TREATMENT Autoimmune Hemolytic Anemia
Severe acute AIHA can be a medical emergency. The immediate treatment almost invariably includes transfusion of red cells. This may pose a special problem because, if the antibody involved is nonspecific, all of the blood units cross-matched will be incompatible. In these cases, it is often correct, paradoxically, to transfuse incompatible blood, with the rationale being that the transfused red cells will be destroyed no less but no more than the patient’s own red cells, but in the meantime, the patient stays alive. A situation like this requires close liaison and understanding between the clinical unit treating the patient and the blood transfusion/serology lab. Whenever the anemia is not immediately life-threatening, blood transfusion should be withheld (because compatibility problems may increase with each unit of blood transfused), and medical treatment started immediately with prednisone (1 mg/kg per day), which will produce a remission promptly in at least one-half of patients. Rituximab (anti-CD20) was regarded as second-line treatment, but it is increasingly likely that a relatively low dose (100 mg/wk × 4) of rituximab together with prednisone will become a first-line standard. It is especially encouraging that this approach seems to reduce the rate of relapse, a common occurrence in AIHA. For patients who do relapse or are refractory to medical treatment, one may have to consider splenectomy, which, although it does not cure the disease, can produce significant benefit by removing a major site of hemolysis, thus improving the anemia and/or reducing the need for other therapies (e.g., the dose of prednisone). Since the introduction of rituximab, azathioprine, cyclophosphamide, cyclosporine, and intravenous immunoglobulin have become second- or third-line agents. In very rare severe refractory cases, either autologous or allogeneic hematopoietic stem cell transplantation may have to be considered.
Paroxysmal cold hemoglobinuria (PCH)
PCH is a rather rare form of AIHA occurring mostly in children, usually triggered by a viral infection, usually self-limited, and characterized by the involvement of the so-called Donath-Landsteiner antibody. In vitro, this antibody has unique serologic features; it has anti-P specificity and binds to red cells only at a low temperature (optimally at 4°C), but when the temperature is shifted to 37°C, lysis of red cells takes place in the presence of complement. Consequently, in vivo there is intravascular hemolysis, resulting in hemoglobinuria. Clinically the differential diagnosis must include other causes of hemoglobinuria (Table 10-6), but the presence of the Donath-Landsteiner antibody will prove PCH. Active supportive treatment, including blood transfusion, is needed to control the anemia; subsequently, recovery is the rule.
Cold agglutinin disease (CAD)
This designation is used for a form of chronic AIHA that usually affects the elderly and has special clinical and pathologic features. First, the term cold refers to the fact that the autoantibody involved reacts with red cells poorly or not at all at 37°C, whereas it reacts strongly at lower temperatures. As a result, hemolysis is more prominent the more the body is exposed to the cold. The antibody is usually IgM; usually it has an anti-I specificity (the I antigen is present on the red cells of almost everybody), and it may have a very high titer (1:100,000 or more has been observed). Second, the antibody is produced by an expanded clone of B lymphocytes, and sometimes its concentration in the plasma is high enough to show up as a spike in plasma protein electrophoresis, i.e., as a monoclonal gammopathy. Third, because the antibody is IgM, CAD is related to Waldenström’s macroglobulinemia (WM) (Chap. 18), although in most cases, the other clinical features of this disease are not present. Thus, CAD must be regarded as a form of WM (i.e., as a low-grade mature B cell lymphoma) that manifests at an earlier stage precisely because the unique biologic properties of the IgM that it produces give the clinical picture of chronic HA.
In mild forms of CAD, avoidance of exposure to cold may be all that is needed to enable the patient to have a reasonably comfortable quality of life; but in more severe forms, the management of CAD is not easy. Blood transfusion is not very effective because donor red cells are I positive and will be rapidly removed. Immunosuppressive/cytotoxic treatment with azathioprine or cyclophosphamide can reduce the antibody titer, but clinical efficacy is limited, and in view of the chronic nature of the disease, the side effects may prove unacceptable. Unlike in AIHA, prednisone and splenectomy are ineffective. Plasma exchange will remove antibody and is, therefore, in theory, a rational approach, but it is laborious and must be carried out at frequent intervals if it is to be beneficial. The management of CAD has changed significantly with the advent of rituximab; although its impact on CAD is not as great as on AIHA, up to 60% of patients respond, and remissions may be more durable with a rituximab-fludarabine combination. Given the long clinical course of CAD, it remains to be seen with what schedule or periodicity these agents will need to be administered.
