The adaptation of spinal and brain stem synergies to an individual's goals and limitations (i.e., context) requires a knowledge of the relations of the body segments to each other, the situation of the body in space and the gravitational field, and the presence of environmental hazards and aids to balance and locomotion. This information is largely derived from the somatosensory, vestibular, and visual senses. Generally, one sensory system is adequate to orient a person and permit normal balance and gait. Balance and gait difficulties arise if environmental information is inadequate because of reduced sensory input or if a mismatch between the sensory information provided by the three sensory systems occurs because the environmental clues are ambiguous or the information is distorted by diseased sensory systems.
Sensory ataxia arises from a proprioceptive sensory loss in the lower extremities and may be produced by diseases affecting large myelinated peripheral nerves, dorsal roots, or posterior columns. The gait disorder is often first recognized when the patient is suddenly deprived of vision, such as occurs when walking into a dark room or closing the eyes to wash the face. At this mild stage, the only abnormalities detectable on examination may be a slight unsteadiness when the person stands with the feet together and the eyes closed (Romberg's sign), walks with the eyes closed, walks backward, or makes sudden turns. With more severe sensory loss, the patient shows increased sway when standing and walks with a widened base and irregular steps. The feet may be raised too high, thrown too far forward, and brought down too quickly. At this stage, the gait resembles that of a person with cerebellar ataxia. The patient's eyes
are focused on the feet and the ground immediately in front of them. Diverting the eyes away from the feet may cause the patient to fall. Reactive or protective postural responses as induced by the pull test may be delayed because proprioceptive input to signal movement of the center of mass relative to the base of support is absent. A cane may aid in walking, perhaps through proprioceptive cues appreciated in the hands. With severe proprioceptive sensory loss, the patient may not be able to stand unaided, even though strength is retained. Neurological examination reveals a profound loss of position sense at the toes and ankles and generally also at the knees and hips in patients with ataxia on the basis of proprioceptive sensory loss. Other aspects of the neurological examination help in differentiating the peripheral nerve and spinal cord posterior column origin of the sensory loss.
Vestibular ataxia is seen in patients with vestibular dysfunction; its severity depends on the speed with which dysfunction develops, the extent of the lesion, and the degree of compensation. The person with longstanding complete loss of vestibular function, for example from aminoglycoside antibiotics, may appear completely normal under most circumstances. The effects of the vestibular deficit may emerge only when the person is in an environment in which vision and proprioceptive clues are reduced or deceptive. Subtle signs in these patients include an en bloc appearance, which restricts the movement of the head during walking. Ataxia may be elicited in these patients by asking them to rotate the head from side to side while walking. The ability to balance on one foot or to walk in tandem with the eyes open or closed may also be impaired.y
Vestibular function that is present but distorted causes more difficulty with balance and gait. Patients are very dependent on visual information, but their deficient vestibular-ocular reflexes make it difficult for them to differentiate between self and environmental movement. For example, such patients find it difficult to walk on a busy city sidewalk because of the surrounding movement of pedestrians and vehicular traffic. These patients generally describe unsteadiness but not vertigo. The gait may be characterized by staggering and veering in response to erroneous perceived self-motion. Acute vestibular lesions are generally associated with vertigo and produce prominent difficulties with gait and balance. The sudden onset of vertigo may be associated with an inability to walk or even to stand.
Historical features suggestive of vestibular disorders include the following: (1) spontaneous vertigo, (2) vertigo induced by movement, Valsalva maneuvers, noise, exercise, and heat, (3) visual blurring induced by head and body movements, and (4) disequilibrium induced by body movements and movements in the surrounding environment. Findings on examination that suggest vestibular dysfunction include spontaneous or positional nystagmus, en bloc gait (particularly minimizing head movement), ataxia with head movement, and difficulty in balancing on one foot or on a compliant surface (e.g., foam) with the eyes closed. y
Visual ataxia is unsteadiness caused by visual deficits. Under normal circumstances, humans are very dependent on vision for balance; foveal vision appears to be most important, but peripheral vision also contributes to balance. Disturbances in visual acuity or visual fields increase body sway and predispose the person to falls. People adjusting to new bifocals may feel unsteady or even fall. Vision may also be affected by abnormalities of eye movements. Limitation of eye movements, particularly downward movement, or an imbalance between the extraocular muscles, producing diplopia, can cause balance difficulties and falls. y
Multisensory disequilibrium occurs with deficits in multiple sensory systems. No single deficit is sufficient to cause a problem, but the sum causes difficulties in orientation in space. y
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