Tuesday, August 16, 2011

Laboratory Diagnosis of meningitis: Macroscopic examination

Cerebrospinal fluid (CSF): is a clear body fluid that occupies the subarachnoid space and the ventricular system around and inside the brain. Essentially, the brain "floats" in it.More specifically,CSF occupies between the arachnoid mater (the middle layer of the brain cover, meninges) and the pia mater (the layer of the meninges closest to the brain). Moreover , it constitutes the content of all intra-cerebral (inside the brain, cerebrum) ventricles, cisterns and sulci (singular sulcus) as well as the central canal of the spinal cord. It is an

approximately isotonic solution and acts as a "cushion" or buffer for the cortex. Also providing a basic mechanical and immunological protection to the brain inside the skull.

Amount and constitution:
CSF is produced at a rate of 500 ml/day. Since the brain can only contain from 135-150 ml, large amounts are drained primarily into the blood through arachnoid granulations in the superior sagittal sinus. The CSF contains approximately 0.3% plasma proteins or 15 to 40 mg/dL depending on sampling site. CSF pressure ranges from 60 - 100 mmH2O or 4.4 - 7.3 mmHg with most variations due to coughing or internal compression of jugular veins in the neck ( Dixon et al.,2002).

CSF has many putative roles including mechanical protection of the brain, distribution of neuroendocrine factors and prevention of brain ischemia. The prevention of brain ischemia is made by decreasing the amount of CSF in the limited space inside the skull. This decreases total intracranial pressure and facilitates blood perfusion.
(b) - CSF collection :

CSF is usually collected by an experienced medical officer or health worker by lumbar puncture through aseptically inserting a needle into the subarachinoid space usually at the level of lumber spine .The procedure may be dangerous if the intracranial pressure ( ICP) is raised , so the clinician should be check that there is no papilloedema before the proceeding.

About 6 ml is collected in fresh sterile screw-capped container and divided into 2 parts. A part for culture and should be incubated at 37◦c.The other part should be transported on ice and examined for other tests as cell count, microscopically examination and biochemical analysis including measurement of glucose , protein and for antigen detection.

Lumbar puncture should be avoided in patients with depressed levels of consciousness and shock. Contraindications to lumbar puncture include the following: prolonged or focal seizures , focal neurological signs,widespread purpuric or petechial rash ,pupillary dilatation or asymmetry , impaired oculocephalic reflexes , abnormal posture or movement - decerebrate or decorticate movement or cycling , signs of impending brain herniation ( inappropriate low pulse, raised blood pressure, irregular respiration) , coagulation disorder , papilledema and hypertension.

(c) - Storage and transport :

According to Wong et al .,( 2000) the specimens which will be examined for viral detection may be stored by freezing up to 5 days at 4 c and for 6 days or more at -20◦ c after proper mixing in viral transport media. Viral transport media are used to transport small volume of fluid specimen, small tissue, scrapings and swab specimen.These media contain serum , albumin and gelatin to stabilize virus and antimicrobial agents as penicillin,streptomycine and a more potent mixture include vancomycine,gentamycine and amphotricine.CSF is potentially highly infectious and must be handled and transported with great caution .

(I) – Nonspecific discriminatory tests:

A-Macroscopical examination:

Normal CSF is clear and colorless like water. A yellow color "xanthochromic" may result from subarachnoid hemorrhage and traumatic lumbar puncture .

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