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Why might cerebrospinal fluid be examined in someone with later-stage syphilis?

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Scanning electron micrograph of Treponema pallidum, the spirochete bacterium that is the causative agent of syphilis. — syphilis Scanning electron micrograph of *Treponema pallidum*, the spirochete bacterium that is the causative agent of syphilis.

In the later, asymptomatic stage of syphilis, examining the cerebrospinal fluid is the most reliable method for determining whether the central nervous system has been affected by the infection.1

While most syphilis testing is performed on blood serum, the Venereal Disease Research Laboratory (VDRL) test can also be conducted on a sample of cerebrospinal fluid obtained via a lumbar puncture. This diagnostic step is critical because untreated syphilis can progress to neurosyphilis, which may lead to severe and irreversible neurological damage, such as paresis or tabes dorsalis. Identifying central nervous system involvement is essential for guiding appropriate treatment and follow-up care, as medical intervention can eliminate the causative organism, Treponema pallidum, but cannot reverse damage that has already occurred.12345