[Ametropia and heterophoria "against the rules": a solution with prisms]

This study presents the results of the prescription of small prisms in ametropic patients whose glasses increase the associated heterophoria (which we have called heterophoria "against the rule"). It concerns the myopic eso (group I), the hyperopic exo (group II) and the presbyopic exo (group III) as a particular subgroup of the former. In a population of 211 patients wearing prisms, we have selected 53 in the first group, 34 in the second group and 20 in the third group. The 3 groups were compared according to the age and the fusional complaints of the patients. In 70 to 80% of the cases the associated ametropia was very small. In more than 60% of the cases the total prismatic prescription never exceeded 2 PD. A decrease or a suppression of the prisms was possible in 50% of the cases in groups I and II and in 15% in group III. Respectively 87%, 89% and 86% of the cases became asymptomatic in each category after a mean follow-up of 6.8--4.7--5.8 years (range: 2-12). No complaint was increasing during this period and no prismatic prescription exceeded a total amount of 5 PD. A familial factor was present in 18% of the cases. This study underlines the fact that the limit of tolerance of a latent phoric unbalance is more important that the amount of heterophoria itself. This observation is fundamental for understanding the apparent paradoxal behaviour of these patients and the efficacy of an apparent "homeopathic" prescription of prisms.