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Perspectives |
H-O Karnath, MD, PhD, is Associate Professor for Neurology, Department of Cognitive Neurology, Hertie Institute for Clinical Brain Research, University of Tuebingen, Hoppe-Seyler-Str 3, D-72076 Tuebingen, Germany (Karnath{at}uni-tuebingen.de).
D Broetz is Physical Therapist, Department of General Neurology, Hertie Institute for Clinical Brain Research, University of Tuebingen
Address all correspondence to Dr Karnath
| Abstract |
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Key Words: Hemiparesis Pusher syndrome Spatial neglect Spatial orientation Thalamus
| Introduction |
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| Pusher SyndromeDistinctive Disorder or Catch-all for Different Expressions of Postural Instability Following Stroke? |
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Use of the term "pusher syndrome" for a number of different postural instability symptoms that occur in patients after brain damage (for an overview, see Schädler and Kool5) should be avoided. The term should be used for the distinctive disorder of actively pushing away from the nonhemiparetic side as defined by Davies1 and illustrated in Figure 1. Until recently, the pathophysiological mechanism leading to pusher syndrome and the particular brain structure damaged were unknown.
| Is Contraversive Pushing Caused by Hemineglect and Thus a Typical Disorder of the Right Hemisphere? |
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Davies,1 however, also observed that pushing behavior is not almost exclusively associated with right brain damage, as is the case for patients who exhibit spatial neglect.9,10 Pusher syndrome frequently occurs also with lesions of the left hemisphere and is not related to neglect but rather to aphasia.1 A study of 327 patients with acute stroke and hemiparesis who were investigated within the first couple of days after onset of stroke led to the observation that left and right hemisphere damage occurs with equal frequency in patients with contraversive pushing (left brain damage: 47%; right brain damage: 53%).2 Moreover, there was no evidence for a regular co-occurrence of pathological pushing with spatial neglect, anosognosia, aphasia, or apraxia.2
In agreement with Pedersen et al,2 Karnath and co-workers11 found that hemispatial neglect is not the cause of contraversive pushing. In their sample of 23 patients with pusher syndrome, they found a large proportion who had left brain damage and thus aphasia but no neglect. Sixty-five percent of their patients with contraversive pushing had right-side lesions, and 35% had left-side lesions. Although contraversive pushing within the group of patients with right-side lesions was highly associated with spatial neglect (80% of these patients also had neglect), neglect did not appear to be the cause of pushing behavior. The reason for this observation was that 20% of the patients with right brain damage who exhibited contraversive pushing and 100% of the patients with left brain damage who exhibited contraversive pushing showed no symptoms of spatial neglect. All of the patients had pusher syndrome due to left-sided brain lesions rather had aphasia.11
We conclude that both neglect and aphasia are highly associated with pushing behavior after right-side brain damage (
neglect) and after left-side brain damage (
aphasia), but that both disorders neglect and aphasia cannot be the underlying cause of pusher syndrome. Symptoms such as neglect and anosognosia after right-sided lesions and aphasia after left-sided lesions frequently exist with contraversive pushing because the relevant brain structures associated with these functions lie in close proximity to each other. Neither neglect nor aphasia, however, is causally related to contraversive pushing.
| What Is the Brain Structure Typically Damaged in Patients With Pusher Syndrome? |
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When this assumption was studied for the first time, the data showed that the parietal cortex is not the neural correlate of pusher syndrome.11 In a sample of 23 patients with severe contraversive pushing who were consecutively admitted to a neurology department, the authors identified brain lesions by magnetic resonance imaging or computed tomography. The overlap area of infarction in the patients with pusher syndrome was determined and compared with that of a sample of 23 patients with stroke admitted in the same period who did not exhibit contraversive pushing but were similar with regard to age, etiology of lesion, presence of hemiparesis, spatial neglect, and aphasia. The analysis revealed that the brain structure typically damaged in patients with pusher syndrome is the left or right posterolateral thalamus. This finding suggests that the posterolateral thalamus is involved in our control of upright body posture.
