A 1-Diopter Vertical Prism Induces a Decrease of Head Rotation: A Pilot Investigation.

Clinical studies in non-specific chronic arthralgia and back pain seem to indicate an association between vertical heterophoria (VH - latent vertical retinal misalignment) and asymmetrical head rotation. Such clinical observations suggest a link between VH and head rotation, but this was never tested. The purpose of this study was to simulate a VH in healthy subjects and examine its influence on the amplitude of active head rotation during 3D motion capture in upright stance. Subjects were asked to rotate their head three times from the straight ahead position and then to the right, back to straight ahead, to the left, and back to the straight ahead again. Three randomized conditions were run: normal viewing, with a 1-diopter prism base down on the dominant eye, or the non-dominant eye. The most important finding is that the experimental VH whichever the eye with the prism induces a significant decrease in the mean angle of head rotation compared to the normal viewing condition. This decrease was significant for rotation to the left. We suggest that the prism-induced VH modifies the reference posture and thereby affects head rotation; further studies are needed to confirm this effect and to extend to other types of dynamic activities.

Active ocular vergence improves postural control in elderly as close viewing distance with or without a single cognitive task.

Performance of the vestibular, visual, and somatosensory systems decreases with age, reducing the capacity of postural control, and increasing the risk of falling. The purpose of this study is to measure the effects of vision, active vergence eye movements, viewing distance/vergence angle and a simple cognitive task on postural control during an upright stance, in completely autonomous elderly individuals. Participated in the study, 23 elderly subjects (73.4 ± 6.8 years) who were enrolled in a center dedicated to the prevention of falling. Their body oscillations were measured with the DynaPort(®) device, with three accelerometers, placed at the lumbosacral level, near the center of mass. The conditions were the following: eyes open fixating on LED at 20 cm or 150 cm (vergence angle 17.0° and 2.3° respectively) with or without additional cognitive tasks (counting down from one hundred), performing active vergence by alternating the fixation between the far and the near LED (convergence and divergence), eyes closed after having fixated the far LED. The results showed that the postural stability significantly decreased when fixating on the LED at a far distance (weak convergence angle) with or without cognitive tasks; active convergence-divergence between the LEDs improved the postural stability while eye closure decreased it. The privilege of proximity (with increased convergence at near), previously established with foot posturography, is shown here to be valid for accelerometry with the center of mass in elderly. Another major result is the beneficial contribution of active vergence eye movements to better postural stability. The results bring new perspectives for the role of eye movement training to preserve postural control and autonomy in elderly.

Incidence of vertical phoria on postural control during binocular vision: what perspective for prevention to nonspecific chronic pain management?

Vertical heterophoria (VH) is the latent vertical misalignment of the eyes when the retinal images are dissociated, vertical orthophoria (VO) when there is no misalignment. Studies on postural control, during binocular vision in upright stance, reported that healthy subjects with small VH vs. VO are less stable, but the experimental cancellation of VH with an appropriate prism improves postural stability. The same behavior was recorded in nonspecific chronic back pain subjects, all with VH. It was hypothesized that, without refraction problems, VH indicates a perturbation of the somaesthetic cues required in the sensorimotor loops involved in postural control and the capacity of the CNS to optimally integrate these cues, suggesting prevention possibilities. Sensorimotor conflict can induce pain and modify sensory perception in some healthy subjects; some nonspecific pain or chronic pain could result from such prolonged conflict in which VH could be a sign, with new theoretical and clinical implications.

Spherical lenses and prisms lead to postural instability in both dyslexic and non dyslexic adolescents.

