A low vision assessment determines how a pathology has affected a person’s visual function and how these changes affect their daily life.
Case history
The most important part of a low vision assessment is obtaining the case history, which is more in-depth than for a standard eye examination. During this conversation, the practitioner will ask the patient about visual history and general health. They will also delve deeper into the patient’s social situation to find out if they are currently receiving any support from social services or third sector organisations at home, work or school if appropriate.
The practitioner will enquire about specific difficulties the patient may be having. As an example, reading encompasses a wide variety of tasks and most low vision aids (LVAs) are task specific, so the aid that is needed for reading a book may well be different to the aid for reading small print on food packets.
Daily living tasks such as cooking, socialising, hobbies and taking medication safely also need consideration. It is important to discuss lighting, including the habitual lighting in the patient’s home, and what lighting conditions cause them problems.
Practical assessments
A full assessment of visual function is then carried out, starting with the monocular and binocular distance visual acuities. The use of a LogMAR chart is preferred for low vision assessment for several reasons1. Each row has the same number of letters, with each letter being scored individually, and there is a uniform progression of letter sizes so the chart can be used at different distances, as close as 0.5 metres if necessary. For adults with learning difficulties and children, a LogMAR Kay Picture test can be used.
Near acuities are then assessed using a reading chart appropriate for patients with low vision. Standard reading charts often don’t have large enough text to be useful in the low vision clinic, and the use of a chart such as the Bailey-Lovie, which starts with a text size of N80, is preferred2.
Some charts use unrelated words, which can give a more accurate picture of the patient’s acuity threshold, as they are not able to guess words from the context of the sentence. A Bailey-Lovie chart also allows for recording near acuity in LogMAR notation if used at 25cm. Near acuities can also be assessed using any LVAs that the patient currently uses.
As mentioned previously, contrast sensitivity is very important to assess the patient’s real world functional ability. It is usually assessed in the low vision clinic using a chart such as the Pelli-Robson chart3, which has rows of letters of equal size but reducing contrast. Letters of similar contrast are grouped in threes, with two groups per line. The chart is used at one metre, and the patient’s contrast threshold is recorded as the last group of letters they were able to see.
It may be useful to assess the patient’s visual fields to give an understanding of how any loss might affect their orientation and mobility or interfere with near vision tasks. However, full threshold tests can be long winded and stressful for people with impaired vision. For most patients, a confrontation test will give adequate insight regarding peripheral defects. For central vision, the Amsler grid can be used, although it’s reliability has been questioned4.
Goal setting
Once we have a clearer picture of the patient, their circumstances including existing support, their visual function and what causes difficulties for them in their daily life, we can agree on some goals to work towards. It is important at this stage to manage a patient’s expectations, and to explain that LVAs are not a ‘cure’ for their visual loss.
Simple interventions such as providing some suitable sunglasses to alleviate glare, or supplying a handheld magnifier to read small print on packets may be a “quick win”, but often a variety of different solutions will be needed.
References
1. Bailey IL and Lovie JE. New design principles for visual acuity letter charts. Am. J. Optom. Physiol. Opt. 1976; 53:740-745.
2. Bailey IL and Lovie JE. The design and use of a new near- vision chart. Am. J. Optom. Physiol. Opt. 1980; 5:378-387.
3. Pelli DG, Robson JG and Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin. Vis. Sci. 1988;2:187-199.
4. Schuchard RA. Validity and interpretation of Amsler grid reports. Archives of Ophthalmology 1993;111:776-780.
Abi Crutcher is an extended services contact lens optician with a professional certificate in low vision. In addition to working in practice, Abi represents dispensing opticians on her regional optical committee and, through that, on the Welsh Optometric Committee. She represents optometry on her local primary care cluster committee – and is ABDO’s regional lead in Wales.