Traditionally, the posterolateral part of the thalamus was thought to serve simply as a "relay structure" of the vestibular pathway on its way from the brain stem to the cortex. The findings of Karnath and colleagues,11 however, showed that this is not the only task of the posterolateral thalamus. The ventral posterior and lateral posterior nuclei of the posterolateral thalamus rather seem to be fundamentally involved in our control of upright body posture. Patients exhibiting severe contraversive pushing showed a clear overlap of their infarctions in this portion of the thalamus.11 This structure is anatomically distinct from the "vestibular cortex" identified by Brandt and co-workers12 in the posterior insula. In addition, the clinical findings in patients with such posterior insula lesions are different. While a lesion of the human "vestibular cortex" leads to a tilt of the perceived visual vertical but not to contraversive pushing,12 a lesion of the posterolateral thalamus in patients with pusher syndrome induces the opposite pattern. The patients with contraversive pushing show normal perception of visual vertical, but they exhibit a severe tilt of perceived body posture in relation to gravity.13 Thus, both graviceptive systems not only appear to use distinct anatomical structures but also seem to process afferent sensory information from peripheral input sources differently.
Future studies are needed to investigate the possible role of diaschisis.14 Lesions of those thalamic nuclei (ventral posterolateral, ventral posteromedial, and lateral posterior) that were found to be affected in patients with contraversive pushing11 might lead to additional functional or metabolic abnormalities in some of the structurally intact regions of the cortex. Thalamocortical axons arising in the ventral posterolateral and ventral posteromedial nuclei project to the primary somatosensory cortex in the postcentral gyrus (Brodmann areas 3a, 3b, 1, and 2), to the secondary somatosensory cortex in the parietal operculum, and to the insula.15 The lateral posterior nucleus projects to the posterior parts of areas 5 and 7 of the superior and inferior parietal lobules.15 Imaging and other metabolic studies might help to assess whether additional critical substrates in the cortex are present and relevant in patients with pusher syndrome.
| Which Mechanism Leads to Pusher Syndrome? |
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Alternatively, it is possible that the pushing behavior is a secondary response to the patients' unexpected experience that they lose lateral balance when trying to get up and sit upright. The experiment of Karnath et al13 revealed that the patients' perceived "upright" orientation was tilted about 18 degrees toward the ipsilesional side. Thus, when patients try to get up and orient the body to what they perceive to be "upright," they become laterally instable because the center of mass is shifted too far to the ipsilesional side. Pushing the body to the opposite (contralesional) side might be the ensuing reaction to this experience.13 Therefore, no pushing occurs when patients sit immobilized by lateral stabilization in the cushioned safety of the experimental chair that was used in the study.13
In the future, researchers may want to further investigate these possible interpretations. Nevertheless, the study of Karnath et al,13 clearly showed for the first time that contraversive pushing is due to a severe misperception of body orientation in relation to gravity. Moreover, the data suggest that, for the purpose of rehabilitation, the preserved ability to align the body axis to earth vertical with the help of visual cues might be helpful. Although patients with pusher syndrome are not spontaneously able to use the visual input to control upright body posture, this might become possible when training procedures apply this ability as part of conscious strategies to control posture in these patients.
The discrepancy of a pathologically tilted postural vertical concurrent with an unimpaired perception of the visual vertical shows that patients with contraversive pushing manifest a selective disturbance of control of upright body posture.13 Although they are no longer able to determine when their body is oriented in an erect position, they have no problems correctly determining the orientation of the visual world around them. Patients with lesions of the vestibular system behave exactly the opposite. They show visual-vestibular dysfunction with a perceptual tilt of the visual vertical but have no problems orienting their body to an earth-vertical, upright position.12,16,17
These dissociations provide evidence for a separate pathway in humans for sensing the orientation relative to gravity that is apart from the well-known pathway for orientation perception of the visual world. For this reason, Karnath et al11 posited that the brain structure typically damaged in patients with pusher syndromethe posterolateral thalamusmight constitute the neural representation of this second graviceptive system in humans.
