial (Nonidiopathic) Pulmonary Fibrosis		+	++	+	-	Connective-tissue disorder Pneumoconiosis	+	+	+	+	Wheezing S3 P2 End-inspiratory rales	Increased A-a gradient	Elevated ESR Serologic testing for ANA, RF, ANCA & ASCA may be positive	Reticular and/or nodular opacities Honeycomb appearance (late finding)	Bilateral reticular and nodular interstitial infiltrates	↓	Video-assisted thoracoscopic lung biopsy
Lymphocytic Interstitial Pneumonia		+	+	+	+	Autoimmune Lymphoproliferative disorders	-	+	-	-	Wheezing Rales	Increased A-a gradient	Polyclonal hypergammaglobulinemia Increased LDH	Bibasilar interstitial or micronodular infiltrates	Determines the degree of fibrosis Cysts (characterstic)	N	Open lung biopsy
Obesity		+	+	-	-	Overweight Diabetes mellitus Asthma	-	-	-	+	Wheezing	-	Increased hematocrit	X ray findings are often limited due to body habitus	CT findings are variable and depends upon severity of obesity	N	Clinical
Pulmonary Eosinophilia		+	+	+	+	Infections Parasitic Fungal Mycobacterial	+	-	+	+	Wheezing Rales	Increased A-a gradient	Leukocytosis with eosinophilia (> 250/µL)	Interstitial or diffuse nodular densities	Determines extent and distribution of the disease Interstitial infiltrates Cysts and nodules	↓	Biopsy of lesion (skin or lung)
Neuromuscular disease	Scoliosis	-	+	-	-	Postural abnormality	-	-	-	-	Decreased breath sounds	In severe scoliosis, the rib cage may press against the lungs making it more difficult to breathe.	R/0 genetic conditions Marfan's syndrome Edward's syndrome Total lymphocyte count (should be >1500/μL) Nutritional status must be assessed	Accurate depiction of the true magnitude of the spinal deformity can be assessed by supine anteroposterior (AP) and lateral spinal radiographs	N/A	N	Clinical Radiographs
	Muscular dystrophy	-	+	-	-	Proximal muscle weakness	-	-	-	-	Decreased breath sounds	Rash	Elevated CPK and aldolase +ANA +Anti-Jo abs Elevated ESR, CRP and RF	N/A	N/A	N	Muscle biopsy
	ALS	-	+	-	-	Muscle weakness Neurological deficit	-	-	-	-	Decreased breath sounds	Symptoms begin with limb involvement diue to muscle weakness and atrophy.  Cognitive or behavioral dysfunction Sensory nerves and the autonomic nervous system are generally unaffected	N/A	Not significant/diagnostic	Not significant/diagnostic	-	Clinical diagnosis Nerve conduction studies and needle electromyography (EMG)
	Myasthenia gravis	-	+	-	+	H/O of difficulty getting up from chair Combing Difficulty in swallowing	-	-	-	-	Decreased breath sounds	Extraocular, bulbar, or proximal limb muscles. Breathing as rapid and shallow Respiratory muscle weakness can lead to acute respiratory failure may require immediate intubation.	Anti–acetylcholine receptor (AChR) antibody (Ab) test +	Thymoma as an anterior mediastinal mass.	Thymoma as an anterior mediastinal mass.	N	Electromyography

References

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