The causative organism of syphilis is Treponema pallidum. AIDS patients and IV drug abusers are at high risk of developing neurosyphilis. Two major forms of parenchymal neurosyphilis are general paresis and tabes dorsalis. In tabes dorsalis, the primary infection is in the posterior nerve roots of the lumbosacral cord. In the posterior nerve roots, a chronic inflammation develops with fibrosis of the leptomeninges and degeneration of myelin.
General paresis is a chronic meningoencephalitis.
Grossly, the brain is atrophic, chiefly the frontal lobes. The leptomeninges are thickened and adherent to the cortical surface. The ventricles are dilated. Histologically, the leptomeninges are infiltrated with lymphocytes and plasma cells. Perivascular infiltrations are found in the cerebral cortex and subcortical gray structures.