There is controversy as to whether dyslexic children present systematic postural deficiency. Clinicians use a combination of ophthalmic prisms and proprioceptive soles to improve postural performances. This study examines the effects of convergent prisms and spherical lenses on posture. Fourteen dyslexics (13-17 years-old) and 11 non dyslexics (13-16 years-old) participated in the study. Quiet stance posturography was performed with the TechnoConcept device while subjects fixated a target at eye-level from a distance of 1_m. Four conditions were run: normal viewing; viewing the target with spherical lenses of -1 diopter (ACCOM1) over each eye; viewing with -3 diopters over each eye (ACCOM3); viewing with a convergent prism of 8 diopters per eye. Relative to normal viewing, the -1 lenses increased the surface of body sway significantly whereas the -3 diopter lenses only resulted in a significant increase of antero-posterior body sway. Thus, adolescents would appear to cope more effectively with stronger conflicts rather than subtle ones. The prism condition resulted in a significant increase in both the surface and the antero-posterior body sway. Importantly, all of these effects were similar for the two groups. Wavelet analysis (time frequency domain) revealed high spectral power of antero-posterior sway for the prism condition in both groups. In the ACCOM3 condition, the spectral power of antero-posterior sway decreased for non dyslexics but increased for dyslexics suggesting that dyslexics encounter more difficulty with accommodation. The cancelling time for medium range frequency (believed to be controlled by the cerebellum), was shorter in dyslexics, suggesting fewer instances of optimal control. We conclude that dyslexics achieve similar postural performances albeit less efficiently. Prisms and lenses destabilize posture for all teenagers. Thus, contrary to adults, adolescents do not seem to use efferent, proprioceptive ocular motor signals to improve their posture, at least not immediately when confronted to convergence accommodation conflict.

Face Piercing (Body Art): Choosing Pleasure vs. Possible Pain and Posture Instability.

Piercings (body art, i.e., with jewelry) are more and more widespread. They can induce various complications such as infections, allergies, headaches, and various skin, cartilage, or dental problems, and represent a public health problem. We draw attention to possible side effects resulting from face piercing complications observed on four young adults such as eye misalignment, decreased postural control efficiency, and non-specific chronic back pain with associated comorbidity. We found that the origin was pierced jewelry on the face. Removing the jewelry restored eye alignment, improved postural control, and alleviated back pain in a lasting way. We suggest that pierced facial jewelry can disturb somaesthetic signals driven by the trigeminal nerve, and thus interfere with central integration processes, notably in the cerebellum and the vestibular nucleus involved in postural control and eye alignment. Facial piercings could induce sensory-motor conflict, exacerbate, or precipitate a pre-existing undetermined conflict, which leads pain and complaints. These findings are significant for health; further investigations would be of interest.

Postural control during the Stroop test in dyslexic and non dyslexic teenagers.

Postural control in quiet stance although simple still requires some cognitive resources; dual cognitive tasks influence further postural control. The present study examines whether or not dyslexic teenagers experience postural instability when performing a Stroop dual task for which their performances are known to be poor. Fifteen dyslexics and twelve non-dyslexics (14 to 17 years old) were recruited from the same school. They were asked to perform three tasks: (1) fixate a target, (2) perform an interference Stroop test (naming the colour or the word rather than reading the word), (3) performing flexibility Stroop task: the subject performed the interference task as in (2) except when the word was in a box, in which case he had to read the word. Postural performances were measured with a force platform. The results showed a main task effect on the variance of speed of body sway only: such variance was higher in the flexibility task than for the other two tasks. No group effect was found for any of the parameters of posture (surface, mediolateral and anteroposterior sway, variance of speed). Further wavelet analysis in the time-frequency domain revealed an increase in the spectral power of the medium frequency range believed to be related to cerebellum control; an accompanying increase in the cancellation time of the high frequency band related to reflexive loops occurred for non-dyslexics only. These effects occurred for the flexibility task and could be due to its high cognitive difficulty. Dyslexics displayed shorter cancellation time for the medium frequency band for all tasks, suggesting less efficient cerebellar control, perhaps of eye fixation and attention influencing body sway. We conclude that there is no evidence for a primary posture deficit in 15 year old teenagers who come from the general population and who were recruited in schools.

Vertical heterophoria and postural control in nonspecific chronic low back pain.

