GASH is an acronym that means Glare, Arcing, Starbursts and Halos. GASH occurs after refractive surgery for many reasons, ranging from residual refraction, to a too-small or misplaced optical zone, to astigmatism.
Glare is a sharp reflection, such as when the sunlight reflects off a car windshield into your eyes causing you to be distracted.
Arcing is a type of starring, such as when looking at Christmas lights or a chandelier and the starburst from each bulb seems to curve right into the starburst from the next bulb.
Starring or starbursting is a sunburst that occurs around a light source.
Halos are rings of soft light that appear around light sources.
When GASH is a result of residual refraction (not hitting a correction of 20/20) or regular astigmatism it usually can be reduced or eliminated with glasses and/or contact lenses and/or additional surgery. Additional surgery usually comes with a risk level similar to the original surgery.
When GASH is a result of a small or misplaced correction, or when the laser leaves an uneven corneal surface (irregular astigmatism), it usually cannot be reduced or eliminated with glasses or soft contact lenses. There is no reliable surgical fix for GASH due to a small optical zone or uneven corneal surface at this time, although technology continues to advance. Glasses and soft contacts usually cannot mask these problems. Rigid gas permeable prosthetic contact lenses (GPs, aka hard lenses) are the primary option to mask GASH from some surgical results. Fitting RGPs after refractive surgery can be challenging (due to the changed curvature of the cornea) and requires the expertise of a specially trained lens fitter. Not everyone is able to wear these lenses successfully.