Timing and method of intervention is critical for managing intermittent exotropia, unlike other types of childhood-onset strabismus. A recent study evaluated the efficacy of using overminus lenses with prism spectacles in young children with intermittent exotropia and found that this combination can significantly improve both intermittent exotropia and stereopsis.

The study included 60 hyperopic patients aged three to six years with intermittent exotropia who were randomly divided into treatment and observation groups. The treatment group received overminus lenses of -2.50D with 2PD base-in prisms on each side. The researchers measured ocular alignment, status of binocular vision and refraction changes at one, three, six and 12 months follow-up.

At 12 months, the researchers reported a mean refractive error of 1.42D in the observation group and 1.43D in the treatment group. The mean exotropia control scores for the observation and treatment groups were 5.72 and 1.75, respectively. Mean stereoacuities were 2.16 and 1.91 log arcsec, respectively.

The researchers pointed out that because children have a strong adaptability to prescription lenses, overcorrection bears a risk of myopia. “In our clinical trial, we chose -2.50D incorporated with the 2PD base-in prisms as the initial power of overminus lenses, as it allowed the children to adapt to the lenses to maintain a constant accommodative demand and clear visual quality,” they explained. “We calculated that 4.00D of the base-in prism could provide as many accommodative convergence benefits as would 1.00D of additional overminus, assuming a normal accommodative convergence to accommodation ratio. Thus, -2.50D of overminus with 4.00D of base-in prism could achieve the same effect as that of 3.50D of overminus; this is theoretically less likely to cause asthenopia, especially at near work.”

The researchers concluded that their randomized clinical trial demonstrated that overminus spectacles with prism significantly improve control of intermittent exotropia and stereopsis by reducing the angle of strabismus in the patient population. They noted that this treatment didn’t appear to induce myopia. “A further randomized trial is warranted to assess the effect of overminus spectacles with prism after the treatment has been discontinued,” they noted.