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Acute lymphocytic leukemia (ALL)—also known as acute lymphoblastic leukemia—is a malignant disease caused by the abnormal growth and development of early nongranular white blood cells, or lymphocytes. The leukemia originates in the blast cells of the bone marrow (B-cells), thymus (T-cells), and lymph nodes. ALL occurs predominantly in children, peaking at 4 years of age. ALL is seen more frequently in industrialized nations, and it is slightly more common among white children and boys.

ALL often is diagnosed after a patient experiences a 4- to 6-week period of illness. Initial symptoms may include a nonspecific infection (e.g., respiratory infection) that persists or recurs despite antibiotic therapy. During this period, the person may start to experience aching bone pain in the back, limbs, and/or joints. Walking difficulties may be seen in some children who have extreme swelling of the large joints. But the symptoms that most often suggest referral for a blood count (measure of the number of blood cells within the blood) are a purplish-brown rash or the onset of excessive bruising.

If ALL is T-cell in type, the thymus is involved. Leukemia-related enlargement of the thymus may lead to coughing, shortness of breath, or compression of the superior vena cava (SVC), the large vein that carries blood from the head and arms back to the heart). Such venous blockage may induce head and arm swelling and may cause a life-threatening condition known as SVC syndrome.

Both children and adults with ALL are at risk of developing complications due to central nervous system (CNS) involvement. CNS invasion is especially likely among patients with the L3 subtype of ALL. When leukemic cells infiltrate the CNS, they can cause increased pressure within the skull and paralysis of cranial nerves that connect the brain with other organs, muscles, etc.

Age is an important prognostic factor in ALL. Studies have suggested that patients who are younger than 35 years of age fare better than older patients; however, this observation may be related to the higher incidence of the Philadelphia chromosome (Ph1) among older ALL patients - a subgroup that has a poorer chance of survival. Fortunately, though, 60% to 80% of children and adults with ALL will achieve complete remission of the disease after completing appropriate therapy

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There is no standard staging system for ALL. Rather, ALL is categorized according to a system known as the French-American-British (FAB) Morphological Classification Scheme for ALL:

L1—Mature-appearing lymphoblasts (T-cells or pre-B-cells). Cells are small with uniform genetic material, regular nuclear shape, nonvisible nucleoli (round bodies within the nucleus, the site of RNA synthesis), and little cytoplasm (substance of a cell, excluding the nucleus).
L2—Immature and pleomorphic (variously shaped) lymphoblasts (T-cells or pre-B-cells). Cells are large and variable in size, with variable genetic material, irregular nuclear shape, one or more large nucleoli, and variable cytoplasm.
L3—Lymphoblasts (B-cells; Burkitt's cells) are large and uniform; genetic material is finely stippled and uniform; nuclear shape is regular (oval to round); there are one or more prominent nucleoli; and cytoplasm is moderately abundant.