Vertical phoria and postural control in upright stance in healthy young subjects
Introduction
Postural control in quiet upright stance requires the central integration of visual, vestibular, cutaneous and muscle proprioceptive sensory inputs and their rapid processing (Nashner, 1976). To maintain the centre of body mass in equilibrium, the central nervous system performs appropriate coordinate transformations of these inputs (lvanenko et al., 1999) and permanently generates muscular response adapted as corrective torque through the action of a feedback control system (Horak and Macpherson, 1996, Peterka, 2002).
On the other hand, visual stabilization of posture decreases when the distance to target fixation increases; this was initially attributed to decreased angular size of retinal slip induced by body sway (Bles et al., 1980, Paulus et al., 1984, Brandt et al., 1986, Paulus et al., 1989). Kapoula and Le (2006) showed that in addition to retinal slip, the ocular motor signals from the converging eyes, and perhaps related neck muscle activity, are involved in postural stabilization at a close distance. Le and Kapoula (2007) examined posture and vergence angle at several distances and suggested that at intermediate and far distances (i.e. beyond 90 cm) the central nervous system would use mostly internal signals (vestibular, proprioceptive, and somatosensory), while at close distance both visual and horizontal vergence oculomotor signals would be significant.
Vertical heterophoria (VH) is a relative deviation of the visual axes reduced via binocular vision mechanisms (Amos and Rutstein, 1987). VH exists in normal subjects, inferior to 1 diopter, on average 0.16 ± 0.01° corresponding to 0.28 diopter (van Rijn et al., 1998).
Numerous patients without a precise anatomical diagnosis, without neuropathy or rheumatism, suffer from postural disorders and complaints such as chronic neck pain, low back pain, headache (e.g. Lápossy et al., 1995; Hagen et al., 2006), vertigo (e.g. Bucci et al., 2006, Treleaven et al., 2008) or proprioceptive impairment for instance at spine, ankle or knee levels (e.g. Morningstar et al., 2005; Treleaven, 2008). Clinical study of the management of chronic pain syndrome suggested an association with VH and balance problems that were clinically evaluated (Matheron et al., 2005). Indeed, Matheron et al. (2005) reported that in patients with chronic pain syndrome associated with VH, a specific proprioceptive physiotherapy acting on oropharynx, temporomandibular joint and/or pelvis most of the time restored vertical orthophoria (VO) immediately and diminished pain (evaluated with the subjective visual analog scale – VAS – introduced by Huskisson, 1974); moreover, such physical therapy improved mobility of spinal and peripheral joints, and normalized behavior in the balance tests. These clinical observations were corroborated by a laboratory study in which VH was artificially induced in healthy subjects by the insertion of a vertical prism; the prism modified postural stability (Matheron et al., 2007).
This study was designed to test the hypothesis that the quality of postural performance in quiet upright standing could be lower, even for normal healthy subjects with vertical heterophoria within the physiological range, than those with vertical orthophoria.
The results confirmed this expectation; moreover, an additional study showed that the cancellation of the small vertical heterophoria with a prism led to the improvement of postural stability.
Section snippets
Subjects
Twenty-six healthy young subjects (15 females, 11 males) in the age range of 22–34 years (27.04 ± 3.29 years) were recruited among the laboratory co-workers, without neurological, otoneurological or ophthamological symptoms, with no medication or musculoskeletal problem. They all had normal vision with no history of strabismus, double vision, nor any other manifest ocular disease. The subjects with glasses were not included, as their glasses can already have some prismatic effects, and thus
Main experiment
For the main experiment, means and standard errors are shown in Table 2 for each group of subjects (VO and VH) and for each distance (close and far) for all postural parameters.
Next, we will present the results of ANOVA evaluating the effects of vertical phoria conditions and distance conditions on postural parameters, i.e. the surface area of the CoP excursions, the standard deviations of lateral (SDx) and anterio-posterior (SDy) body sways, and the variance of speed.
Subjects with physiological vertical heterophoria are less stable
The most important finding is that healthy young adults with a small VH within the physiological range, i.e. less than 1 diopter (0.57°), showed greater postural instability than the subjects with VO. The link between vertical phoria and posture was confirmed by the additional experiment in which the cancellation of the VH by an appropriate prism improved postural stability. VH contributes to the variability of postural performances among healthy subjects. Indeed, variability has been reported
Acknowledgements
The authors thank the subjects and Mildred Aknin for English revision, and thank the anonymous reviewers for their constructive comments. E.M. thank Dr. France Mourey and Pr Pierre Pfitzenmeyer for their constant support.
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