What condition might an inability to bring all keratometer mires into focus indicate?

Inability to bring all keratometer mires into focus typically indicates irregular astigmatism. In regular astigmatism, the cornea has a consistent curvature along two principal meridians, allowing for a more uniform focus where the mires appear clear and defined. However, when the corneal shape deviates from this regular curvature, such as in cases of irregular astigmatism, the shape may have multiple steep or flat areas that do not align properly for simultaneous focus of the mires.

This situation can occur due to various conditions, including corneal scarring, keratoconus, or post-surgical changes, where the surface of the cornea is not smooth or uniformly curved. As a result, the keratometer cannot establish a consistent focus across all mires when assessing the corneal curvature, leading practitioners to suspect irregular astigmatism as the underlying issue.

The other conditions listed, such as regular astigmatism, myopia, and hyperopia, would not typically present with this specific difficulty in focusing all keratometer mires simultaneously, as they involve more straightforward relationships between curvature and focus.