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M. V. Cigada, W. Albisetti, S. Simonetta, M. Piras, F. D'Amico, L. Mosca, R. Ratiglia; Posture and Visual System: The Role of Phoria. Invest. Ophthalmol. Vis. Sci. 2008;49(13):753. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The relationship between extraocular muscles and body posture was studied through the evaluation of the postural response to a prism used to compensate subjects' phoria.
Surface Electromiogram (SEMG) was acquired from gastrocnemius medialis (GM), sternocleidomastoideus (SCM), longissimus thoracis (LT) and trapezius (TR), in 45 eterophoric subjects (18-45 years old; 20 males).Each measure was repeated:1.with closed eyes2.with open eyes3.with open eyes and a prism compensating the subjects' phoria.During condition 2 & 3 the subjects were photographed with a superimposed reference plumb-line. On each photograph the distances of C7 (VII processus spinosus vertebrae cervicalis) from the plumb line and the differences in height of the shoulders (AS - acromion scapulae) were measured with a commercial software (Adobe Photoshop) using a 10 cm square as a reference to transform pixels in millimeters.The SEMG was acquired 3 times for each stimulation. For the first 3 seconds subjects had their eyes closed, than they started reading a visual acuity chart for 12 sec (open eyes period - OEP). From all the signals was extracted the 0-400 Hz component of the OEP, the measurements were rectificated and integrated over all the 12 sec, in order to obtain a single index, quantifying muscular activity during the OEP; a log transformation was performed to best fit the normal distribution. Phoria was measured on each subject with a maddox rod and a rotating prism. SEMG data were analyzed with multifactor ANOVA, photo measures with paired t-test and sign test; a p value less than 0,05 was considered significant.
Analyzing SEMG data with ANOVA, near phoria effect was significant in most of the trials (all except Right SCM), far phoria was significant only for Right TR and prism effect was significant only for Right SCM; while photo analysis gave a positive significant effect for a prism compensating the phoria, both for C7 distance from the plumb line and for AS level. The effect was significant with paired t-test: C7 (p=0.026), AS (p=0.01) and with sign test C7 (p=0.005), AS (p=0.04).
Etherophoric subjects have a different postural response than orthophoric ones, which confirms our previous results (ARVO 2006 # 2465), yet it is not easy to detect this difference using SEMG, probably because postural muscles are deeper than those detectable with surface electrodes. Prism compensation of phoria improves posture.
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