Immune Mediated Haemolytic Anaemia

Also known as: IMHA — Autoimmune haemolytic anaemia (AIHA) — Pure red cell aplasia (PRCA)

Introduction

Immune-mediated haemolytic anaemia (IMHA) is the result of a type II antibody-mediated immune response directed against molecules expressed on the surface of erythrocytes. The clinical presentation of the disease depends on the isotype of antibody produced and the severity of the anaemia.

IgM antibodies are capable of fixing complement on the surface of red blood cells leading to the assembly of a membrane attack complex. This complex causes the direct lysis of erythrocytes and intravascular haemolysis. Since IgM antibodies have a high avidity, they are also able to co-ordinate the formation of large numbers of red blood cells into aggregates, a phenomenon known as auto-agglutination.

Some types of IgG antibody are able to directly activate the complement cascade but, in most cases, these antibodies are not able to cause intravascular haemolysis of agglutination and they are therefore described as incomplete antibodies. These antibodies act as opsonins and, through their interaction with Fc receptors expressed by cells of the hepatosplenic monocyte-phagocyte system (MPS), they promote the uptake and destruction of the red blood cells to which they are bound. These types of antibody therefore cause extravascular haemolysis.

IMHA may occur as a primary disease with no apparent cause or it may be secondary to another systemic insult. Possible secondary causes of IMHA include bacterial and parasite infections (including Babesia canis in dogs and Mycoplasma haemofelis in cats), adverse drug reactions, neoplasia (especially myeloproliferative and lymphoproliferative disease) and live vaccines, although the association between vaccination and immune-mediated disease remains controversial.

The majority of cases of IMHA affect only the circulating red blood cells resulting in a strongly regenerative anaemia as the bone marrow stem cells respond to the disease. In a small number of cases, antibodies are produced that affect the stem cells of the erythroid lineage in the bone marrow, resulting in a non-regenerative anaemia that still bears many of the same clinical features as IMHA. Although the two diseases have been considered separately in the past, they really represent two ends of a spectrum of immune-mediated disease directed at cells of the erythroid line.

The widespread lysis of red blood cells causes disease in the following ways:

• Blood oxygen carrying capacity is greatly reduced causing exercise intolerance, collapse and tissue hypoxia.
• The release of endogenous procoagulant molecules from lysed cells increases the risk of thromboembolism in various tissues, particularly the lungs, spleen and liver.

Signalment

Primary IMHA occurs with greater frequency in Cocker spaniels, Old English sheepdogs and standard Poodles but any breed of dog may be affected. Middle-aged, entire female animals have been shown to be at increased risk of developing the disease in some studies.

Diagnosis

Clinical Signs

Affected animals often present acutely with sudden-onset collapse or severe exercise intolerance. On examination, the following signs may become evident:

• Pallor and/or icterus of the mucous membranes as the release of bilirubin from lysed red blood cells may result in pre-hepatic jaundice in severely affected animals.
• A haemic heart murmur and a hyperdynamic peripheral pulse due to the reduction in viscosity of the blood.
• Hepatosplenomegaly may be apparent in cases of extravascular haemolysis where the activity of the MPS is greatly increased.
• Tachypnoea frequently occurs as animals attempt to compensate for reduced tissue oxygenation and blood oxygen carrying capacity.
• Dyspnoea may occur in animals which develop pulmonary thromboemboli or disseminated intravascular coagulation.

Laboratory Tests

On presentation, full biochemical and haematological analysis of blood samples are indicated to confirm the diagnosis and to obtain a baseline measurement to assess the efficacy of future treatment. Several tests are also available that have a higher specificity for the diagnosis of IMHA.

Haematology

Affected animals have a reduced packed cell volume (PCV) or haematocrit (HCT) and often a reduced haemoglobin concentration. IMHA causes a strongly regenerative anaemia and evidence of macrocytosis (increased MCV) should be apparent after 48-72 hours in dogs. A blood smear is extremely useful in evaluating cases of IMHA as spherocytes are often visible. These small, dense red blood cells are formed due to partial phagocytosis of red blood cells by the MPS. Polychromasia should also be visible on a blood smear from an animal undergoing regeneration and reticulocytosis can be confirmed using a supravital stain such as new methylene blue.

Reactive thrombocytosis and leucocytosis may be present with any cause of anaemia.

Biochemistry

Serum bilirubin concentration will be elevated and this may be sufficiently severe to cause icterus. The serum urea concentration may also be elevated as the kidneys receive less oxygen than normal causing a pre-renal azotaemia. A similar process of tissue hypoxia may also result in elevations in liver enzymes such as ALT and AST.

Other Tests

An in-saline agglutination test may be used to diagnose cases of IMHA that involve auto-agglutination. A drop of whole blood is mixed with a drop of plain saline on a glass slide and agitated for 30-60 seconds. A positive result is recorded if evident aggregates form but the slide should be evaluated under a microscope as rouleaux formation may result in a similar gross appearance.

