The central nervous system (CNS) includes the brain and accompanying spinal cord. Because the CNS contains fragile and sensitive organs, it is protected by a physiological blood-brain barrier. In addition, the brain and spinal cord are contained in compartments and immersed in a sustaining cerebrospinal fluid (CSF). Although the CSF includes glucose and proteins, abnormalities in the amounts of these can indicate disease.
Blood-Brain Barrier
The blood capillaries that supply the central nervous system are unique in that specialized capillary endothelial cells form tight junctions blocking most substances from entering the CNS. The specialized function of these cells is termed the blood-brain barrier. Large molecules, toxins, many proteins, and other blood products are kept from crossing the barrier. However, transport mechanisms allow vital nutrients such as glucose, vitamins and certain proteins to cross the barrier. Although the protective properties of the blood-brain barrier are usually beneficial, they also prevent the delivery of drugs that could otherwise to be used to treat diseases of the central brain and spinal cord.
Cerebrospinal Fluid
The brain and spinal cord are immersed in a protective fluid called cerebrospinal fluid. The fluid serves as a cushion to absorb physical pressure and keep shock from causing injury. In addition, the CSF contains vital nutrients such as glucose and proteins. Cerebrospinal fluid is produced from blood by specialized ventricles in the brain.
Protein in Cerebrospinal Fluid
According to the Neuropathology Web, the CSF in the lumbar region that contains the spinal cord normally contains 15-45 mg/dL of protein. Since the amount of protein allowed across the blood-brain barrier is highly regulated, abnormalities in the protein content of the CSF may indicate a serious medical problem. Abnormally high levels of protein in the CSF may indicate that blood is spilling into the CSF. Blood leakage occurs when the blood-brain barrier has been compromised either by inflammation or physical injury. Protein levels in the CSF are often increased in conjunction with CNS fungal infections and meningitis.
Glucose in Cerebrospinal Fluid
The CSF surrounding the spinal cord also normally contains 50-80 mg/dL of glucose. Fluctuations in the CSF glucose level can indicate the presence of disease. High traces of glucose in the CSF may be a reflection of higher blood glucose, otherwise known as hyperglycemia. In contrast, lower levels of glucose in the CSF may indicate increased glucose use, either by white blood cells or bacteria. Lower CSF glucose levels are often found in conjunction with infections of the central nervous system.
Spinal Tap
A lumbar puncture, or "spinal tap," is performed by inserting a needle between two vertebrae in the lumbar region of the back. The purpose of this procedure is to collect a sample of cerebrospinal fluid. The sample can then be analyzed to determine whether there are abnormalities in the amounts or types of CSF contents. An abnormality in the amount of glucose or protein in the CSF is not a definitive indicator of disease. These abnormalities may be attributed to a variety of health conditions. Results of the lumbar puncture are often used in conjunction with other laboratory tests and the presence of symptoms to make a diagnosis.
