Axial myopia induced by hyperopic defocus in guinea pigs: A detailed assessment on susceptibility and recovery.

This study investigated whether adolescent guinea pigs can develop myopia induced by negative lenses, and whether they can recover from the induced myopia. Forty-nine pigmented guinea pigs (age of 3 weeks) were randomly assigned to 4 groups: 2-week defocus (n=16), 4-week defocus (n=9), 2-week control (n=15) and 4-week control (n=9). A -4.00D lens was worn in the defocus groups and a plano lens worn in the control groups monocularly. The lenses were worn from 3 weeks to 5 weeks of age in the 2-week treatment groups with the biometry measured at 2, 4, 6, 10 and 14 days of lens wear. The lenses were worn from 3 weeks to 7 weeks of age in the 4-week treatment groups with the biometry measured immediately and at 2, 4, 6, 10 and 14 days after lens removal. Refractions in the defocused eyes developed towards myopia rapidly within 2 days of lens wear, followed by a slower development. The defocused eyes were at least 3.00D more myopic with a greater increase in vitreous length by 0.08 mm compared to the fellow eyes at 14 days (p<0.05). The estimated choroidal thickness of the defocused eyes decreased rapidly within 2 days of lens wear, followed by a slower decrease over the next 4 days. Relative myopia induced by 4 weeks of negative-lens treatment declined rapidly following lens removal. A complete recovery occurred 14 days after lens removal when compared to the fellow controls. The refractive changes during the recovery corresponded to a slower vitreous lengthening and a rapid thickening of the choroid. The plano-lens wearing eyes showed a slight but significant myopic shift (<-0.80D) with no associated biometrical changes. Guinea pigs aged 3 weeks can still develop negative lens induced myopia and this myopia is reversible after removal of the lens. The myopia and recovery are mainly due to changes in vitreous length and choroidal thickness.