The purpose of this study was to test postural control during quiet standing in nonspecific chronic low back pain (LBP) subjects with vertical heterophoria (VH) before and after cancellation of VH; also to compare with healthy subjects with, and without VH. Fourteen subjects with LBP took part in this study. The postural performance was measured through the center of pressure displacements with a force platform while the subjects fixated on a target placed at either 40 or 200 cm, before and after VH cancellation with an appropriate prism. Their postural performance was compared to that of 14 healthy subjects with VH and 12 without VH (i.e. vertical orthophoria) studied previously in similar conditions. For LBP subjects, cancellation of VH with a prism improved postural performance. With respect to control subjects (with or without VH), the variance of speed of the center of pressure was higher, suggesting more energy was needed to stabilize their posture in quiet upright stance. Similarly to controls, LBP subjects showed higher postural sway when they were looking at a target at a far distance than at a close distance. The most important finding is that LBP subjects with VH can improve their performance after prism-cancellation of their VH. We suggest that VH reflects mild conflict between sensory and motor inputs involved in postural control i.e. a non optimal integration of the various signals. This could affect the performance of postural control and perhaps lead to pain. Nonspecific chronic back pain may results from such prolonged conflict.

Analysis of postural control in elderly subjects suffering from Psychomotor Disadaptation Syndrome (PDS).

PDS is a geriatric affliction, described in 1999, characterized by postural impairments, including backward disequilibrium, freezing, a deterioration in the ability to anticipate postural adjustments, anxiety and fear of falling, inducing loss of autonomy. This study compared 10 subjects suffering from PDS, aged 87.3+/-4.9 years, with 10 control subjects, aged 85.4+/-7.9 years concerning postural control (body sway amplitude). In all participants, postural control was assessed using the SwayStar system in natural (spontaneous) and standardized stances, eyes open and eyes closed over a period of 40 s. It was found that: (1) with eyes open, subjects with PDS showed greater body sway amplitude than did controls whatever the position (natural or standardized) and the plane (sagittal or frontal) considered (F(1,16)=6.05; p=0.026), (2) with eyes closed, subjects with PDS showed greater body sway amplitude than did controls in the natural stance whatever the plane (F(1,18)=7.65; p=0.013). In conclusion, PDS has a negative effect on postural control. This data must be taken into account during the rehabilitation of patients with this syndrome.

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

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 200cm. 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.

Effects of ocular dominance on the vertical vergence induced by a 2-diopter vertical prism during standing.

This study examined the eye movement responses to vertical disparity induced by a 2-diopter vertical prism base down while in standing position. Vertical vergence movements are known to be small requiring accurate measurement with the head stabilized, and was done with the EyeLink 2. The 2-diopter vertical prism, base down, was inserted in front of either the non-dominant eye (NDE) or dominant eye (DE) at 40 and 200 cm. The results showed that vertical vergence was stronger and excessive relative to the required value (i.e. 1.14 degrees ) when the prism was on the NDE for both distances, but more appropriate when the prism was on the DE. The results suggest that sensory disparity process and vertical vergence responses are modulated by eye dominance.

Effects of a two-diopter vertical prism on posture.

Postural control in upright stance requires the central integration of visual, vestibular, somatosensory (as cutaneous receptors) and proprioceptive (as joint receptors) inputs. Clinical studies seem to indicate an association between vertical heterophoria (VH) and balance control. The purpose of the study was to simulate a VH and examine its influence on body stabilisation in quiet stance. We studied 15 healthy subjects (25.6+/-3.0 years). The postural stability was measured with a platform under the following conditions: normal viewing, with a two-diopter prism base down placed on the dominant eye (PDE) or the non-dominant eye (PNDE). Both eyes were open in all conditions. All conditions were run at two distances: 200 and 40 cm. The results showed: (i) PNDE increased the antero-posterior body sway for both distances; this result could be related to sensory processing of disparity and/or to inappropriate eye movement response to the disparity induced by the prism; (ii) PDE improved the postural stability only at far distance (reduction of the center-of-pressure excursion area and of the lateral body sway). Such positive result could be due to appropriate sensory processing of disparity and/or eye movement response; the latter would reduce vertical disparity and modify the dynamic and tonic eye muscle activity relative to normal viewing at far distance. We conclude that: (i) experimentally induced vertical phoria does indeed influence postural control; (ii) vertical prisms, even of small power, can have complex effects, positive or negative, depending on the eye wearing it and at the distance fixated.