The normal cerebro-spinal fluid is clear and colourless, has a specific gravity of 1004–1008, and contains a trace of serum globulin and albumose, some chlorides, and a substance which reduces Fehling's solution. Microscopically, it may contain some large endothelial cells and a few lymphocytes, or may be entirely devoid of cells. It does not contain the antitoxins and opsonins which are normally found in the plasma and lymph, hence the liability to infective meningitis after injuries and operations on the central nervous system. With a view to diminishing these risks, hexamine, which is excreted into the cerebro-spinal fluid, is administered for its antiseptic properties in cases of head injury and before intra-cranial operations.
Diagnostic Puncture.—Examination of the fluid withdrawn has proved useful in diagnosis in cases of intra-cranial and intra-spinal hæmorrhage, in various forms of meningitis, in cerebral abscess, and in some cases of cerebral tumour.
The first few drops should be discarded, as they may be stained with blood from the puncture, and about 5 c.c. collected in each of two sterile tubes. To determine whether blood in the fluid is due to the puncture or to a pre-existing intra-cranial or intra-thecal hæmorrhage, the fluid should be centrifugalised; in the former case the supernatant fluid is clear and limpid, in the latter it retains a yellow tinge. In extra-dural hæmorrhage there is no blood in the cerebro-spinal fluid.
In acute meningitis the fluid is turbid, and contains an excess of albumin. Organisms also are present, such as the diplococcus intracellularis in acute cerebro-spinal meningitis; staphylococci, streptococci, and pneumococci, particularly in the intra-cranial complications of middle ear disease. In all cases of acute microbic infection, and especially in the suppurative forms, polynuclear leucocytes are found in the fluid; while in chronic affections, such as tubercle and syphilis, there is an excess of lymphocytes (Purves Stewart). The detection of the tubercle bacillus is confirmatory of a diagnosis of tuberculous meningitis, but, as it is often difficult to find, its absence does not negative this diagnosis. In tuberculous meningitis the clot which forms floats in the centre of the fluid, and is translucent, grey, and flaky; in the pyogenic forms it is yellow, and sticks to the side of the vessel.
In a few cases of malignant tumour of the spinal cord and its membranes, characteristic cells have been found in the fluid after centrifugalising.
In uræmia there is a diminution of chlorides, and an increase of phosphates and sulphates.
The Wasserman test is sometimes positive in the cerebro-spinal fluid, when it is negative in the blood.
Therapeutic Puncture.—In certain cases of cerebral tumour, and of tuberculous meningitis associated with an excessive quantity of fluid in the arachno-pial space, temporary relief of such symptoms of increased intra-cranial tension as headache, vertigo, blindness, or coma, has followed the withdrawal of from 30 to 40 c.cm. of the fluid. Terrier and others have found this measure useful in relieving pain in the head, delirium, and even coma, in cases of basal fracture. Carrière has found it beneficial in some cases of uræmia. The quantity withdrawn must not exceed 40 c.cm., lest the ventricles be emptied and pressure be exerted directly on the basal ganglia (Tuffier). In a number of cases sudden death has followed the withdrawal of cerebro-spinal fluid.
This route is sometimes selected for the induction of spinal anæsthesia, and for the injection of antitoxin in cases of tetanus.