Prism bars

These can range from blurred and double vision, difficulties in concentration during close-work tasks, skipping words and lines while reading, and, occasionally, difficulty tolerating relatively small changes in their spectacles.
 
In some cases, the cause of the patient’s symptoms can be obvious and can be diagnosed as significant refractive error, poor convergence, reduced amplitudes of accommodation, large or small phorias that are decompensating, or corneal, retinal and lenticular pathology.
 
However, sometimes despite a very thorough refraction, examination and assessment using fixation disparity tests, the cause of what at first appears to be an eye-related problem can be elusive. 
 
This is the point in my evaluation of the patient that I reach for my old friend – a well-used box that pre-dates me, containing a pair of old-fashioned prism bars. I use these to measure distance and, more often, near prism fusion ranges.  

I use the horizontal prism bar most frequently to test the horizontal prism fusion range, but occasionally, especially in the presence of anisometropic patients with asthenopia, I will use the vertical prism bar in order to check the vertical prism fusion ranges.  

"They may not be high tech, but they provide information that no other test can. They are also cheap to buy and last a lifetime"

Prism process

When using the prism bars, the patient is simply asked to report when a row of words or letters of an appropriate size first becomes blurred – N5 is generally used at near for patients with normal vision and accommodation – and they cannot see the text clearly after the lens is placed in front of it. The prism power is then slowly increased.

The traditional technique sees the practitioner introduce the lowest powered prism lens in front of the patient’s eye, which is usually the right for convenience, with the prism bases nasally. The prism power is then slowly increased, while the practitioner watches to ensure that the patient’s eyes make the appropriate vergence movement and are making an effort to fixate. This is often called measuring the negative prism fusion range (ie divergent), however, those who have trained in the US tend to call it the ‘Base In’ reserve.  

This process effectively measures how much effort the patient can use in order to maintain single clear binocular vision while having to diverge.   

Once the patient reports the first blur, they are then asked to report when diplopia is noted and then when the target becomes clear and single, as the prism power is decreased at a constant rate. 

The test is repeated holding the prism bar in the opposite direction in order to measure the positive prism fusion range, known as ‘Base Out.’ This assesses how well the patient can converge without experiencing blur or diplopia.

Different settings 

When in orthoptic practice, the prism break point is often used as a measure of the patient’s motor-fusion ability. However, the point of first sustained blur also gives an indication of how quickly the patient is likely to experience symptoms when the motor-fusion system is required to sustain prolonged activity. In symptomatic patients, it will also show how the low fusion range to blur will affect their ability to maintain the effort in order to process visual information.
 
Low prism fusion ranges in patients with asthenopic symptoms after a period of close work often provides an explanation where no other abnormal test results are identified. 

The test takes no longer than a minute or two, and treatment with simple orthoptic type exercises often resolves the problem in just a few weeks. 

I could not manage without my prism bars. They may not be high tech, but they provide information that no other test can. They are also cheap to buy and last a lifetime. Furthermore, a set of basic prism bars could be yours courtesy of eBay from as little as £60, so what are you waiting for?

Watch OT's skills guide videos on Motility, Fusional range, The cover test and Accommodative facility featuring Paul Adler.

Image credit: Louis Stone