In which direction does a GP lens typically orient when used for irregular astigmatism?

In the case of irregular astigmatism, a GP (gas permeable) lens typically orients against the rule. This is primarily because irregular astigmatism is often characterized by variations in the corneal surface that lead to an asymmetrical refractive error. When a GP lens is fitted to address this condition, the design and fitting characteristics of the lens allow it to provide a more stable vision correction by aligning in a manner that counters the natural corneal steepness and flattening, which is usually seen in irregular astigmatism.

When lenses are oriented against the rule, they are designed to align with the steepest meridian of the cornea, which often falls perpendicular to the rule of regular astigmatism. This orientation maximizes the optical performance of the lens and provides better stability during blinking and eye movement, leading to improved visual acuity and comfort.

In contrast, other orientations like with the rule, vertically, or horizontally would not appropriately address the specific irregularities caused by the condition, making against the rule the most suitable choice in managing irregular astigmatism effectively.