A number of chemicals with oxidative potential, whether medicinal or not, can cause hemolysis even in people who are not G6PD deficient (see above). Examples are hyperbaric oxygen (or 100% oxygen), nitrates, chlorates, methylene blue, dapsone, cisplatin, and numerous aromatic (cyclic) compounds. Other chemicals may be hemolytic through nonoxidative, largely unknown mechanisms; examples include arsine, stibine, copper, and lead. The HA caused by lead poisoning is characterized by basophilic stippling; it is in fact a phenocopy of that seen in P5N deficiency (see above), suggesting it is mediated at least in part by lead inhibiting this enzyme.
In these cases, hemolysis appears to be mediated by a direct chemical action on red cells. But drugs can cause hemolysis through at least two other mechanisms. (1) A drug can behave as a hapten and induce antibody production; in rare subjects, this happens, for instance, with penicillin. Upon a subsequent exposure, red cells are caught, as innocent bystanders, in the reaction between penicillin and antipenicillin antibodies. Hemolysis will subside as soon as penicillin administration is stopped. (2) A drug can trigger, perhaps through mimicry, the production of an antibody against a red cell antigen. The best known example is methyldopa, an antihypertensive agent no longer in use, which in a small fraction of patients stimulated the production of the Rhesus antibody anti-e. In patients who have this antigen, the anti-e is a true autoantibody, which then causes an autoimmune HA (see below). Usually this will gradually subside once methyldopa is discontinued.
Severe intravascular hemolysis can be caused by the venom of certain snakes (cobras and vipers), and HA can also follow spider bites.
Paroxysmal nocturnal hemoglobinuria (PNH)
PNH is an acquired chronic HA characterized by persistent intravascular hemolysis subject to recurrent exacerbations. In addition to hemolysis, there is often pancytopenia and a distinct tendency to venous thrombosis. This triad makes PNH a truly unique clinical condition; however, when not all of these three features are manifest on presentation, the diagnosis is often delayed, although it can always be made by appropriate laboratory investigations (see below).
PNH has about the same frequency in men and women and is encountered in all populations throughout the world, but it is a rare disease; its prevalence is estimated to be approximately 5 per million (it may be somewhat less rare in Southeast Asia and in the Far East). There is no evidence of inherited susceptibility. PNH has never been reported as a congenital disease, but it can present in small children or as late as in the seventies, although most patients are young adults.
The patient may seek medical attention because, one morning, she or he passed blood instead of urine (Fig. 10-9). This distressing or frightening event may be regarded as the classical presentation; however, more frequently, this symptom is not noticed or is suppressed. Indeed, the patient often presents simply as a problem in the differential diagnosis of anemia, whether symptomatic or discovered incidentally. Sometimes, the anemia is associated from the outset with neutropenia, thrombocytopenia, or both, thus signaling an element of bone marrow failure (see below). Some patients may present with recurrent attacks of severe abdominal pain defying a specific diagnosis and eventually found to be related to thrombosis. When thrombosis affects the hepatic veins, it may produce acute hepatomegaly and ascites, i.e., a full-fledged Budd-Chiari syndrome, which, in the absence of liver disease, ought to raise the suspicion of PNH.
Consecutive urine samples from a patient with paroxysmal nocturnal hemoglobinuria (PNH). The variation in the severity of hemoglobinuria within hours is probably unique to this condition.
The natural history of PNH can extend over decades. Without treatment, the median survival is estimated to be about 8–10 years; in the past, the most common cause of death has been venous thrombosis, followed by infection secondary to severe neutropenia and hemorrhage secondary to severe thrombocytopenia. Rarely (estimated 1–2% of all cases), PNH may terminate in acute myeloid leukemia. On the other hand, full spontaneous recovery from PNH has been documented, albeit rarely.
Laboratory investigations and diagnosis
The most consistent blood finding is anemia, which may range from mild to moderate to very severe. The anemia is usually normomacrocytic, with unremarkable red cell morphology. If the MCV is high, it is usually largely accounted for by reticulocytosis, which may be quite marked (up to 20%, or up to 400,000/μL). The anemia may become microcytic if the patient is allowed to become iron deficient as a result of chronic urinary blood loss through hemoglobinuria. Unconjugated bilirubin is mildly or moderately elevated; LDH is typically markedly elevated (values in the thousands are common); and haptoglobin is usually undetectable. All of these findings make the diagnosis of hemolytic anemia compelling. Hemoglobinuria may be overt in a random urine sample; if it is not, it may be helpful to obtain serial urine samples, because hemoglobinuria can vary dramatically from day to day and even from hour to hour. The bone marrow is usually cellular, with marked to massive erythroid hyperplasia, often with mild to moderate dyserythropoietic features (not to be confused with myelodysplastic syndrome). At some stage of the disease, the marrow may become hypocellular or even frankly aplastic (see below).
