There are 3 types of prism commonly available to people with PSP.  Hamblin glasses (sometimes known as lazy glasses) are cheap and easily purchased via the internet. They produce a 90 degree shift in the image position. This allows the user to see what is in their lap without needing to bend their neck. This type of prism may be useful for people with PSP when performing tasks such as reading, eating or using mobile devices. They may also allow the wearer to see what is on the ground in front of them without looking down. This may have some advantages for mobility. However, as also they occlude vision straight ahead, the wearer cannot see where they are going, so they may create additional issues for mobility.

Prism lenses are commonly used to correct double vision in ophthalmologic problems such as strabismus by aligning the image eye with the image in the preserved eye. They typically produce a much smaller shift in the image position than Hamblin glasses. Glass prisms have high optical clarity so are suitable for tasks that need acute vision, such as reading, using computers or watching TV. They are worn alongside normal prescription so do not occlude vision. However, they are much more expensive than Hamblin glasses and require a specialist to grind and fit the prism. Furthermore, the maximum shift is typically ~10 diopters (this is equivalent to ~5 degrees of visual angle) and it is unclear whether the image shift would be sufficient to provide a functionally useful change in vision in PSP.

Adjustable prism lenses [1] are a custom-made device reported by Sadowski (2000).  The patient required 20 diopter prism for eating and a 30 diopter lens for eating. Sadowski mounted the prism lens on an adjustable arm in front of the usual prescription. This allowed the user to flexibly adjust the image position on the retina and thus more easily move between eating and reading, while reducing the overall weight and preserving visual acuity. This solution was flexible but the glasses had to be custom made and were still quite heavy, requiring a headband to ensure they did not slip off.

Fresnel Prisms are soft plastic prisms that can be attached to existing eye wear. They are typically fitted to the periphery of the glasses, leaving the centre clear. When fitted to the blind side of hemianopes they can expand the visual field sufficiently to help to avoid obstacles in the blind hemifield[1]. People with PSP might benefit from Fresnel prisms fitted to the lower part of their spectacles to increase visibility of the area around their feet, which might help with mobility (although no studies have actually tested this). They are cheap and easy to use and have a maximum deviation of 40 diopters (~25 degrees of visual angle). However, the optical quality is relatively poor so they are probably of limited use for tasks that require precise vision such as reading, watching TV or using computers.

 

 

[1]Sadowski, B. (2000). An optical device for patients with downgaze problems shown in a patient suffering from Steele-Richardson-Olszewski syndrome. Neuro-Ophthalmology, 24(1), 311-314. doi:10.1076/0165-8107(200008)24:1;1-v;ft311

 

[2]Bowers, A. R., Keeney, K., & Peli, E. (2014). Randomized Crossover Clinical Trial of Real and Sham Peripheral Prism Glasses for Hemianopia. Jama Ophthalmology, 132(2), 214-222. doi:10.1001/jamaophthalmol.2013.5636