A person reading using Hamblin or ‘lazy reader ‘ prism glasses

People with PSP often experience problems with their vision. A very common issue is a paralysis of the eyes which prevents the person from looking downwards. This vertical gaze paralysis makes it hard to do everyday tasks that usually require looking down; such as reading, using an electronic device (e.g., phone, tablet, or computer keyboard), and cutting up food. One solution that is often recommended is to use light-bending prism glasses, shown in the top figure.

There are several types of prism glasses (click HERE for more information), but the ones most freqently recommended for people with PSP are called Recumbent prisms. Developed in 1935 by Andrew Mckie Ried as a visual aid for people who were unable to sit up due to spinal injuries (Keeler, Singh, & Dua, 2010), they comprise two prisms mounted on a frame with the base facing the eye. Light entering the prism undergoes a double internal reflection before entering the eye, producing a 90 degree a shift in image position which allows the user to see items such as books when held on the chest. When the wearer is seated in an upright position Hamblin prisms show the user what is in thier lap or on the tabletop, compensating for thier inablity to look downward.  They were manufactured by Messers Hambin of Wigmore street for the 1935 Oxford Ophthalmological Congress, so are often known as Hamblin glasses (Keeler et al., 2010). They are also sometimes called periscope glasses, lazy glasses or lazy readers.

Prism glasses could be useful visual aid that helps people with with PSP to retain some of their independence. However, somewhat surprisingly, there is very little scientific evidence that people with PSP benefit from using prisms glasses. Furthermore, very little is known about how and why people use prism glasses, what kind of instruction or training they are given, and what challenges or issues people with PSP experience when trying to use prisms. To try and start answering these question, Dr Ali Lane and I supervised a research project by two masters students (Dan Ford and James Ireland) in which 35 people with PSP were recruited with the help of the PSPA to tell us about their experiences of using prism glasses. The majority of participants (72%) had heard of prism glasses, and of these participants 51% had actually tried them. The most common source of information about prisms was the PSPA (42%), followed by a neurologist or neuro-opthalmologist (20%), and the most common source of the prisms themselves was the PSPA (31%) , followed by the NHS (19%), and 17% purchased them online. When we asked about the kinds of things prisms helped people with the most common responses were eating, using electronic devices and watching TV. However, when we asked whether people still used the prisms only 22% of people reported persisting with their use. This relatively low adherence may be related to the frequency with which people reported experiencing unpleasant side effects such as disorientation and practical problems with the weight and fit with existing eyewear. We also found that people with gaze paralysis in only one direction (e.g. downward or upwards) found the glasses more useful than people with problems in 2 or more direction.

The results of our survey indicate that while many people with PSP are aware of prism glasses, there is scope to improve patient knowledge of this visual aid. Furthermore, while some people find them useful for eating, reading and using electronic devices, most people with PSP do not appear to benefit from prisms or they find them hard to use. Even those who do find them beneficial do not reliably persist with their use. It is likely that better fitting and more systematic training and practice would improve the usefulness of the prisms and reduce the negative impact of side-effects such disorientation. Indeed, there is evidence from stroke survivors with inattention that training improves adherence and outcomes of prism glasses therapy. However, no such training exists for people with PSP.

It is important to be cautious when drawing conclusions from a relatively small sample, but the findings seem to suggest that Hamblin prisms are, unfortunately, a relatively ineffective assitive tool. We believe this is likely due poorly fitting devices, the weight and inflexibility of over-the-counter Hamblin prisms, a lack of guidance about which patients are most likely to benefit (i.e patients with uni-directional gaze problems) and the types of task that are most likely to benefit (e.g. eating) and a lack of systematic training on prism use. The only way to resolve this issue is to conduct more evidence-based research that can establish the optimal way to use prism glasses for the rehabilitation of PSP. We are currently looking for ways to fund more research in the important area and would be interested in hearing from people who are considering studying for a PhD on this topic.

If you have PSP and would like to try prism glasses, you can request a pair free of charge from the PSPA: helpline@pspassociation.org.uk

You can learn more about our other research into attention and memory in PSP here

Keeler, R., Singh, A. D., & Dua, H. S. (2010). Recumbent spectacles: taking it lying down. British journal of ophthalmology., 94(5), 535.

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