| Diagnosis of Pushing Behavior |
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Spontaneous Body Posture
The most striking feature of patients with contraversive pushing is their spontaneous posture while sitting and standing. Their longitudinal body axis is tilted toward the paretic side. This behavior is best observed without prior instructions, right after changing position (eg, from a supine position to sitting at the bedside). To quantify pathological body posture, we differentiate among 3 intensities (see Appendix): severe contraversive tilt with falling to the side contralateral to the brain lesion, severe contraversive tilt without falling, and mild contraversive tilt without falling. A tilted logitudinal body axis must occur regularly, not just occasionally, due to the normal insecurity in balancing when patients become hemiparetic after stroke.
Abduction and Extension of the Nonparetic Extremities
Another feature of contraversive pushing is the use of the nonparetic extremities to bring about the pathological lateral tilt of the body axis. With the patient sitting on the bedside, we observed that the ipsilesional hand is abducted from the body searching for contact with the surface and the elbow is extended (Fig. 1). In our experience, if the feet have ground contact, the ipsilesional leg will be abducted, and the knee and hip joints will be extended as well. To quantify this characteristic feature, we use visual assessment of abduction and extension of the extremities (see Appendix), depending on whether the movements occur spontaneously even at rest or only on changing position (eg, on moving the patient from the wheelchair to the bed or on standing up after sitting).
Resistance to Passive Correction of Tilted Posture
Evaluating a patient's behavior on being corrected by the investigator to an upright position is the third diagnostic feature for determining the presence of contraversive pushing. It is known that any attempt by the examiner to move the tilted body axis to an upright position by shifting the weight toward the nonparetic side elicits active resistance from the patient. The patient increases the force in the already extended nonparetic extremity. During our clinical examination (see Appendix), we evaluate the occurrence or nonoccurrence of active resistance to being interventionally corrected.
The Appendix summarizes the 3 variables (ie, spontaneous body posture, increase of pushing force by spreading of the nonparetic extremities from the body, and resistance to passive correction of posture) in the form of a scale, published as the so-called "Clinical Scale for Contraversive Pushing (SCP)."13,18 The authors13,18 intended the scale to aid clinicians in diagnosing the presence of pushing behavior and determining its severity. The weighted values that were tentatively assigned to each finding of the examination in the Appendix are still in the process of being validated. For a firm diagnosis of contraversive pushing, we suggest a value of 1 or more (summed over the results for sitting and standing; maximum=2 per variable) for each of the 3 variables. However, further investigation of the scale is needed; lower or higher values might turn out to be more adequate for a firm diagnosis.
| Prognosis of the Disorder |
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| Suggestion for a New Strategy for Treating Pusher Syndrome |
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Because the patients feel erect when they see that they are tilted, and vice versa,13 we believe the first goal of physical therapy should be to demonstrate this, showing the patients that visual information corresponds to reality. While sitting or standing, the patients should be asked to see whether they are oriented upright. We also provided an experience that showed patients that it is beneficial to use visual aids (eg, the therapist's arm as shown in Fig. 3) to give patients feedback about their body orientation. It is our observation that the experience of not falling after attaining the corrected position, combined with seeing that they are upright, increases the patients' confidence and lowers both the presence and the extent of the reaction to abduct and extend the nonparetic extremities to push toward the paretic side.
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In our day-by-day clinical management of patients with pusher syndrome, we see that this procedure produces successful results. However, research is needed involving controlled studies of this new approach to examine the effects of the intervention and whether it shortens the time for inpatient care and accelerates independence in daily living.
| Appendix |
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| Footnotes |
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This work was supported by a grant from the Deutsche Forschungsgemeinschaft awarded to Dr Karnath (Ka 1258/23).
| References |
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