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AIDS, Acquired immune deficiency syndrome; HIV, human immunodeficiency virus; HTLV-I, human T-cell lymphotrophic virus type-1; GTP, guanosine triphosphate.

the motor neurons, producing weakness or paralysis, and would also disrupt the ascending and descending tracts, producing other neurological dysfunction. However, release of locomotor synergies can be seen in patients with damage to the central nervous system. Automatic or spinal stepping can be elicited in humans with clinically complete cervical or thoracic cord transection by having the patients partially supported on a moving treadmill. W Automatic stepping, probably due to stimulation of the brain stem locomotor regions, also occasionally occurs in patients with brain stem lesions and with coma, such as occurs with central herniation. y

Freezing gait, or difficulty in initiating and maintaining locomotion, may represent an interruption of voluntary access to the brain stem locomotor regions or spinal central pattern generators. The feet of a patient with freezing seem to stick to the floor, and the patient may be at a loss as to how to start walking. This gait pattern has been given the graphic term gait ignition failure. Its milder form is termed start hesitation because a hesitation of several seconds

ensues before the patient can begin to walk. When gait is finally initiated, it may begin with small steps that increase in length, sometimes to normal length, a phenomenon termed slipping clutch gait. Turns are accomplished with slow, small steps and sometimes freezing, termed turn hesitation. Any distraction that interrupts the patient's concentration on walking, such as being asked a question or passing through a narrow opening, may precipitate freezing. Freezing may be overcome by tricks that convert walking from an automatic act of moving from one place to another to a cortically directed process of voluntary movements of the legs and feet. Thus, patients may initiate gait by focusing on stepping on a particular spot on the floor, pretending to kick something, or stepping over an object. A plausible but unproved interpretation of these clinical observations is that locomotion in patients with freezing is achieved by cortically directing the individual movements of the limbs until stepping is initiated; then the more automatic stepping generated from the brain stem locomotor regions and spinal central pattern generators can function, but always under the constant surveillance of the cortex. Distraction in these patients brings locomotion to a halt by interfering with the necessary cortical supervision of walking.

Freezing is most commonly seen in patients with Parkinson's disease (seeChapters 16 and 34 . However, frontal cortex and deep white matter lesions sometimes produce freezing with little or no imbalance and with no other parkinsonian signs. Further localization to specific frontal regions or to specific cortical bulbar tracts has not been possible, to date. Patients with these frontal and subcortical lesions may have marked freezing, but once under way they have an almost normal or completely normal stride. Arm swing is frequently preserved, which helps to differentiate isolated gait ignition failure from the freezing characteristic of parkinsonism. The base may be normal, as in parkinsonism, but often is widened, which is atypical of parkinsonism. There may be an exaggerated side-to-side sway of the trunk in an attempt to raise the feet off the floor, another feature not found in parkinsonism. Nevertheless, this freezing gait resembles the more common parkinsonian freezing gait pattern so closely that it is often referred to as lower half parkinsonism. y , y

Balance or equilibrium synergies organized in the brain stem are highly stereotyped and are not context sensitive. In day-to-day living, balance synergies are adapted to the environment, body position in space, previous experience, and expectation by the higher centers. Decerebrate and decorticate posturing and tonic neck reflexes are reflex brain stem postural synergies that are released by extensive cortical and subcortical lesions (see.Chap.teL1 and Chapter.").. Abnormal posturing (fencer's position) may also be produced by mesial frontal lobe epileptic discharges.

Frontal disequilibrium may be seen in patients with lesions of the frontal lobes or deep white matter. This gait abnormality is associated with an imbalance that may represent loss of access to balance synergies, inappropriate selection of synergies, or release of inappropriate brain stem synergies. Hydrocephalus, frontal infarcts, multiple lacunar infarcts in deep white matter, and frontal masses can produce frontal disequilibrium, a profound disturbance of balance that may preclude standing. y ,[18| , y Patients with frontal disequilibrium have difficulty in rising to stand because they do not bring their feet under themselves as they try to rise and may not even place their feet on the ground. Helped to an erect position, they cannot stand independently because they do not bring their weight over their feet. Many hyperextend the trunk and push backward in seeming disregard for their support base. Stepping may be bizarre with crossing of the legs and no coordination between the trunk and the legs. This clinical phenomenon was first described by Bruns in 1892 and is sometimes termed Bruns' ataxia or frontal ataxia. It is important to recognize that this form of ataxia is different from that produced by cerebellar lesions. Frontal disequilibrium is also sometimes termed apraxia of gait; however, this term seems inappropriate because gait is limited by balance, not by locomotor difficulties. y

Frontal gait is a combination of imbalance that does not preclude locomotion, small steps, and freezing. It is more common than the two extremes described previously, frontal disequilibrium and isolated gait ignition failure. Frequently, but not always, the base is widened. y The frontal gait pattern has also been referred to as marche aA petits pas. Frontal disequilibrium and frontal gait disorders are often accompanied by other frontal lobe signs such as grasping, dementia, pseudobulbar palsy, and urinary urgency and incontinence.

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