GBS is a rare disorder where the body’s immune system mistakenly attacks the peripheral nervous system (nerves outside the brain and spinal cord). This autoimmune attack leads to muscle weakness and sometimes paralysis that develops rapidly over days or weeks. Since symptoms can mimic other serious conditions, diagnosis relies on clinical signs and specific laboratory tests. Analysis of the Cerebrospinal Fluid (CSF) is an important procedure for confirming this neurological condition.
The Mechanics of Guillain-Barré Syndrome
GBS is classified as an acute inflammatory polyradiculoneuropathy, meaning it involves widespread inflammation of the nerve roots and peripheral nerves. The syndrome is thought to be triggered by an antecedent infection, such as a respiratory illness or gastrointestinal infection, which confuses the immune system. This immune response instead begins to attack components of the patient’s own nerves through a process called molecular mimicry.
The damage primarily targets the myelin sheath, the fatty layer that insulates the nerve’s axon and allows for rapid signal transmission. Immune cells called macrophages infiltrate the nerve roots and strip the myelin away, a process called demyelination. This damage slows down or blocks the nerve signals traveling to the muscles, resulting in the characteristic rapid onset of weakness and sensory changes. The resulting weakness is typically symmetrical, affecting both sides of the body equally.
The Cerebrospinal Fluid Collection Procedure
Cerebrospinal fluid (CSF) is a clear, protective fluid that surrounds the brain and spinal cord, acting as a cushion against injury. To analyze the CSF, a procedure known as a lumbar puncture (LP), or spinal tap, is performed. The LP is conducted in the lower back, where a thin, hollow needle is inserted between two lower vertebrae, usually the third and fourth lumbar bones.
The needle is carefully guided into the subarachnoid space, the area containing the CSF, where a small amount of fluid is collected for laboratory testing. Patients are typically positioned lying on their side with their knees drawn toward their chest or sitting and bent forward to help open the spaces between the vertebrae. A local anesthetic is used to numb the area before the larger needle is inserted.
Once the CSF is collected, the needle is removed and a bandage is applied to the site. Post-procedure care often includes resting in a flat position for a period of time to help minimize the risk of a post-dural puncture headache, a potential side effect. The collected CSF sample is then sent to a lab for analysis of its composition, including protein levels and cell counts.
Interpreting the Spinal Fluid Results
The characteristic finding in the CSF of a patient with GBS is a pattern known as “albuminocytologic dissociation.” This refers to significantly elevated protein levels in the fluid combined with a white blood cell (WBC) count that is normal or only minimally increased. The concentration of protein, particularly albumin, in the CSF is usually greater than 0.55 grams per liter.
This high protein level occurs because the immune attack damages the nerve roots, which are part of the peripheral nervous system and traverse the subarachnoid space. The inflammation and resulting damage disrupt the blood-nerve barrier, allowing large protein molecules from the bloodstream to leak into the CSF. The protein level often correlates with the severity and duration of the disease.
The second half of the dissociation is the low white blood cell count, typically fewer than 10 cells per cubic millimeter. This finding is important because it helps distinguish GBS from infectious diseases like bacterial meningitis or viral encephalitis, which would show a high WBC count (pleocytosis). A markedly elevated cell count, such as over 50 cells per cubic millimeter, usually suggests an alternative diagnosis.
The protein elevation is time-dependent and may not be present immediately at the onset of symptoms. Only about 50% of patients show elevated protein within the first three days, and this percentage increases to approximately 80% after the first week. Therefore, a normal CSF protein level early in the disease course does not rule out GBS, and the test is often used primarily to exclude other causes of acute weakness.