Is There a Causal Relationship between Myopia and Intraocular Pressure

Aims: To determine if there is causal association between myopia and intraocular pressure at the University of Port Harcourt Teaching Hospital (UPTH), Nigeria.

Study Design: A case control study.

Place and Duration of Study: The study was carried out at the University of Port Harcourt Teaching Hospital (UPTH) between November, 2012 and May, 2013.

Methodology: Eighty consecutive patients of myopes (group A) and emmetropes (group B) were sampled in two groups. Group A was subgrouped into low myopia (−3.0D<Spherical Equivalent (SE)≤ −0.5D), moderate myopia (−3.0D≤ SE-< -6.0D) and high myopia (SE ≥ -6).

Intraocular pressures were taken between 9am -12 mid-day by Perkins applanation tonometer (MK2 Model). Autorefraction was carried out with (Carl Zeiss meditec) while Axial length was measured with A scan ultrasound machine (Pascan 300A Digital biometric reader). Full examination of the fundus was carried out.

Results: 160 eyes of 80 patients each were respectively in groups A and B. The mean age of the myopes was 23.54 ± 12.74 years while that of the controls was 23.62±12.86 years (P=0.968).

Among the myopes, there were 42(52.5%) males and 38(47.5%) females while the control had 32(40.0%) males and 48(60.0%) females. There was no statistical difference in male (p=0.411) nor female (0.416) gender.

The mean axial length of the myopes was 24.03±1.68 mm while that of the control was 23.09±0.87 mm. (P=0.001).

There was no correlation between myopia and IOP (Pearson correlation coefficient: r=0.14, r2=0.02, 95% CI=-0.14-0.18). There was also no correlation between IOP and axial length in both groups. There was however a linear correlation between myopia and axial length(r=0.76, r2=0.57, 95% CI=0.45-0.67.

Conclusion: Myopes have longer axial length than emmetropes in our study, this difference was not accounted for by changes in intraocular pressure.