History Taking and Vision RecordingChapter 1

Among causes of acute fall of vision, central retinal arterial occlusion needs prompt action. Other causes are central retinal venous occlusion, retinal or vitreous haemorrhage in visual axis, optic neuritis, methyl alcohol poisoning, acute angle closure glaucoma, temporal arteritis, etc. Causes of gradual fall of vision are the refractive error, senile cataract, open angle glaucoma, senile macular degeneration, diabetic maculopathies, etc. Important causes of poor vision at night are vitamin A deficiency in children, retinitis pigmentosa, open angle glaucoma, high myopia and early lental opacity. Secondary vitamin A deficiency in adult from liver diseases and choroidal atrophies and degeneration also can produce night blindness.

Visual field defect if involves the macular vision or of acute onset, are usually complained of but slowly progressive defects of peripheral vision are not generally noted by the patients. Field defects are described by patients in many ways. Patients may report as inability to see an object of interest with an eye. This is usually found in macular lesions. Patient may describe floaters or vitreous haemorrhage as some shoots in front of eye. In the event of peripheral field defect they may complain of stumbling on objects lying on the way or colliding with objects on the side. Defects in central field are commonly found in CSR, macular degeneration, optic neuritis, etc. Peripheral field defects are common in open angle glaucoma, retinitis pigmentosa, etc. Vascular lesions of optic pathway, intracranial tumour, toxic neuropathies—all produce characteristic field defects depending on the site of lesions.

Distorted vision, e.g. a straight line may be seen distorted in part of latters may be seen broken in macular degeneration and choroiditis.

Double vision is almost always due to weakness or paralysis of one or more extraocular muscle, decompensated phoria or poor fusional reserve being other common causes. A rapidly developing proptosis or fracture of the orbit may produce double vision by producing anatomical misalignment of the eyes. In early cataract, patients may complain of polyopia or multiple images in the eye.

Always note the location, quality, severity, aggravating and relieving factors and association, if any.

Common causes of acute severe pain in eye are acute glaucoma, exposure to welding arc, etc. Moderetely severe pain may be due to iritis, corneal erosion or injury, ophthalmic zoster, corneal ulcer, bullous keratopathy, scleritis and episcleritis. Mild pain and foreign body sensation are caused by conjunctivitis, blepharitis, foreign body in the cornea or conjunctiva, dacryoadenitis and orbital cellulitis.

Pain and tenderness on the temple in temporal arteritis, at the inner angle and below the eye in acute dacryocystitis, unilateral headache in migraine are important causes of pain around the eye. Brow pain in a postoperative patient may be caused by endophthalmitis. Pain is aggravated by ocular movement in optic neuritis; near work or reading in refractive error and asthenopia, movement of the lid in corneal injury or foreign body, bullous keratopathy, etc.

Pain and headache may be relieved by sleep in asthenopia, phoria and sometimes in chronic congestive glaucoma. Foreign body sensation of conjunctivitis is relieved temporarily by irrigation of the eye. Vomiting may be induced by migraine, acute angle closure glaucoma. Migraine may be associated with scintillating scotoma also.

The thick purulent discharges in neonate that exude from the eye on pressure and fill rapidly after cleansing are suggestive of gonococcal ophthalmic neonaturum. Mucopurulent discharges that glue the lids during sleep and associated with foreign body sensation suggest bacterial conjunctivitis. Thin watery discharges in viral conjunctivitis and keratitis, ropy discharges with itching in allergic conjunctivitis and mucoid discharges on pressure over of inner canthus and below in chronic dacryocystitis are characteristic features. Frothy discharges along lid margin are found in meibomitis. Watering may be due to a foreign body in cornea or conjunctiva, concretion of conjunctiva or denuded epithelium of cornea in abrasion or trichiasis. Iritis and acute angle closure are also associated with watering and photophobia. Reflexive watering on exposure to wind or staring may be early sign of dry eye. Ectropion and blockage of nasolacrimal pathway causes overflow of tear fluid.

The most important cause of painless red eye is subconjunctival haemorrhage. The colour is bright red. Ask for any history of trauma in or around eye, blood dyscrasia, hypertension, diabetes mellitus, etc. Subconjunctival haemorrhage may be found in infections like whooping cough, measles and also in conjunctivitis.

Another system for measuring visual acuity is the ETDRS test. “ETDRS” stands for Early Treatment of Diabetic Retinopathy Study. It is a relatively new system for assessing visual acuity that is gaining rapid acceptance world-wide. The ETDRS test assesses visual acuity in a controlled and standardised, high contrast environment. It is similar to the Snellen's chart, in that, characters of decreasing size are presented to the patient for reading. The key differences are that the ETDRS test does not rely on a flat chart or projected image, but an internally illuminated lightbox on which the characters are painted on one translucent surface.

