What condition should be suspected when there are larger fluctuations in a patient’s refractive error?

When larger fluctuations in a patient’s refractive error are observed, diabetes should be suspected due to its impact on the lens and refractive stability. Diabetes can cause significant changes in blood sugar levels, which in turn can lead to fluctuations in hydration levels of the lens. This change in hydration can alter the shape and, subsequently, the refractive power of the lens, resulting in variable vision. Patients with diabetes may experience episodic changes in visual acuity and refractive error, particularly if their blood sugar levels are not well-controlled.

Cataracts typically lead to a gradual decline in visual acuity and might cause a change in refractive error, but the changes are usually not fluctuating; instead, they become progressively worse. Keratoconus does involve instability in vision but is characterized by a progressive bulging of the cornea, not necessarily large fluctuations in refractive error in quick succession. Astigmatism can cause variations in vision but tends to be more stable unless there are changes in corneal curvature that are typically not rapid. Thus, the presence of fluctuating refractive errors strongly indicates the potential influence of diabetes.