The definitive diagnosis of PNH must be based on the demonstration that a substantial proportion of the patient’s red cells have an increased susceptibility to complement (C), due to the deficiency on their surface of proteins (particularly CD59 and CD55) that normally protect the red cells from activated C. The sucrose hemolysis test is unreliable; in contrast, the acidified serum (Ham) test is highly reliable but is carried out only in a few labs. The gold standard today is flow cytometry, which can be carried out on granulocytes as well as on red cells. A bimodal distribution of cells, with a discrete population that is CD59 and CD55 negative, is diagnostic of PNH. In PNH patients, this population is at least 5% of the total red cells and at least 20% of the total granulocytes.
Hemolysis in PNH is mainly intravascular and is due to an intrinsic abnormality of the red cell, which makes it exquisitely sensitive to activated C, whether it is activated through the alternative pathway or through an antigen-antibody reaction. The former mechanism is mainly responsible for chronic hemolysis in PNH; the latter explains why the hemolysis can be dramatically exacerbated in the course of a viral or bacterial infection. Hypersusceptibility to C is due to deficiency of several protective membrane proteins (Fig. 10-10), of which CD59 is the most important, because it hinders the insertion into the membrane of C9 polymers. The molecular basis for the deficiency of these proteins has been pinpointed not to a defect in any of the respective genes, but rather to the shortage of a unique glycolipid molecule, GPI (Fig. 10-2), which, through a peptide bond, anchors these proteins to the surface membrane of cells. The shortage of GPI is due in turn to a mutation in an X-linked gene, called PIG-A, required for an early step in GPI biosynthesis. In virtually each patient, the PIG-A mutation is different. This is not surprising, because these mutations are not inherited; rather, each one takes place de novo in a hemopoietic stem cell (i.e., they are somatic mutations). As a result, the patient’s marrow is a mosaic of mutant and nonmutant cells, and the peripheral blood always contains both PNH cells and normal (non-PNH) cells. Thrombosis is one of the most immediately life-threatening complications of PNH and yet one of the least understood in its pathogenesis. It could be that deficiency of CD59 on the PNH platelet causes inappropriate platelet activation; however, other mechanisms are possible.
The complement cascade and the fate of red cells. A. Normal red cells are protected from complement activation and subsequent hemolysis by CD55 and CD59. These two proteins, being GPI-linked, are missing from the surface of PNH red cells as a result of a somatic mutation of the X-linked PIG-A gene that encodes a protein required for an early step of the GPI molecule biosynthesis. B. In the steady state, PNH erythrocytes suffer from spontaneous (tick-over) complement activation, with consequent intravascular hemolysis through formation of the membrane attack complex (MAC); when extra complement is activated through the classical pathway, an exacerbation of hemolysis will result. C. On eculizumab, PNH erythrocytes are protected from hemolysis from the inhibition of C5 cleavage; however, upstream complement activation may lead to C3 opsonization and possible extravascular hemolysis. GPI, glycosylphosphatidylinositol; PNH, paroxysmal nocturnal hemoglobinuria. (From L Luzzatto et al: Haematologica 95:523, 2010.)
Bone marrow failure (Bmf) and relationship between Pnh and aplastic anemia (AA)
It is not unusual that patients with firmly established PNH have a previous history of well-documented AA; indeed, BMF preceding overt PNH is probably the rule rather than the exception. On the other hand, sometimes a patient with PNH becomes less hemolytic and more pancytopenic and ultimately has the clinical picture of AA. Because AA is probably an organ-specific autoimmune disease, in which T cells cause damage to hematopoietic stem cells, the same may be true of PNH, with the specific proviso that the damage spares PNH stem cells. PIG-A mutations can be demonstrated in normal people, and there is evidence from mouse models that PNH stem cells do not expand when the rest of the bone marrow is normal. Thus, we can visualize PNH as always having two components: failure of normal hematopoiesis and massive expansion of a PNH clone. Findings supporting this notion include skewing of the T cell repertoire and the demonstration of GPI-reactive T cells in patients with PNH.