These are two major advantages of this system. Firstly, characters are painted in absolutely black colour. Thus, they are not only darker, but are more uniform and more fade-resistant as compared to those on paper chart. Secondly, internal lighting is variable. By adjusting internal illumination, the examiner can compensate for differing amount of ambient room-light, thereby ensuring that the test is consistent with universal contrast standards.

Any given scene will have a measurable range of inherent contrast. This range can be changed in response to the events that are both objective (external factors) and subjective (internal factors, i.e. factors arising from the observer). For example, an outdoor landscape on a sunny day will have high-contrast whereas at night, or through fog or rain, the same scene will have lower-contrast. The same highcontrast landscape may also be perceived as lower-contrast if the 6viewer is experiencing glare, has a cataract, is wearing multifocal contact lens, or has a compromised retino-neural system. In these cases, contrast is reduced by subjective or internal events.

Perception of contrast sensitivity is an essential component of functional vision. The ability to perform normal day-to-day activity can be compromised by poor contrast perception. For this reason, measurement of changes in a person's contrast sensitivity has been increasingly important as a method of assessing any individual's overall functional vision.

There are several methods for measuring contrast thresholds. Some of these methods, like Regan's charts, present alphabet characters at multiple levels of contrast, etc. are collectively known as “Low-contrast letter identification, tests”. However, the principal method for contrast assessment, in use since 1984, is the sine-wave contrast sensitivity test.

Many researchers believe that human visual system is composed of separate neural channels which independently respond to and analyse visual informations based on contrast as well as size and shape. The information for all the channels is combined by the brain to create the images we perceive. Based on this knowledge, researchers developed visual testing procedures which try to measure a person's contrast sensitivity via these separate channels. This may be done by using sine-waves. This is consistent with the goal of creating a sensitive test that assesses the performance of each neural channel individually. As sine-wave contrast sensitivity testing measures the ability to see pure space waves, this is the method found to provide the most sentitive and most specific test of individual's contrast vision channels.

Contrast sensitivity test patterns may be presented on a video monitor, or an illuminated disc in a view box or on a wallchart. The fundamental task is to identify the orientation of slanted bars or to read letters of varying contrast. Sine-waves are waves like space patterns and the graphic representations of them are called “sine-wave gratings”. As changes occur to each of the various characteristics of light (brightness, colour, contrast and others), the sine-waves representing these characteristics change accordingly. In this way, each subtle change in light prompts a subtle change in the sine-wave pattern which in turn stimulates a different neural channel.

The ability to distinguish simple visual patterns corresponds to the ability to distinguish more complex visual patterns. Visual response to sine-wave gratings correlates well to contrast sensitivity and permits accurate and predictable functional vision assessment for diverse objects.

Visual information is isolated into its component parts; each of which is transmitted to the brain via its own channel. These channels are depicted as narrow-curves; each one corresponding to a different component of light. In this way, sine-wave contrast sensitivity tests each channel, separately. The brain recombines these channels into a single, composite image-depicted as the wide-curve and known as “contrast sensitivity function”. It is the sum of all these signals that creates the final image we see.

This is how sine-wave gratings look when used to test contrast. The changes in the character of light prompt changes in the way the sine-wave is depicted on each grating in a typical sine-wave test, different sized gratings measure contrast sensitivity over a range of patterns whose characteristics correspond to those of everyday objects.

7Visiogram: Plotting the results of this testing produces a curve representing contrast sensitivity. This curve is called as “visiogram” or “contrast sensitivity function chart”. This is a highly accurate diagnostic and monitoring tool for the ophthalmologist.

Pelli-Robson chart: The Pelli-Robson chart is an 86 × 63 cm chart that is hung 1 m from the patient's eye. The letters are equivalent to 6/270 Snellen's letters. This letter chart, counting 8 rows of six letters, arranged in groups of three letters (within each triplet the letters have the same contrast) which are of constant size but vary in contrast. The contrast decreases from above down (contrast in each successive triplet decreases by a factor of 0.15 log units) ranging from 100 to 0.9 per cent. The subject is asked to read the letters until two or more errors are made in a group of three. Incorrectly identifying the letter ‘C’ as an ‘O’ is a common error and can be counted as correct. The contrast threshold is taken as the last group in which at least two out of three letters are correctly identified.

