The light that leaks: Brückner and the red reflex.

The Brückner test utilizes the ophthalmoscope to assess symmetry of binocular fixation by qualitative comparison of the red reflexes. Accurate fixation by the patient's eye on the ophthalmoscope light causes darkening of the red reflex, previously believed due to macular pigmentation. By designing and experimenting with a beamsplitter ophthalmoscope, we have demonstrated that if the patient's fovea is not exactly conjugate to the light source, light from the retina spills past the light source at the ophthalmoscope mirror into the examiner's eye, creating the red reflex. Therefore, binocular asymmetry of this conjugate relationship, whether due to anisometropia or strabismus, creates asymmetrical red reflexes. Understanding this principle of conjugacy is critical to optimal utilization of the red reflex in routine clinical examination as well as in microsurgery.