The clinical diagnosis of Guillain-Barré syndrome (GBS) is confirmed through cerebrospinal fluid (CSF) analysis via lumbar puncture and nerve conduction studies (NCS).
Here's a breakdown of each test:
Cerebrospinal Fluid (CSF) Analysis (Lumbar Puncture)
A lumbar puncture, often referred to as a spinal tap, is crucial in diagnosing GBS. The characteristic finding in CSF analysis is albuminocytologic dissociation. This means there's an elevated protein level in the CSF without a corresponding increase in white blood cell count (or a normal white blood cell count). It's important to note that this finding might not be present in the early stages of the disease (first week).
- Procedure: A needle is inserted into the lower back to collect a sample of CSF.
- What it reveals: High protein levels, typically above 45 mg/dL, with a normal or slightly elevated white blood cell count. Other conditions can mimic GBS; CSF analysis can help to rule these out.
- Importance: Essential for confirming the diagnosis, particularly when combined with NCS results. Lumbar puncture is generally indicated in every suspected case of GBS.
Nerve Conduction Studies (NCS)
Nerve conduction studies measure the speed and strength of electrical signals traveling along nerves. They help identify abnormalities in nerve function, which are characteristic of GBS.
- Procedure: Small electrodes are placed on the skin over specific nerves. A mild electrical impulse stimulates the nerve, and the electrodes record the nerve's response.
- What it reveals: NCS can reveal slowed nerve conduction velocity, prolonged distal latencies, reduced compound muscle action potential (CMAP) amplitudes, and conduction block, depending on the GBS subtype. These findings indicate demyelination (damage to the myelin sheath that insulates the nerves).
- Importance: Helps to differentiate GBS from other neurological conditions and to classify the subtype of GBS (e.g., AIDP, AMAN, AMSAN). It can also help assess the severity and prognosis. NCS findings may be normal early in the disease course (first week).
Combining CSF Analysis and NCS
While either test alone can suggest GBS, combining the results of both CSF analysis and NCS significantly increases diagnostic accuracy. The results are considered together with the patient's clinical presentation (progressive weakness, areflexia) to establish the diagnosis.
In summary, to confirm a diagnosis of GBS, clinicians rely on both cerebrospinal fluid analysis (specifically looking for albuminocytologic dissociation) and nerve conduction studies (detecting nerve damage). Lumbar puncture is considered necessary in nearly all suspected cases.
