Elsevier

Clinical Neurophysiology

Volume 119, Issue 10, October 2008, Pages 2314-2320
Clinical Neurophysiology

Vertical phoria and postural control in upright stance in healthy young subjects

https://doi.org/10.1016/j.clinph.2008.06.016Get rights and content

Abstract

Objective

To test the quality of postural performance in quiet upright stance in healthy young adults with vertical heterophoria (VH) within the normal range and without VH (vertical orthophoria, VO).

Methods

Twenty-six subjects took part in this study. The postural stability was measured with a force platform while the subjects fixated a target at eye level in a straight ahead position, placed at either 40 or 200 cm.

Results

The results indicated that the postural control was better for subjects with VO than subjects with VH. Particularly, there was an interaction between vertical phoria and distance: the subjects with VH showed greater instability than the subjects with VO at a far distance only. An additional study showed that the cancellation of VH with a prism improved postural stability.

Conclusions

The quality of postural performance in quiet upright stance was lower in the subjects with VH. We speculate that VH, even when small in size, indicates a perturbation of the somatosensory/proprioceptive loops involved in postural control.

Significance

Vertical phoria could perhaps indicate the capacity of the central nervous system to integrate optimally proprioceptive cues.

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.

References (46)

  • A.M. Wong et al.

    Vertical misalignment in unilateral sixth nerve palsy

    Ophthalmology

    (2002)
  • F.J. Amos et al.

    Vertical deviation

  • S.R. Bharadwaj et al.

    Variation of binocular-vertical fusion amplitude with convergence

    lnvest Ophthalmol Vis Sci

    (2007)
  • W. Bles et al.

    The mechanism of physiological height vertigo II. Posturography

    Acta Otolaryngol

    (1980)
  • T. Brandt et al.

    Vision and posture

  • J.A. Büttner-Ennever
  • E. Casillas Casillas et al.

    Comparison of subjective heterophoria testing with a phoropter and trial frame

    Optom Vis Sci

    (2006)
  • K.M. Daum

    Heterophoria and heterotropia

  • Gagey PM, Weber B. Stabilométrie. In: Masson, editor. Posturologie: régulation et dérèglements de la station debout....
  • E.M. Hagen et al.

    Comorbid subjective health complaints in low back pain

    Spine

    (2006)
  • F.B. Horak et al.

    Postural equilibrium and orientation

  • Y.P. lvanenko et al.

    Effect of gaze on postural responses to neck proprioceptive and vestibular stimulation in humans

    J Physiol

    (1999)
  • Z. Kapoula et al.

    Effects of distance and gaze position on postural stability in young and old subjects

    Exp Brain Res

    (2006)
  • Cited by (0)

    View full text