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M.V. Cigada, W. Albisetti, M. Sette, R. Ratiglia; Postural Modifications: Difference Between Normal and Heterophoric Subjects . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2465.
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© ARVO (1962-2015); The Authors (2016-present)
In a previous paper [ARVO abstracts 2003; 2763/B602] we found that changes in postural conditions can modify phoria measurement. In this work we tried to quantify the response of some postural muscles when the contraction of extrabulbar muscles was stimulated; we also tried to evaluate if pre–existing phoria was related to such muscular response.
Surface Electromiogram (SEMG) was acquired from gastrocnemius medialis (GM), rectus femoris (RF), longissimus dorsi (LD), trapezius (TR), in both right and left sides, on 50 young subjects (20–30 years old; 25 males and 25 females) without any postural problem . SEMG was recorded during the following visual stimulations repeated separately on Right Eye (RE) and Left Eye (LE): no prisms
single prism of 5 pd base 0° (base in= int)
single prism of 5 pd base 180° (base out)
single prism of 2 pd base 55°
single prism of 2 pd base 235°.
For the first 3 seconds each subject had his eyes closed (closed eyes period), than he started reading a visual acuity chart for 12 sec (open eyes period).The 0– 100 Hz component of the open eyes SEMG signal was releaved 3 times; the 3 measurements were integrated and averaged in order to obtain a single index, quantifying muscular activity during the open eyes period. A log transformation was performed so the data could best fit the normal distribution. Phoria was measured on each subject with a maddox rod. Dominant hand and eye were detected. Data were analyzed with multifactor ANOVA; a P value less than 0,05 was considered significant.
We found no significant effect of prism position in front of either eye, while we found a significant effect of phoria on muscular response: heterophoric subjects produce higher contractions. This was more evident in males on TR, RF and LD and in females on GM and RF. Considering the whole sample, the phoria level was significant for all the muscles (P=0.03 or less)
While the presence of a relationship between phoria state and muscular response confirms that there is a link between postural and visual system; the lack of a direct effect of prism on muscular contraction shows that the model requires more than a simple biunivocal relationship. This leaves interesting questions about the real nature of phorias.
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