‘Does Size Matter?’ – An Investigation of Anisometropia, Aniseikonia and Anisophoria

Kristen L Saba, BAppSc(Orth)
Ross Fitzsimons, MB ChB FRANZCO
Marsden Eye Specialists, Strabismus and Orthoptic Clinic, Sydney, Australia

Aim: To examine how acquired anisometropia commonly gives rise to symptoms of diplopia in patients and to differentiate the cause of these symptoms in terms of aniseikonia and anisophoria.

Method: Twenty-one patients with acquired anisometropia >1.00 D and astigmatism <1.00 D were examined. Symptoms of diplopia and cover test in primary position at 6 m and 1/3 m were recorded with the patient wearing spherical equivalent correction and then repeated through the reading position of the lenses, 12 mm below the optical centre. Stereopsis was assessed using the TNO test, and aniseikonia was measured using the Awaya New Aniseikonia Test. Results: Twelve of 21 patients (57%) reported diplopia when viewing through the reading position, but only three (14%) when viewing through the optical centre. Cover test showed an induced vertical heterophoria in the reading position. Binocularity measured by TNO showed stereopsis to be markedly reduced when looking through the reading position of the spectacle lens. There was no obvious relationship between Awaya aniseikonia measurements and patients’ symptoms.

Conclusion: Symptoms of diplopia in acquired anisometropia are more often due to optically induced anisophoria than to the aniseikonia. This finding is contrary to traditional teaching. Recognition of this and simple expedients in management resolve these symptoms for most patients.