title: Fresnel membrane prisms: clinical experience.
abstract: BACKGROUND
  There are few published reports on the clinical application of Fresnel membrane prisms in the treatment of diplopia in adults. The authors describe the use of these prisms in patients with fourth and sixth cranial nerve palsies, restrictive motility caused by thyroid-related orbital disease, and convergence insufficiency.
  
  
  METHODS
  Of 209 patients who had been treated with Fresnel prisms, 141 were selected. The database included patients from a private practice in Montreal, seen from 1988 to 1996, and patients seen by orthoptists in the ophthalmology department of a children's hospital in Montreal between 1992 and 1996. All the patients had diplopia associated with fourth (48 patients) or sixth (43 patients) cranial nerve palsy, thyroid-related orbitopathy (18 patients) or convergence insufficiency (32 patients). After qualitative and quantitative assessment of the ocular misalignment, a Fresnel prism was selected for power and axis and for appropriate location on the spectacle lens. Ocular dominance and side of paresis or restriction were also considered in the placement of the prism. The patient's response to treatment was documented.
  
  
  RESULTS
  The Fresnel prisms were oriented horizontally in 72 patients (51%), vertically in 55 (39%) and obliquely in 14 (10%). They were placed on the spectacle lens before the nondominant eye in 127 cases (90%), either covering the entire lens, or on the upper or lower segment or both. The patients were followed for an average of 15 (range 2 to 96) months. Of the 141 patients 113 (80%) had a successful outcome, with relief of their diplopia. Twenty-seven patients (19%) eventually had the prismatic correction ground into the lens, 70 (50%) chose to wear the Fresnel prism on a permanent basis because incorporation into the lens was not possible or because of cost, and 17 (12%) used the Fresnel prism as a temporary device before or after surgery. Most patients who converted to incorporated prisms did so when the prismatic power became stable, usually after 6 to 8 months. Eight patients (6%) stopped using the prism because of associated side effects, such as blurred vision, persistent diplopia, torsion or optical aberrations.
  
  
  CONCLUSIONS
  The Fresnel prism is an excellent device in treating diplopia in adult patients. It is a reasonable permanent option when incorporating the prism into the spectacle lens is not possible. A fused blurred image caused by a Fresnel prism placed in front of the nondominant eye is preferable to double but clear images.
authors.affiliation: Department of Ophthalmology, McGill University Health Centre, Montreal, Que.
authors.forename: M
authors.initials: M
authors.lastname: Flanders
authors.forename: N
authors.initials: N
authors.lastname: Sarkis
mesh_terms: D020434:Abducens Nerve Diseases / Q000150:complications; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004172:Diplopia / Q000209:etiology / Q000628:therapy*; D005139:Eyeglasses; D005260:Female; D006111:Graves Disease / Q000150:complications; D006801:Humans; D008297:Male; D008875:Middle Aged; D015835:Ocular Motility Disorders / Q000150:complications; D055095:Optics and Photonics; D012029:Refraction, Ocular; D012189:Retrospective Studies; D020432:Trochlear Nerve Diseases / Q000150:complications; D014792:Visual Acuity