Vision Testing Then and Now- from 1920’s to 20/20
Science cafe welcomed Simon Frackiewicz at its May meeting. Simon is a not infrequent presenter at Science Cafe and came to speak about the medical examination of that most important of sensory organs, the human eye. It was put in the context of historical progression pitting modern day techniques against those from 1927, this being the date at which Simon’s private business first established itself in the field of optometry. 1927 was also marked the establishment of the BBC by Royal Charter, the first transatlantic telephone call, the birth year of Ken Dodd and the inauguration of a recognised diploma of expertise for optometrists.
Simon is an optometrist and orthoptist both at Yeovil District Hospital and in private practice. Orthoptics deals with achieving coordination of the eyes for good binocular vision (binocularity being the key to good 3-dimensional vision) and the purpose of optometry can be divined from its Greek roots, op... for eye and ..metro for measurement. In essence, optometry is concerned with examining the eye and making objective measurements to ensure continued ocular health and optimum optical performance, and from a scientific point of combines the study and application of both physics and biology.
2 Aspects of Measurement
Various aspects of ocular assessment were considered:
Visual Acuity (sharpness of vision)
Simon started with the fundamental metric of vision, visual acuity or sharpness of vision. The ancient Greeks, being keen and competent astronomers, were particularly interested in this and defined ‘normal’ vision (now known as 6/6 or 20/20 vision) as being able to separate the stars Mizar and Alcor in the Great Bear (Ursa Major). But how does this apply to an opticians’ test chart and how do we interpret degrees of acuity in sight? The ‘normal’ level of vision was set at being able to distinguish 2 points of light with an angle of 1’ minute of arc (the so-called minimum angle of resolution).
The Snellen chart is the most familiar of the optometrist’s tool kit and named after its developer, Dutch ophthalmologist, Herman Snellen (1834-1908). Snellen experimented with abstract symbols across a 5 x 5 grid. A year later he started using the alphanumeric capitals as we know them today. Snellen developed charts using symbols based in a 5×5 unit grid, each unit subtending an angle of 1 minute of arc and the whole letter being 5 minutes of arc (see Fig.1).
Fig. 1 Construction of Snellen letters (optotypes)
Snellen defined “standard vision” as the ability to recognize one of the optotypes when it subtends an angle of 5 minutes of arc (i.e. each limb subtends 1 minute or arc). Considering the whole chart:
Snellen Fraction = Distance at which test is carried out .
Distance at which the smallest optotype seen subtends an angle of 5’ of arc
For example, 6/60 means the ability to see an object only at 6 meters which should be normally seen at 60 meters. The Snellen chart incorporates variable letters per line, size decreases down the chart with little consideration given to visibility of individual letters (c.f. ‘M’ and ‘I’- see Fig.2). 6/6 in metres equals 20/20 in imperial.
Fig. 2: LHS Snellen charts, the first being a dated serif font and the second sans serif. RHS the more modern LogMAR chart designed by Ian Baily and Jan Love-Kitchin in 1976
Lack of standardisation of the Snellen has led to a search for a better means of assessing acuity. Many ophthalmologists and vision scientists now use the improved LogMAR chart. Visual acuity if scored, not by a Snellen fraction, but by the logarithm of the minimum angle of resolution (MAR). Typeface, size progression, size range, number of letters per row and spacings are selected in an endeavour to achieve a standardisation of the test task. Counterintuitively, when using the LogMAR chart, lower scores indicate better vision and vice-versa
For children, or non-letter readers, the 4-position tumbling E chart can be used, or pictorial cards such as Cardiff or Kay charts incorporating pictures of animals or geometrical shapes can be presented for recognition, always ensuring that the animal or shape is drawn with reference to its minimum resolution angle. Most recently Lea Symbols (see Fig.3) have been adopted using four symbols which are more similar and more universally acknowledged than letters.
Fig.3: LEA Visual Test System using four different optotypes
Field of Vision (Width of Vision)
The other major parameter of vision is the so-called field of vision, in other words how widely a person can see. Ophthalmologist, Henry Traquair (1895-1954), likened the sensitivity of the retina across its surface to an ‘island of vision’ (see Fig. 4) in a ‘sea of darkness’. With a high sensitivity in the centre of the retina (the macula) and a lessening of sensitivity towards the periphery (this is a function of the packing of rod and cone cells in different parts of the retina and the way their impulses are combined in visual processing). Except for the physiological blind spot, the hill is distorted, diminished or missing (in whole/part) where there is disease. Consequently, various medical conditions such as glaucoma, stroke, pituitary disease, or neurological deficits can be detected.
Fig. 4: The ‘normal’ island of vision (after Traquair). The hill is highest at fixation, where visual sensitivity is greatest. The height of the hill of vision declines toward the periphery as visual sensitivity diminishes.
Clinically, this island of vision can be explored by either static or dynamic perimetry (see Fig. 5). This is performed clinically by keeping the subject's gaze fixed while presenting objects at various places within their visual field.
Fig. 5: In static perimetry, the intensity of a stationary target of constant size is varied to determine the sensitivity of specific locations in the field of vision. In kinetic perimetry, a stimulus of set size and intensity is moved from non-seeing to seeing areas of the visual field. The island of vision is approached horizontally, and isopters, depicting areas of equal retinal sensitivity, are plotted.