A Coomb's test can be used to diagnose cases of IMHA that are caused by incomplete antibodies. The red blood cells from a patient are mixed with Coomb's antiserum (IgG antiobies directed against IgG) and, in cases where the patient has IMHA with antibodies attached to the surface of the erythrocytes, the antiserum will result in the formation of aggregates of cells. The titre of the test should be evaluated as weakly positive results may occur with other diseases.

In cases of pure red cell aplasia, a Coomb's test may still be positive but, experimentally, a definitive diagnosis can only be made by transfusing the serum of one animal to another and documenting the development of anaemia in the recipient.

A diagnosis of PRCA is made more easily by examining bone marrow aspirates (or core biopsy) for evidence of erythroid hypoplasia and a reduction in the erythroid: myeloid ratio of the marrow stem cells.

Diagnostic Imaging

Imaging may be indicated to rule out other potential causes of the signs observed but it is not necessary to make a diagnosis of IMHA. Hepatosplenomegaly will be the major findings on both radiographs and ultrasound scans.

Treatment

Affected animals often present acutely and may require intensive care. The ultimate aim of long-term treatment for IMHA is to control the autoimmune response.

Stabilisation

In animals that have lost a large percentage of their PCV acutely, it is likely that a blood transfusion will be required. Since animals with IMHA are not usually hypovolaemic, packed red blood cells are often transfused to replace erythrocytes without significantly expanding the plasma volume. In emergency situations, whole blood or synthetic haemoglobin molecules (such as Oxyglobin [tm]) may be used to support the oxygen carrying capacity of the patient. Oxygen may be provided to tachypnoeic and dyspnoeic patients by facemask, nasal catheter or flow-by.

Immunosuppressive Therapy

Whatever supportive measures are taken, the autoimmune response must be controlled to prevent the continuing lysis of red blood cells. The following types of drug are typically used to achieve this goal:

• Corticosteroids such as prednisolone and dexamethasone are used universally as a first-line treatment for IMHA as they are frequently effective, act rapidly and are available in a variety of preparations. Traditionally, very high doses of corticosteroids have been used to try to control the autoimmune response but dose rates beyond 2-3 mg/kg/day may be associated with significant adverse effects. More recently, there has been a move to add further immunosuppressive agents (see below) in a polypharmaceutical approach which allows the clinician to keep the corticosteroid dose rate at a reasonable level. Corticosteroids act to control both the cell- and antibody-mediated immune responses.
• Azathioprine is frequently used in the management of IMHA and it has effects on both cell- and antibody-mediated immune responses. It may take 3-4 weeks of treatment before the patient experiences the maximal effects of the drug. Azathioprine is a cytotoxic drug and gloves should be worn to administer tablets. Owners or keepers should be made aware that the active drug or its metabolites may be present in the saliva and other secretions of animals receiving the drug.
• Ciclosporin A is a fungal metabolite that inhibits a signaling molecule (ciclophilin) involved in T cell activation. Since B cells require T cell help to become activated, differentiate into plasma cells and produce antibodies, there is a rationale for the use of ciclosporin in cases of IMHA but there are currently no reports of its relative efficacy compared to the other immunosuppressive drugs.
• Other less commonly-used drugs include danazol (a steroid related to testosterone), mycophenolate mofetil and cyclophosphamide. Cyclophosphamide was used widely in the management of IMHA but two separate studies have shown that its use is associated with higher rates of mortality.
• Human gamma globulin is a variably popular product that is thought to act by occupying Fc receptors on cells of the MPS and therefore preventing phagocytosis of opsonised red blood cells. Studies investigating the effects of this drug have produced variable results, with some showing that it makes little difference to outcome and others suggesting that it may be useful in the short-term control of cases that are refractory to other immunosuppressive regimes. Human gamma globulin is not widely available in veterinary practices and it is very expensive.
• Due to the risk of thrombo-embolism caused by the release of endogenous procoagulant molecules, low doses of heparin or aspirin are often administered to dogs with IMHA to try to prevent the formation of thrombi in the lungs, liver or spleen. There is some evidence to suggest that, if a dose of heparin is used which is individualised for a particular patient, there may be beneficial effects on survival. This effect is not observed if a constant dose of heparin is used for all patients.

Adjunctive Therapy

Animals with IMHA frequently suffer from concurrent vomiting, regurgitation and diarrhoea while hospitalised. These conditions are frequently managed with gastro-protectant drugs such as sucralfate, ranitidine and omeprazole to prevent the development of gastro-duodenal ulceration and oesophagitis.

Antibiotics are frequently administered to animals which present with acute haemolytic crises but these should be used judiciously on a case-by-case basis.

Prognosis

Most mortality occurs in the first two weeks after presentation and, overall, 30-50% of animals would be expected to survive for at least one year after initial treatment for IMHA.

Literature Search

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