Sensory (Tabetic, or Proprioceptive) Gait Ataxia
A loss of proprioception—as occurs in patients with severe large-fiber polyneuropathy, posterior nerve root lesions (e.g., tabes dorsalis, lumbosacral root compression), or interruption of the posterior columns in the spinal cord (multiple sclerosis, vitamin B12 deficiency, spondylotic, or tumor compression)—abolishes or seriously impairs the capacity for independent locomotion. After years of training, such patients still have difficulty in initiating gait and in forward propulsion. As J. Purdon Martin described in the severe form of the disorder, they hold their hands slightly in front of the body, bend the body and head forward, walk with a wide base and irregular, uneven steps, but still rock the body. If they are tilted to one side, they fail to compensate for their abnormal posture. If they fall, they cannot rise without help; they are sometimes unable to crawl or to get into an “all fours” posture. They have difficulty in getting up from a chair. When standing, if instructed to close their eyes, they sway markedly and fall (Romberg sign).
The principal observable features of sensory-ataxic gait are the brusqueness of movement of the legs and in its extreme, stamping of the feet as the foot is forcibly brought down onto the floor (apparently to detect the location of the foot as a substitute for proprioception). The feet are placed far apart to correct the instability, and patients carefully watch both the ground and their legs. As they step out, their legs are flung abruptly forward and outward, in irregular steps of variable length and height. The body is held in a slightly flexed position, and some of the weight is supported on the cane that the severely ataxic patient usually carries. To use Ramsay Hunt’s characterization, the patient with this gait disorder is recognized by his “stamp and stick.” As mentioned, the most specific feature is that the ataxia is markedly exaggerated when the patient is deprived of visual cues, as in walking in the dark. Such patients, when asked to stand with feet together and eyes closed, show greatly increased swaying and usually, the fully expressed Romberg sign with falling off to one side. It is said that in cases of sensory ataxia, the shoes do not show wear in any one place because the entire sole strikes the ground at once. Examination usually discloses a loss of position sense in the feet and legs and usually of vibratory sense as well. The peripheral or central location of the sensory lesions can be further determined by the state of the tendon reflexes.
Whatever the location of the lesion, its effect is to deprive the patient of perception of the position of his limbs and, more relevant to gait, to interfere with a large amount of afferent proprioceptive and related information that does not attain conscious perception. A sense of imbalance is usually present but these patients do not describe dizziness. They are aware that the trouble is in the legs and not in the head, that foot placement is awkward, and that the ability to recover quickly from a misstep is impaired. The resulting disorder is characterized by varying degrees of difficulty in standing and walking; in advanced cases, there is a complete failure of locomotion, although muscular power is retained.
Formerly, a disordered gait of this type was observed most frequently with tabes dorsalis, hence the term tabetic gait; but it is also seen in Friedreich ataxia and related forms of spinocerebellar degeneration, subacute combined degeneration of the spinal cord (vitamin B12 deficiency), a large number of sensory polyneuropathies, sensory ganglionopathies of various causes, and those cases of multiple sclerosis or compression of the spinal cord in which the posterior columns are involved.