Tabes Dorsalis pathophysiology

The syphilis disease is the sole cause of tabes dorsalis. Treponema pallidum is usually transmitted via direct contact with the infected lesion (sexual contact) or blood transfusion (rare). It is understood that tabes dorsalis is caused by tertiary syphilis from treponema pallidum infection. Tabes dorsalis is a manifestation of invasion of treponema pallidum spirochete to the dorsal column of spinal cord in tertiary syphilis. In tabes dorsalis, the preganglionic portion of the dorsal roots of spinal nerves is infiltrated with lymphocytes and plasma cells, and invasion of treponema pallidum spirochetes to posterior columns of the spinal cord makes it atrophic. The demyelination of the axones of the neurons is the main cause of symptoms and it affects the neurons in the dorsal root ganglia and posterior columns of the spinal cord.

• The syphilis disease is the sole cause of tabes dorsalis.
• The pathogenesis of syphilis may be described in the following steps:^{[1][2][3][4][5][6][7][8][9][10][11]}
  • Treponema pallidum is usually transmitted via direct contact with the infected lesion (sexual contact) or blood transfusion (rare).
  • The incubation period varies with the size of innoculum (9-90 days).

• Following transmission, Treponema pallidum uses the intact or abraded mucous membrane to enter the body.
• It then disseminates to the lymphatics and blood stream to gain access to any organ of the body.

• Syphilis uses fibronectin molecules to attach to the endothelial surface of the vessels in organs resulting in inflammation and obliteration of the small blood vessels causing vasculitis (endarteritis obliterans).
• Organism has slow replication rate (30-33 hrs) and evades the initial host immune response.
• It may seed to different organs of the body especially the cardiovascular system and central nervous system resulting in tertiary syphilis.

Different stages of syphilis results from the interaction between the antigen and the host immune response.^[1][2]

• The initial infection in primary syphilis is limited due to T helper 1 response and lack of the antibody response.
• It is speculated that there is a shift from T helper 1 to Th2 response during secondary syphilis.

• Cytotoxic T cells and an incomplete humoral response is mainly responsible for persistence of infection and tissue damage in tertiary syphilis.
• Ineffective type 4 delayed hypersensitivity reaction containing macrophages and sensitized T cells is mainly responsible for the gumma formation in various organs.

• It is understood that tabes dorsalis is caused by tertiary syphilis from treponema pallidum infection.
• Tabes dorsalis is a manifestation of invasion of treponema pallidum spirochetes to the dorsal column of spinal cord in tertiary syphilis.
• In tabes dorsalis, the preganglionic portion of the dorsal roots of spinal nerves is infiltrated with lymphocytes and plasma cells, and invasion of treponema pallidum spirochetes to posterior columns of the spinal cord makes it atrophic.
• The demyelination of the axones of the neurons is the main cause of symptoms and it affects the neurons in the dorsal root ganglia and posterior columns of the spinal cord.

There is no known genetic mutation that is associated with syphilis. However, neurosyphilis may be associated with the gene polymorphism for IL-10 production with increased levels seen in the patients with neurosyphilis.^[11]

• Syphilis is associated with increased transmission of HIV.^[9]
  • The underlying mechanism may be related to the accumulation of dendritic cells containing CCR5 co-receptors at the site of infection, the same receptor entity binding the HIV.

On microscopic histopathological analysis, characteristic findings of tabes dorsalis are:^[1]

• Small vessel inflammation (endarteritis obliterans)
• Granulomatous lesions (gumma) containing central necrosis, inflammatory cells, such as lymphocytes, macrophages, plasma cells and fibroblasts.

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