Regan chart: The Regan's low-contrast acuity charts are an example of a low-contrast acuity test. Four charts, one each for high-contrast (96%), intermediate-contrast (25%), low-contrast (11%) and very low-contrast (4%) may be presented to the patient under constant illumination. Typically, the normal eye will lose about four lines of Snellen's acuity from the high-contrast (96%) to the low-contrast (11%) chart and seven lines from the high-contrast (96%) to the very low-contrast (4%) chart. A patient's loss of acuity between the higher- and lower-contrast charts is then compared to the standardised nomogram.

10 per cent (Michelson) contrast Bailey-Lovie chart: Baily and Lovie adopted ten 5:4 aspect ratio letters—DEFHNPRUVZ—which has been shown previously to have relatively equal legibility. The chart has a constant number of letters, namely five, on every line and has a characteristic V shape. The letter size progression is constant on the Baily-Lovie chart and geometric, such that the letters decrease in size by a factor of 10√10, which equals 1.2589 and approximates a ratio of 5:4. This progression may also be expressed as 0.1 log₁₀ units, and the charts are frequently referred as “log MAR charts” because of this constant logarithmic size progression. This choice of progression means that a 10-line progression down the chart corresponds to a 10 x reduction. Similarly, a 3-line change corresponds to a 2 x reduction, a 5-line change approximates a 3 x reduction, and a 7-line change corresponds to a 5 x reduction. In order to equalise any contour interaction effect, the spacing between letters on a given line is also constant, being equal to the letter's width. The spacing between two adjacent lines is equal to the height of the letters on the lower line and, therefore, equal to the width of the letters of the upper line (Fig. 1.2).

The goals of visual acuity testing is to determine the smallest high-contrast letter size a person can see. But visual acuity testing alone has several limitations owing to the relatively small number of retinal cells excited by this test and the large and non-specific range of channels tested by alphabet letters. On the other hand, the goal of sine-wave contrast sensitivity testing is to measure visual response over a wide range of visual stimuli. Sine-wave testing assesses the health of multiple visual channels, one channel at a time. In this way, the contrast perception of 8sine-wave gratings more meaningfully corresponds to our everyday perception.

In addition to sine-wave grating charts, low-contrast letter identification test charts have also been developed to assess contrast. These charts range from having single-sized letter formats (the Pelli-Robson chart) to one or more variable-contrast acuity charts (the Baily-Lovie and Regan charts). Although these tests represent improvements over the traditional Snellen's acuity test charts, they are less sensitive and less specific to contrast loss than are sine-wave gratings.

One method of testing contrast sensivity used since 1984 is the VCTS chart. VCTS stands for Vision Contrast Test System and is sometimes known as the “Ginsburg's chart”.

A VCTS chart measures contrast sensitivity using sine-wave gratings of varying sizes and contrast to simultaneously test both visual acuity and contrast sensitivity. For both near and farvision testing, patients must correctly identify the orientation of the lowest-contrast grating they can see.

The visual acuity is a measure of the resolution limit of the visual system. With the VCTS, acuity corresponds to the highest patch that can be seen on the chart. The bracketed contrast sensitivity values on the graph represent the average values for the last patch a person having the equivalent acuity, could see. VCTS chart consists of 5 rows of 9 circular sine-wave grating patches; spatial frequency remains constant across a row and increases down a column from 1.5 to 18.0 cycles per degree. The contrast increases from right to left. The grating is either vertical or tilted clockwise or counter-clockwise 15 degrees, the last patch in each row is blank. The chart is uniformly illuminated and subject is instructed to indicate the orientation of the grating or to respond blank if it is not visible. Contrast threshold is taken as the contrast prior to the first incorrect or blank response.

A new version of the Vistech chart (Fig. 1.3) incorporates some of the design features suggested by the American Academy of Ophthalmology which was missing in the original. in addition, a spin-off from the Vistech, the CVS-1000 has recently been shown to have superior repeatability to the original Vistech.

There may be difference between the acuity values obtained using the VCTS and those obtained through other means. These differences may occur for two reasons:

It is important to note that, if an individual can read any 20/20 letters, then he must have sensitivity at 18 cycles per degree. However, his threshold at 18 cycles per degree may be above the contrast levels available on the VCTS. The patient may have to skip back to the previous line to be able to see a grating.

The diagnostic specificity of this test is low, as it cannot detect specific disease. Abnormalities have been associated with a variety of diseases including cataract. Patient with cataract have better contrast sensitivity when tested in dimmer light than when tested in brighter light.

The following systems are used to test glare disability.