General analysis of cerebrospinal fluid

A general analysis of cerebrospinal fluid includes counting the number and composition of formed elements of the blood. Normally, 1 μl of cerebrospinal fluid contains 4-6 cells (lymphocytes). In pathological processes (inflammation of the meninges, encephalitis, volumetric processes, acute cerebral circulatory failure), the number of cellular elements increases. In purulent meningitis, neutrophils appear in large quantities in the cerebrospinal fluid (up to several tens of thousands in 1 μl), in serous meningitis, the number of cells increases from several tens to 1-2 thousand due to lymphocytes. The ratio of neutrophils and lymphocytes is calculated as a percentage (cytogram). The number of erythrocytes is often detected and counted. In addition, eosinophils (in parasitic diseases of the central nervous system), macrophages (in protracted inflammatory processes), and atypical cells (in tumors of the meninges, leukemia) can be detected in the cerebrospinal fluid. If the presence of atypical cells is suspected, a smear of the cerebrospinal fluid is examined by a cytologist. Counting the cytosis and cytogram is not only of diagnostic value, but also allows one to evaluate the effectiveness of antibacterial therapy in bacterial meningitis.

Cytochemical methods are used as additional tests, which allow us to judge the functional state of the cells of the cerebrospinal fluid (determination of the glycogen content and activity of myelopyroxidase in neutrophils, activity of alkaline phosphatase in lymphocytes, etc.).

It is desirable to count cells in the cerebrospinal fluid within 1-2 hours after the puncture. At later stages, the cellular composition may change significantly due to cell lysis, precipitation, and formation of fibrin clots. Since erythrocytes in the cerebrospinal fluid are quickly lysed, they are determined only in the presence of fresh blood in the subarachnoid space: after traumatic punctures, subarachnoid hemorrhages, parenchymatous hemorrhages with penetration of erythrocytes into the cerebrospinal fluid pathways, in venous thrombotic occlusions with swelling of the veins and secondary diffusion of blood cells through the venous wall.

The upper limit of the normal number of leukocytes in the cerebrospinal fluid is 5 in 1 μl. However, some syphilologists consider the upper limit of the norm to be not 5, but 9 cells. A slight pleocytosis of up to 20 in 1 μl is usually observed after pyelography, spinal anesthesia, and stroke. Incomparably more severe changes are observed in infectious diseases of the central nervous system. Acute bacterial meningitis is usually accompanied by a much more pronounced pleocytosis than aseptic meningitis. Thus, in most bacterial meningitis there is a pleocytosis of more than 1000 in 1 μl; however, in the early stages or in the case of partially treated meningitis (!) pleocytosis may be less. In aseptic meningitis, such a high pleocytosis is rare. In cases where pleocytosis is particularly high (5,000-10,000 in 1 μl), in addition to meningitis, a rupture of an intracerebral or perimeningeal abscess may be suspected; in this case, a lightning-fast increase in clinical symptoms is usually observed. An increase in the concentration of polymorphonuclear leukocytes is usually observed in bacterial meningitis. An increased lymphocyte content is usually observed in chronic infections (tuberculous and fungal meningitis), untreated bacterial infections, viral infections, non-infectious inflammatory processes (for example, exacerbation of multiple sclerosis). Eosinophilia is rare and indicates helminthiasis, including cysticercosis, and is also sometimes observed in tuberculous meningitis, CNS lymphomas, and foreign bodies.

Immunological methods

Methods based on the determination of pathogen antigens and antibodies are used. The RLA method is most often used to detect antigens of meningococcus, pneumococcus and Haemophilus influenzae type b. Enzyme-linked immunosorbent assay (ELISA) is used to diagnose tuberculous meningitis; if herpes encephalitis is suspected, specific antibodies are determined in the cerebrospinal fluid.

Polymerase chain reaction (PCR) is widely introduced into practice, as it allows identifying the vast majority of neuroinfection pathogens and, in practical conditions, establishing the etiology of neuroinfection in 90% of patients. The advantages of the method are high sensitivity and specificity, the ability to detect fragments of the pathogen genome during treatment, and determine the microbial load if necessary. To reduce costs, it is recommended to initially conduct a reaction with primers of common pathogens (meningococcus, pneumococcus, Haemophilus influenzae type b, enteroviruses), and then with rarer pathogens (gram-negative bacteria, borrelia, mycobacteria tuberculosis, herpes viruses, viruses - pathogens of childhood droplet infections, etc.). The picture of the cerebrospinal fluid depends on the timing of the study and the treatment.

Cytological examination of cerebrospinal fluid sometimes allows identification of atypical cells present even in minimal quantities. This is a fairly important method for detecting tumor lesions of the central nervous system.

Inflammatory processes accompanied by leukocytosis may also have certain cytological characteristics. Thus, lymphocytes appearing in the cerebrospinal fluid in response to a viral infection may have clearly visible nuclei, due to which they are sometimes confused with malignant cells. Herpes encephalitis may be accompanied by the appearance of large intranuclear inclusions in lymphocytes or ependymocytes; such a finding is pathognomonic. In cryptococcal infections, yeast-like colonies may be detected either in a free state or intracellularly in macrophages. Subarachnoid hemorrhage leads to the appearance of macrophages (erythrophages) stretched by multiple vacuoles. Macrophages are initially filled with erythrocytes and lipid products of their decay, and subsequently with hemosiderin. In some storage diseases, such as Tay-Sachs disease, macrophages with foamy cytoplasm filled with ganglion cell breakdown products are detected. Identification of tumor cells is based on the detection of a number of cytological signs characteristic of the neoplastic process. The reliability of the cytological diagnosis of the tumor is greater, the more neoplastic signs are detected. Most often, cytological studies of the cerebrospinal fluid are used to diagnose CNS damage in acute leukemia and lymphomas, which usually disseminate into the subarachnoid space. Special antibodies against B- and T-lymphocytes are used for immunodiagnostics. Thus, in general inflammatory processes, T-lymphocytes predominate, and in malignant processes, predominant proliferation of pathological clones of B-lymphocytes is observed. Immunohistochemical studies are used to identify a specific form of leukemia. It should be borne in mind, however, that in cases of leukemia, accompanied by the release of pathological cells into the bloodstream, the results of the study of the cerebrospinal fluid may be false positive due to the entry of these cells with the bloodstream. It is important to remember that cytological examination of the cerebrospinal fluid is effective only in those malignant processes in which the meninges are involved. Carcinomatosis of the meninges most often occurs with metastasis of cancerous tumors of the lung, mammary gland, abdominal cavity and melanoma.

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