What is cerebrospinal fluid testing?

Cerebrospinal fluid (CSF ) is the fluid that surrounds the spinal cord and brain (“nerve fluid”). The CSF serves a protective function, as this layer of fluid cushions the sensitive nervous structures. Furthermore, cerebrospinal fluid is believed to play important roles in the brain’s immune defense and metabolism. In ALS, the cerebrospinal fluid findings are “normal.” No immune cells or harmful metabolic byproducts are detectable. Cerebrospinal fluid analysis therefore serves to confirm normal findings and rule out changes typical of other diseases (“exclusion diagnosis”). In addition to ruling out immunological and metabolic changes, the detection of biomarkers is becoming increasingly important.

The protein“neurofilament light chain”(NfL) is a biomarker that indicates damage to nerve extensions (axons). The majority of ALS patients have elevated levels of NfL in their cerebrospinal fluid. The NfL concentration reflects the disease process and serves as a prognostic factor for the progression of ALS. A higher NfL level is statistically associated with increased ALS progression. Conversely, a lower NfL level is considered a favorable prognostic factor. For this reason, NfL is also referred to as an ALS progression marker. NfL analysis is not yet part of “routine diagnostics” and is primarily available at specialized ALS centers.

Cerebrospinal fluid (CSF) testing requires a lumbar puncture and is therefore associated with some discomfort. During the lumbar puncture, a fine needle (puncture needle) is inserted at the level of the lumbar spine through the skin and between two vertebral bodies of the lumbar spine into the subarachnoid space. Cerebrospinal fluid is drawn through the puncture needle into diagnostic tubes and collected for subsequent analysis. The experience of the lumbar puncture procedure varies greatly: While some patients perceive the procedure as painful and stressful, the majority of patients tolerate it well. Prior to a lumbar puncture, a physician always provides a detailed explanation of the specific procedure, the risks, and the expected results of the cerebrospinal fluid analysis.