In 1927, the standard method was the Bjerrum screen (after Danish Ophthalmologist, J.K. Bjerrum 1851- 1920), a two-metre black, square screen where an operator wearing a black glove would discretely move a white/coloured target to determine the contours of the visual field. This was used in hospitals into the 1980’s. In the 1990’s, electronic static perimeters (e.g. Friedmann perimeter) were introduced. The dynamic arc calibrated bowl projection instrument, such as the Goldmann (in progressive development since the 1940’s), is able to measure the full extent of the visual field and is still considered a gold standard in perimetry. The size and intensity of targets may be varied to plot different isopters kinetically and determine local static thresholds. Whilst computerised plotting is both standard and highly effective, arguments do rage as to which is the better algorithm to use in assessment (e.g. SITA, GATE, ZEST etc).
This is determining an accurate prescription for maximal vision. Refraction can be objective and subjective. The former is designed to permit a prescription to be obtained objectively, without input from a patient, useful for children and perhaps the uncertain, by using a hand-held retinoscope, an instrument available as much today as in 1927. More recently, such objective determinations have been surrendered to technology whereby a refractometer can determine the prescription with a high degree of accuracy. Subjective assessment requires involvement from the patient responding to questions as to which of alternative lenses offered is the best. It is carried out using a range of lose, trial case lenses, again the standard method in 1929 as well as in 2023, though nowadays phoropters are available, which instead of lenses being lose, they are contained in a unit adjacent to the eye and lens changes are made electronically (see Fig. 6). Such methodology can be faster but is certain circumstances accuracy might be affected because the bulk of a phoropter against the face can feel somewhat different to a trial frame and lenses.
Fig. 6: Determining a prescription LHS the traditional method of trial lenses and a trial frame, still very much in use. RHS a phoropter containing banks of lenses.
Examination inside the eye is as important today as it was in 1927, though as expected the mode of achieving this is somewhat different (see Fig.7). The direct ophthalmoscope, the standard instrument to examine the fundus, was invented by Hermann von Helmholtz in 1851 and made portable by Francis Welch and William Allyn in 1915. Welch Allyn is still a major producer of diagnostic instruments. The direct ophthalmoscope gives an excellent view of the central section of the retina but viewing the more peripheral areas, of relevance regarding tears and detachments, is not possible. In 1927, Simon’s forebears would have made full use of this method. It can be more effective if the pupils are dilated, in 1927, probably with atropine (belladonna), but today with safer drugs such as tropicamide.
Fig.7: Direct and Indirect Ophthalmoscopy
More recently, new optical systems have given rise to indirect ophthalmoscopy permitting a wider view of the back of the eye allowing the retina to be viewed almost to its extreme periphery (this is where detachments and tears often occur). Image capture can be included during examination providing a permanent record to attached to a patient record. The less than perfect nature of imaging lead to the missing of a retinal detachment in 5 year old Leif Anderson. His father Douglas, an engineer, developed a new imaging system that allows the full extent of the retina, right to the periphery, to be imaged even in uncooperative subjects such as young children and without needing pupil dilation (see Fig. 8). The technology is known as Optos and is available at some optometrists. Another recent introduction is the Ocular Coherence Tonometry (see Fig. 8). Traditional optical imaging gives a 2-D view of the retina. What goes on in deeper retinal layers may have to be inferred. The OCT cleverly takes vertical slices through the retina allowing many more conditions to be detected earlier and is one of those technologies which really is a game changer.
Fig.8 LHS an optos image showing a retinal detachment. RHS OCT and Optical images compared. Vertical sectioning shows viability of all retinal layers lacking in the 2-D optical image.
The eye is, by necessity, rigid to present a good imaging surface, but being made of soft tissues, it requires to be pressurised to achieve this rigidity, the pressure being maintained by the balance of the creation and drainage of fluid from the eye. If drainage is compromised, ocular pressure becomes too high, and irreparable damage to the retina can occur with consequent blindness, such a condition being the disease of glaucoma. Historically, eye specialists would palpate the eye to determine ocular pressure, gently pushing a finger into the upper part of the globe to assess rigidity and therefore pressure in much the same way that a car driver might consider the pressure is a car tyre. It is unlikely such a method was particularly accurate except in detecting extremes of pressure. In the later 19th century, Norwegian eye surgeon, Hjalmar Schiotz (1850-1927) developed an instrument to measure the pressure objectively (see Fig. 9). As can be seen, applying the instrument to the eye was a serious undertaking and would have been the operative method of checking IOP’s in 1927. Thankfully, such an instrument was applied to an anaesthetised cornea, in the 1920’s possibly using cocaine as the anaesthetising agent.
Fig.9: LHS the Schiotz tonometer in use. RHS a more modern, gentler version of the same instrument
In the Schiotz, a weighted plunger indents the cornea, the indentation being converted to pressure via the indicator.
In the USA, the use of the Schiotz (because it contacted the eye) was considered a medical procedure and required optometrists to refer the patient to an ophthalmologist for a pressure measurement with added inconvenience and cost. This led to the development in the 1970’s of the non-contact tonometer or NCT (the ‘airpuff’ instrument) which allowed optometrists to measure pressure without infringing local medical laws. The Gold Standard tonometer today is the Goldmann which sits as an adjunct on the slit lamp and like the Schiotz is a touch applanation instrument but much gentler in action.
Optometry, both old and new, provides good evidence of sound scientifically based clinical techniques, of taking on board new technology, and ultimately providing an overall benefit to the ocular health of the population.