Assessing the visual field.
Assessing the visual field.
From quite an early period various instruments existed to detect a patient's blind spot and to detect pathological change in his visual field. Even today a simple target fixation test may be employed, using nothing more sophisticated than a pen, for example.
The first attempt to achieve a diagnostic result dates from 1856 when the Berlin ophthalmologist Von Graefe (1828-1870) used a campimeter to examine the visual field of a patient with amblyopia.
In 1857 Herman Aubert constructed the first dedicated investigational perimeter (an arc-shaped instrument later perfected by Richard Förster to whose name this type became attached).
An arc perimeter is used to measure the angle of extent of the patient’s visual field by moving targets along the arc until they can no longer be seen and recording the result. The targets may be an integral part of the instrument running along a groove in the arc or they may be coloured discs on the end of hand-held wands that the optometrists holds against the arc in various places. The arc revolves so that the field can be assessed across any meridian. The instrument can also be used to identify the characteristics of the visual field such as scotoma (of which the physiological blind spot is an example that we all have).
From the 1860s some perimeters were built with automatic recording devices.
In 1872 Scherk introduced the hemispherical perimeter. This type remained a much less popular version until after the Second World War when larger electronic visual field plotters were designed along superficially similar lines.
In 1882 McHardy introduced the quarter arc perimeter.
In 1888 a hand-held version by Schweigger was pioneered for use at the patient's bedside. This was quite small, the arc was just 20cm in size, in contrast to the heavy clinical instruments that often filled huge crates. Many hand perimeters survive from the early-mid 20th century, the example illustrated had a celluloid scale and was supplied by J & R Fleming Ltd in the early 1930s. The 'angle of arc' could be read off a round scale by means of a pointer. Some models, including this one, were also supplied with a detachable base for use on a table.
In the 1920s illuminated arcs were introduced. Some models now projected the test object thereby avoiding giving the patient a giveaway hint of where to look due to mechanical noise.
The Lister perimeter, supplied by Theodore Hamblin Ltd, consisted of a rotating arc with sliding colour targets. There was a lamp holder with a directional shade. Note also the adjustable double chin rest, a round chart holder and an integrally mounted case of white, green and red colour targets.
In 1945 Goldmann published on the principles of quantitative perimetry and his instrument set the standard for most post-war models.
Modern perimeters are often computerised. Some provide something akin to a sealed environment for the patient's head. Instructions on how to look for the lights and how to record spotting them are given by a recorded voice which may also offer encouragement along the lines of 'You're doing really well'.