Until this point, only a few genomic imbalances had been found in the hairy cells, such as trisomy 5 had been found. The expression of genes is also dysregulated in a complex and specific pattern. The cells underexpress 3p24, 3p21, 3q13.3-q22, 4p16, 11q23, 14q22-q24, 15q21-q22, 15q24-q25, and 17q22-q24 and overexpress 13q31 and Xq13.3-q21. It has not yet been demonstrated that any of these changes have any practical significance to the patient. Diagnosis
The diagnosis of HCL may be suggested by abnormal results on a complete blood count (CBC), but additional testing is necessary to confirm the diagnosis. A CBC normally shows low counts for white blood cells, red blood cells, and platelets in HCL patients. However, if large numbers of hairy cells are in the blood stream, then normal or even high lymphocyte counts may be found. On physical examination, 80–90% of patients have an enlarged spleen, which can be massive. This is less likely among patients who are diagnosed at an early stage. Peripheral lymphadenopathy (enlarged lymph nodes) is uncommon (less than 5% of patients), but abdominal lymphadenopathy is a relatively common finding on computed tomography scans. The most important laboratory finding is the presence of hairy cells in the bloodstream. Hairy cells are abnormal white blood cells with hair-like projections of cytoplasm; they can be seen by examining a blood smear or bone marrow biopsy specimen. The blood film examination is done by staining the blood cells with Wrights stain and looking at them under a microscope.
Fourth cranial nerve palsy or trochlear nerve palsy, is a condition affecting cranial nerve 4 (IV), the trochlear nerve, which is one of the cranial nerves. It causes weakness or paralysis of the superior oblique muscle that it innervates. This condition often causes vertical  or near vertical double vision as the weakened muscle prevents the eyes from moving in the same direction together. Because the trochlear nerve is the thinnest and has the longest intracranial course of the cranial nerves, it is particularly vulnerable to traumatic injury. To compensate for the double-vision resulting from the weakness of the superior oblique, patients characteristically tilt their head down and to the side opposite the affected muscle. When present at birth, it is known as congenital fourth nerve palsy. See also
Harada–Ito procedure

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