Santosh Chhetri, B. Optom;
Suraj Thapa Magar, B. Optom
M. Optom Student, Tilganga Institute of Ophthalmology, Nepal
Age-related macular degeneration (AMD) is a degenerative ocular condition that is a leading cause of vision loss and blindness worldwide, especially in developed countries with ageing populations. (1) Dry AMD is more common in people aged 45 to 64, while wet AMD is more common in people aged 65 and up. (2)
Patients with AMD often view non-centrally (eccentric viewing), using a preferred retinal location (PRL) outside the scotoma but still close to the fovea, because resolution worsens rapidly with greater distance from the foveal centre. Such patients are sometimes recognizable by their lack of normal eye contact and their need to look sideways at objects, which can cause postural problems. Eccentric viewing is necessary for optimising visual functioning in the presence of a central scotoma but sometimes people with AMD often fail to optimise their use of eccentric viewing. However, training in eccentric viewing to develop these skills is time-consuming and of unknown effectiveness because such training has not been subjected to systematic evaluation. (3) The use of a prism, which is expected to move the retinal picture away from the scotoma to a PRL without requiring conscious effort from the patient, is a potentially cost-effective remedy.
Figure: Image relocation on the retina with prism.
(Picture Courtesy: E-Scoop)
The choice of prism power for the standard group was based on a previous study reporting that the most common power was 6 prism dioptres. (4) However, the study suggested that patients with greater retinal lesions and worse visual acuity (VA) might require more powerful prisms. In a group of patients with AMD and log MAR VA ranging from 1.1 to 1.7, an average eccentric viewing angle of 7.8° was found that would equate to a prism of approximately 13.5 prism dioptre. In the study of Heather et.al, (5) this was reduced to 10 prism dioptres, which was the highest prism prescribed by Romayananda et. al (4) Patients with higher-powered prisms in a later study (6) reported excessive weight, distortion, and dizziness, whereas the subjects included by Romayananda et al appeared to adapt well to the 10-prism dioptre.
The prism base up direction for the standard group was chosen because
- By comparison, studies that have used simulated scotomas in normally sighted subjects show clear effects of PRL location on reading rate, with a right PRL enabling faster reading than a left PRL (7) and a below PRL enabling faster reading than a left PRL. (8)
- In everyday tasks and mobility, it is generally more important to see objects in the lower than the upper field. (9)
- Most subjects have been reported to adopt a PRL below the scotoma. (10) It has been suggested theoretically (11) and observed clinically (9) that reading is more effective if the scotoma is displaced vertically when training eccentric viewing.
The study by Ramayananda recorded an improvement in near vision, using a prismatic scanning technique in 59 patients with macular lesions. (4) In a controlled clinical trial, Rosenberg reported a functional improvement in over 90% of patients in whom bilateral prism correction was given for distance, intermediate, and near vision, as compared with 60% in the control group. (6) Using high prismatic lenses, Verezen noticed an improvement in visual capacities in 61% of treated patients. The study of Maurizio et. al also showed that mono-lateral prismatic correction may be a viable means to improve visual function in patients affected by advanced bilateral AMD. (12)
In conclusion mono-lateral prismatic correction may be regarded as a viable method of improving visual function in patients with severe bilateral age-related macular degeneration. Further study is also required for systematic evaluations of Prism practices to develop evidence-based methods in low-vision services.
Acknowledgment: We thank Rupesh Poudel, Senior consultant Optometrist, Department of Optometry and vision science clinic, Tilganga Institute of Ophthalmology, Nepal for his priceless guidance.
Declaration of interest: The blog is written solely for educational purposes, with no financial support or conflicts of interest.
- Ly, A., Nivison‐Smith, L., Zangerl, B., Assaad, N., & Kalloniatis, M. (2017). Self‐reported optometric practise patterns in age‐related macular degeneration. Clinical and Experimental Optometry, 100(6), 718-728.
- Thapa, R., Paudyal, G., Shrestha, M. K., Gurung, R., & Ruit, S. (2011). Age-related macular degeneration in Nepal. Kathmandu University Medical Journal, 9(3), 165-169.
- Nilsson, U. L., Frennesson, C. H. R. I. S. T. I. N. A., & Nilsson, S. E. (1998). Location and stability of a newly established eccentric retinal locus suitable for reading, achieved through training of patients with a dense central scotoma. Optometry and vision science: official publication of the American Academy of Optometry, 75(12), 873-878.
- Romayananda, N., Wong, S. W., Elzeneiny, I. H., & Chan, G. H. (1982). Prismatic scanning method for improving visual acuity in patients with low vision. Ophthalmology, 89(8), 937-945.
- Smith, H. J., Dickinson, C. M., Cacho, I., Reeves, B. C., & Harper, R. A. (2005). A randomized controlled trial to determine the effectiveness of prism spectacles for patients with age-related macular degeneration. Archives of ophthalmology, 123(8), 1042-1050.
- Rosenberg, R. O. B. E. R. T., Faye, E. L. E. A. N. O. R., Fischer, M. I. C. H. A. E. L., & Budick, D. E. B. R. A. (1989). Role of prism relocation in improving visual performance of patients with macular dysfunction. Optometry and Vision Science: Official Publication of the American Academy of Optometry, 66(11), 747-750.
- Rayner, K., Well, A. D., & Pollatsek, A. (1980). Asymmetry of the effective visual field in reading. Perception & Psychophysics, 27(6), 537-544.
- Petre, K. L., Hazel, C. A., Fine, E. M., & Rubin, G. S. (2000). Reading with eccentric fixation is faster in inferior visual field than in left visual field. Optometry and Vision Science, 77(1), 34-39.
- Nilsson, U. L., Frennesson, C., & Nilsson, S. E. G. (2003). Patients with AMD and a large absolute central scotoma can be trained successfully to use eccentric viewing, as demonstrated in a scanning laser ophthalmoscope. Vision research, 43(16), 1777-1787.
- Bowers, A. R., Woods, R. L., & Peli, E. L. I. (2004). Preferred retinal locus and reading rate with four dynamic text presentation formats. Optometry and Vision Science, 81(3), 205-213.
- Peli, E. (1986). Control of eye movement with peripheral vision: implications for training of eccentric viewing. Am J Optom Physiol Opt, 63, 113-8.
- Parodi, M. B., Toto, L., Mastropasqua, L., Depollo, M., & Ravalico, G. (2004). Prismatic correction in patients affected by age-related macular degeneration. Clinical rehabilitation, 18(7), 828-832.