Eosinophilia overview

# Overview
Eosinophilia refers to elevated eosinophils in the peripheral blood.  It is important to remember that eosinophilia is a symptom, not a disease. Eosinophilia can be a manifestation of a variety of different disease processes, including neoplastic, infectious, allergic, and idiopathic processes.
When the eosinophil count is markedly elevated and the patient has symptoms attributable to the eosinophils (and not explained by the underlying condition, if any), it is referred to as a hypereosinophilic syndrome.  Most patients with eosinophilia do not have a hypereosinophilic syndrome.

# Historical Perspective
- As mentioned above, eosinophilia is not a unified disease entity, but rather a manifestation of many different disease processes.
- The eosinophil itself was discovered in 1879  as a granulocytic cell that stained with the red dye eosin.  Granulocytes in general were described several decades earlier.
- The hypereosinophilic syndrome was first described in 1968. 

# Classification
- Major disease categories that can cause eosinophilia: 
- Allergy/Atopy
- Infections
Parasitic: particularly associated with helminths, most protozoan parasites do not cause eosinophilia (Toxoplasma is an exception)
Certain fungal organisms: Coccidiomycosis
Viral (HIV, HTLV, Herpes)
- Parasitic: particularly associated with helminths, most protozoan parasites do not cause eosinophilia (Toxoplasma is an exception)
- Certain fungal organisms: Coccidiomycosis
- Viral (HIV, HTLV, Herpes)
- Malignancies
Hematologic: primary eosinophil neoplasia, B and T cell lymphomas, CML, Hodgkin's Lymphoma
Solid tumors: breast, lung
- Hematologic: primary eosinophil neoplasia, B and T cell lymphomas, CML, Hodgkin's Lymphoma
- Solid tumors: breast, lung
- Myelodysplastic and myeloproliferative disorders
- Systemic Mastocytosis
- Addison's Disease
- Drug Reactions (DRESS)
- Vasculitis (Churg-Strauss)
- Idiopathic

# Pathophysiology
- Eosinophilia is defined by blood eosinophil count > 500/microL.
- The pathogenesis is variable.  Eosinophila can be caused by a proliferative neoplasm (as in eosinophilic leukemia), or as a reaction to another process (parasitic infection, drug reaction, solid tumor) that causes elevated levels in cytokines that promote eosinophil maturation in the bone marrow, or it may be idiopathic.

# Causes
- Eosinophilia can be caused by numerous different processes listed above.

# Differentiating eosinophilia from other Diseases
- If the eosinophil count is elevated, eosinophilia is present by definition.  Care should be taken to discover the correct cause for the eosinophilia.

# Epidemiology and Demographics
- Eosinophilia is not a specific disease entity, epidemiologic numbers are availble for major causes of eosinophilia
- Hypereosinophilic syndromes are extremely rare, estimated at 0.36 to 6.3 people per 100000 in the US.  Adults are most commonly affected, however children and the elderly can be affected as well.

# Risk Factors
- Common risk factors in the development of eosinophilia are allergies and parasitic infections.  Eosinophilia is most commonly associated with allergic processes in developed countries and with parasitic infections in developing countries.

# Natural History, Complications and Prognosis
- Highly variable based on the underlying etiology.
- The diseases eosinophilia can be associated with have a wide range of natural histories and prognoses.
- An asthmatic patient with eosinophilia has a very different prognosis than a cancer patient with eosinophilia
- Hypereosinophilic syndromes have a poor prognosis and high mortality, mostly from the cardiac effects of hypereosinophilia

# Diagnosis

## Diagnostic Criteria
- The diagnosis of eosinophilia is made when the peripheral blood eosinophil count is greater than 500 cells/microL.
- Hypereosinophilic syndrome usually has eosinphil counts greater than 1500 cells/microL.  Hypereosinophilic syndrome also requires the presence of organ dysfunction that is caused by the eosinophils and cannot be explained by another disease process .

## Symptoms
- Symptoms seen in eosinophilia are highly variable and reflect the underlying disease.
- Hypereosinophilia can cause symptoms due to the release of destructive compounds from eosinophils.  Symptoms attributable to the elevated eosinophil count include:
Dermatologic manifestations insluding rash, urticaria
Muscle and joint pains
Heart failure and endocardial fibrosis
Cough and shortness of breath
Thrombotic and embolic events
Encephalopathy or neuropsychiatric disturbances
Peripheral neuropathies
Abdominal pain, nausea/vomiting, diarrhea, weight loss
Any organ can be affected by hypereosinophilia and manifestations are variable
- Dermatologic manifestations insluding rash, urticaria
- Muscle and joint pains
- Heart failure and endocardial fibrosis
- Cough and shortness of breath
- Thrombotic and embolic events
- Encephalopathy or neuropsychiatric disturbances
- Peripheral neuropathies
- Abdominal pain, nausea/vomiting, diarrhea, weight loss
- Any organ can be affected by hypereosinophilia and manifestations are variable

## Physical Examination
- Physical exam findings in eosinophilia are highly variable and reflect the underlying disease.
- Physical exam findings in a hypereosinophilic syndrome are also highly variable, since the disease can affect any organ system.  Possible manifestations include:
Signs of volume overload related to heart failure: peripheral edema, pulmonary edema, engorged neck veins, etc.
Rash, skin ulcerations, lichenification
Altered mental status
Respiratory distress
- Signs of volume overload related to heart failure: peripheral edema, pulmonary edema, engorged neck veins, etc.
- Rash, skin ulcerations, lichenification
- Altered mental status
- Respiratory distress

## Laboratory Findings
- There are no specific laboratory findings associated with eosinophilia aside from the elevated eosinophil count.
- Anemia and thrombocytopenia or thrombocytosis may be seen if the eosinophilia is related to a clonal proliferative process in the bone marrow. 

## Imaging Findings
- There are no imaging findings associated with eosinophilia unless it is secondary to a disease process that causes imaging findings.

# Treatment

## Medical Therapy
- Medical treatment for eosinophilia should be directed at the underlying cause (for example treating parasitic infection or malignancy).
- Eosinophilia without evidence of organ dysfunction may be observed with monitoring for evidence of organ involvement.
- Treatment of hypereosinophilic syndrome has a number of options including corticosteroids, hydroxyurea, interferon-alpha, imatinib and other tyrosine kinase inhibitors, IL-5 antibodies, and other cytotoxic and immunomodulatory agents.  

## Surgery
- Surgery may be part of the treatment for certain causes of eosinophilia, such as solid organ malignancies.  However most conditions associated with eosinophilia are not treated surgically.

## Prevention
- There are no primary preventive measures available for eosinophilia.
- Primary prevention should be targeted as specific disease entities.