Chapter 4: Somatic Sensation: Spinal Mechanosensory Systems

CLINICAL CASE | Neurosyphilis and Loss of Vibration Sense and Limb Proprioception

A 36-year-old man was admitted to the hospital for unsteadiness of gait and several other sensory and motor signs, including pain and limb strength impairments. Sensation to touch, pinprick, and temperature was normal. Perception of vibration and limb position sense were absent. When he was asked to stand upright with his eyes closed, he swayed and lost balance. This is a positive "Romberg sign." His gait was broad based, clumsy, and staggering. MRI of his brain was normal but MRI of his spinal cord showed an intense signal in the dorsal columns, bilaterally (Figure 4–1A), that appeared to be the same as CSF.

The man had an untreated syphilitic infection for 10 or more years. He was diagnosed with neurosyphilis, also called tabes dorsalis, on the basis of several laboratory and neurological tests, including the MRI and sensory loss indicated above. This is the advanced stage of syphilis, when it infects the nervous system.

Try to answer the following questions based on your reading of the chapter, inspection of the images, and consideration of the neurological signs.

1. What functional system becomes impaired when there is neuronal degeneration in the dorsal columns?

2. Why does the patient demonstrate Romberg's sign?

Syphilis is normally treated with penicillin. Left untreated, the infectious agent—the spirochete, Treponema pallidum—infects the nervous system. In time, this can result in dysfunction or degeneration of its neuronal targets. Common target neurons are the dorsal root ganglion sensory neurons, which are important for mechanosensation. Especially vulnerable is limb position sense (or limb proprioception), which is signaled by muscle and joint sensory receptors, and vibration sense, which is signaled by Pacinian corpuscles. At autopsy, tabetic patients can show degeneration of the dorsal columns. This can be revealed by staining histological sections of the spinal cord for myelin. Oligodendrocytes also degenerate in regions where axons have degenerated, therefore showing demyelination (Figure 4–1B). The region of intense signal on the MRI demonstrates damaged dorsal column fibers.

Both of these senses are mediated by dorsal root ganglion neurons that have a large-diameter axon, which project rostrally within the dorsal columns. In the absence of limb proprioceptive afferents, patients rely on vision to compensate for the loss of sensory awareness of their limbs. This explains the loss of balance when the patient closed his eyes. Touch is preserved in this patient; there may be diminished sensitivity, but this was not tested. Dorsal root ganglion mechanosensory neurons with a small-diameter axon may play more of a role in touch after degeneration of dorsal root ganglion neurons with large-diameter axons. This residual sensation is termed crude touch.

Stepper F, Schroth G, Sturzenegger M. Neurosyphilis mimicking Miller-Fisher syndrome: A case report and MRI findings. Neurology. Jul 1998;51(1):269-271.