Eosinophilic lung disease forms a rare spectrum of idiopathic and secondary conditions that may be acute or chronic processes. Eosinophilic pneumonia is the most common form. Clinically, eosinophilic lung diseases are classified by etiology, the largest group being idiopathic. Idiopathic eosinophilic lung diseases are divided into simple pulmonary eosinophilia (which has no real pathologic counterpart), acute eosinophilic pneumonia, chronic eosinophilic pneumonia, and idiopathic hypereosinophilic syndrome. Eosinophilic lung diseases of known cause include drug-related eosinophilic pneumonias (with no histologic differences from the idiopathic group), a group of infectious processes (allergic bronchopulmonary aspergillosis/fungal disease, bronchocentric granulomatosis, parasitic infections), and vasculitis (allergic angiitis and granulomatosis, or Churg-Strauss syndrome). The latter group of diseases is discussed in other chapters in this book. Table 26.1 outlines the clinical classification of eosinophilic lung disease.
In cases of pathologically documented eosinophilic lung disease, the most common is idiopathic chronic eosinophilic pneumonia, followed by drug-induced eosinophilic pneumonia and acute eosinophilic pneumonia. The largest relatively recent series of histologically documented eosinophilic pneumonia described five acute and six chronic cases, and a distinction between idiopathic and drug-induced disease was not made.1
Eosinophilic infiltrates in the bronchioles and bronchi may also result in respiratory symptoms and are generally considered separately from eosinophilic pneumonia as eosinophilic bronchitis or bronchiolitis. It has recently been demonstrated that eosinophilic pneumonia and bronchitis may coexist, however.2
Simple Pulmonary Eosinophilia
Simple pulmonary eosinophilia is sometimes called “Löffler syndrome,” although in the initial reports Löffler himself included parasitic infection.3 Because simple pulmonary eosinophilia is rarely biopsied, the pathologic features are not well characterized, but are typically described as interstitial eosinophils and pulmonary edema.4 Patients are by definition asymptomatic and are found incidentally to have migrating lung opacities that are nonsegmental, peripheral, and with ill-defined margins and that spontaneously clear within 1 month. The diagnosis rests on the finding of peripheral eosinophilia in conjunction with lack of symptoms, imaging findings, and absence of other conditions, including infections and parasitic disease. Some patients eventually developed symptoms leading to a specific diagnosis, such as parasitic lung disease, drug-induced lung disease, or allergic bronchopulmonary lung disease.5
TABLE 26.1 Classification of Eosinophilic Lung Diseases
Eosinophilic lung diseases of unknown cause
Simple pulmonary eosinophilia (Löffler syndrome)
Acute eosinophilic pneumonia (idiopathic)
Chronic eosinophilic pneumonia (idiopathic)
Idiopathic hypereosinophilic syndrome
Eosinophilic lung diseases of known cause
Drug reactions (usually eosinophilic pneumonia, acute or chronic)
Allergic bronchopulmonary fungal disease
Bronchocentric granulomatosis
Parasitic infections
Eosinophilic vasculitis (Churg-Strauss syndrome)
Etiology
As noted above, eosinophils may accumulate in the lung due to infectious, allergic reactions, or toxic insults or be a response to unknown stimuli. The list of exposures and conditions associated with pulmonary eosinophilia is long (Table 26.2).6
The mechanism by which eosinophils cause tissue damage involves the Charcot-Leyden crystal protein, an eosinophil lysophospholipase encoded by the CLC gene, in addition to eosinophil cationic protein, a ribonuclease that possesses neurotoxic, helminthotoxic, and ribonucleolytic properties.4,9
Acute Eosinophilic Pneumonia
Clinical Features
The incidence of eosinophilic pneumonia is unknown, because the diagnosis is not often pathologically documented. It is considered rare, as most series contain only a few cases.1,10 The original description of Allen et al. entailed a total of four patients.11 Series of over six patients are based on clinical, imaging, and bronchoalveolar lavage (BAL) findings alone with histologic documentation.2,12
TABLE 26.2 Causes of Eosinophilic Pneumonia
Inhalational
Scotchgard
Methylchloroform
Stainless steel dust containing 0.1% nickel
Dust
Fireworks
Other exposures
Cigarettes smoke
Red spider allergens
Radiographic contrast
L-tryptophan (eosinophilia-myalgia syndrome)
Drugs
acetaminophen
aspirin
bleomycin
carbamazepine
chlorpromazine
chlorpropamide
cromolyn sodium
dantrolene
daptomycin
gold
hydralazine
loxoprofen
methotrexate
minocycline
naproxen
nitrofurantoin
penicillin
pentamidine
quinidine
salazopyrin
sulfadoxine
sulfasalazine
tamoxifen
tolfenamic acid
Infectious
Parasitic infections
Toxocara canis
Filariasis
Ascaris lumbricoides
Paragonimus westermani
Fungal infections
Aspergillus species
Trichosporon terrestre
Trichosporon cutaneum
Trichoderma viride
Viral/chlamydial infections
HIV
Coxsackie A2 virus
Chlamydia (“pertussoid” eosinophilic pneumonia)
Bacterial infections
Mycobacterium tuberculosis
Pseudomonas maltophilia
Immunologic diseases
Rheumatoid diseases
Sarcoidosis
From references 4, 6, 7, 8.
TABLE 26.3 Criteria for the Clinical Diagnosis of Eosinophilic Pneumonia
The criteria for chronic eosinophilic pneumonia is duration more than 1-2 months, or recurrence. Otherwise the disease is considered acute. ABPA, allergic bronchopulmonary aspergillosis.
aMany or most publications currently accept BAL eosinophilia in the absence of biopsy findings.
Adapted from Matsuda Y, Tachibana K, Sasaki Y, et al. Tracheobronchial lesions in eosinophilic pneumonia. Respir Investig. 2014;52(1):21-27.
Most patients are adults, with a wide range from the third to the eighth decades, with a mean age about 30 years.1,12 There is no apparent gender predilection.12 Over one-half of patients are current or recent smokers.12 A series of three children with acute eosinophilic pneumonia has been reported.10
Acute eosinophilic pneumonia is a febrile illness characterized by diffuse pulmonary infiltrates and eosinophilia in BAL fluid (usually > 25% eosinophils).6 The clinical diagnostic criteria are outlined in Table 26.3. The duration of illness is generally <1 month (classically < 1 week), with severe hypoxemia (usually < 60 mm Hg on room air). Infectious processes exclude the diagnosis of eosinophilic pneumonia.11,12 The mean white count is 22 × 109/L with a mean eosinophil count of 5%.12 Respiratory failure may progress to ventilatory dependence.6 There is typically spontaneous improvement especially to treatment with steroids, without relapses. By definition, if relapses occur, a diagnosis of chronic eosinophilic pneumonia is made.
In contrast to other forms of eosinophilic lung disease, peripheral eosinophil counts are normal in about 50% of patients.1,4,10 A peripheral eosinophil count over 10 × 109/L (10,000/µL) has been reported in a patient with fulminant eosinophilic pneumonia.6
A history of medication use is common; four of five patients in the series of Umeki et al. were on antibiotics.1 A causative association between medication use and eosinophilic lung injury can only be presumed if there is a positive in vitro drug-stimulated lymphocyte transformation test,13 or if there is a positive drug challenge after resolution of symptoms.1 The term “idiopathic” is generally not applied to cases of eosinophilic pneumonia if there is a suspicion of drug reaction.12
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