Why each eye is tested separately

Why is it more of a strain on the extrinsic and intrinsic eye muscles to look at close objects than at far objects? Using these muscles will constantly cause strain. But looking at far objects don't need to converge and accommodate so you don't use these muscles making less strain on the eyes. Explain why vision is lost when lights hits the blind spot? Explain why each eye is tested separately when using the Snellen eye chart.

Explain the difference between binocular and panoramic vision. Standard adjustments for presbyopia are available based on age alone. If the eyelid or lashes obstruct the visual axis, the lid may be taped to the forehead to lift it out of the way. During Humphrey Visual Field HVF testing, the patient places his head in the chinrest and fixes his gaze toward a central fixation point in a large, white bowl.

As stated above, this test is an example of static perimetry. It assesses the ability to see a non-mobile stimulus which remains for a brief moment ms in the visual field.

When the patient sees a presented stimulus, he presses the button on a handheld remote control. Stimuli vary in size and luminous intensity. The luminous intensity of the stimuli can be varied over a range of 0. It is reported in decibels dB of attenuation, or dimming, extending from 0 dB the brightest, unattenuated stimulus to 51 dB the dimmest, maximally attenuated stimulus. SITA is a forecasting procedure that uses Bayesian statistical properties that is similar to the methods used for providing weather information and predictions.

SITA allows for more rapid analysis than would be possible without forecasting. By taking into account a user's results in nearby locations, stimuli that are unlikely to be seen, or extremely likely to be seen are not tested exhaustively.

Instead the stimuli that are likely near threshold are tested. All of the information provided on the visual field printout is important. Patient identity information and the specific test and stimulus size are located near the top of the analysis. It is important to verify that the patient's birthdate was properly entered as an error will result in comparisons with normals in the wrong age group. Beneath the patient's name is a statement giving information about the testing parameters, such as "Central Threshold Test.

The next number indicates how the grid of points is aligned to the visual axis. The number "1" indicates that the middle points are overlying the horizontal and vertical meridians.

The number "2" indicates that the grid of points straddles these meridians. This is the setting most commonly used, as it is easier to assess whether visual field defects respect the horizontal or vertical midline. Next on the report are the reliability indices, including fixation losses, false positives, and false negatives.

Fixation losses occur when the patient reports seeing a stimulus that is presented in the predicted area of the physiologic blind spot. False positives occur when a patient presses the button when no stimulus is presented. Eager-to-please participants sometimes struggle with high false positive rates i. False positives can often be corrected by providing a simple statement that many stimuli will not be seen even with normal vision. False negatives occur when a patient fails to see a significantly brighter stimulus at a location than was previously seen.

False negatives are usually the result of attention lapses or fatigue and are difficult to correct. The threshold values of each tested point are listed in decibels in the sensitivity plot. Higher numbers mean the patient was able to see a more attenuated light, and thus has more sensitive vision at that location. To the right of the numerical sensitivity plot is the grayscale map.

This map presents sensitivity across the patient's visual field with lighter regions indicating higher sensitivity and darker regions reflecting lower sensitivity. The sensitivities are not compared to any normative database. Therefore the map may draw attention to an irregularity within a field, but may minimize field loss if loss is more homogenous across the field. Caution should be used as it can be misleading based on where the machine chooses to make the cutoff between the different shades of gray.

The raw threshold data should always be assessed in conjunction with the grayscale representation. The numerical total deviation map compares the patient's visual sensitivity to an average normal individual of the same age. It is useful to compare with age-matched normal thresholds as sensitivity normally decreases gradually with age. Positive values represent areas of the field where the patient can see dimmer stimuli than the average individual of that age.

Negative values represent decreased sensitivity from normal. The numerical pattern deviation map shows discrepancies within a patient's visual field by correcting for generalized decreases in visual sensitivity. It is useful to show localized areas of sensitivity loss hidden within a field that is diffusely depressed. For example, a person with dense cataracts may have decreased threshold across the entire visual field and this may obscure more focal losses due to coexisting disorders like glaucoma.

Rather than comparing the patient's threshold values with a normative database, the pattern deviation analysis finds the patient's 7th most sensitive 85th percentile non-edge point and gives it a value of zero [6]. Each other test location is then compared with this value to correct for any generalized depression. It has been demonstrated that this method is the best for separating widespread or diffuse loss from localized loss.

The bottom-most probability plots are grayscale versions of the total deviation and pattern deviation maps. These maps may be useful to visually represent the statistical significance of the total and pattern deviation calculations. The grayscale maps should only be interpreted in conjunction with the numerical maps to avoid extrapolations. On the right side of the printout are several useful numbers. The glaucoma hemifield test GHT compares groups of corresponding points above and below the horizontal meridian to assess for significant difference which may be consistent with glaucoma.

Mean deviation MD is the mean deviation in the patient's results compared to those expected from the age-matched normative database. This calculation weighs center points more highly than peripheral points. Pattern standard deviation PSD is a depiction of focal defects.

It is determined by comparing the differences between adjacent points. Higher values represent more focal losses, while lower values can represent either no loss or diffuse loss. Short-term fluctuations SF are a calculation portraying the variability between repeated measurements of the same test location.

High SF decreases the reliability of the test. Along the bottom of the HVF printout is a gaze tracker. The patient's pupil is monitored during testing, and each time the pupil moves representing a loss of fixation or head alignment , an upstroke is recorded.

Losses of fixation decrease the accuracy of visual field testing because abnormalities will not correspond with the expected anatomic region of the retina and some may be missed entirely.

When the gaze tracker loses view of the pupil representing a blink or droopy upper eyelid , a downstroke is recorded. Pupillary obstruction can also decrease the accuracy of results. Goldmann visual field GVF perimetry is not as widely available as HVF because it requires skilled perimetrists who manually map the visual field without the aid of a computer algorithm.

Light is projected into a white bowl with a standardized background light intensity. The projected light forms a fairly circular stimulus. Six stimulus sizes are available, ranging from 0. The overall field mapping technique used is a form of kinetic perimetry, where a stimulus is moved into the field of vision. When the patient sees the stimulus, he indicates so with a low-tech method.

At the University of Iowa a washer is given to the patient, with instructions to tap the table with the washer whenever the stimulus is seen. The perimetrist then makes a mark at the point where the stimulus was seen. To account for reaction time, a good perimetrist consistently adjusts the location of the mark. Your doctor may use a computerized refractor to estimate your prescription for glasses or contact lenses.

Or he or she may use a technique called retinoscopy. In this procedure, the doctor shines a light into your eye and measures the refractive error by evaluating the movement of the light reflected by your retina back through your pupil. Your eye doctor usually fine-tunes this refraction assessment by having you look through a masklike device that contains wheels of different lenses phoropter. He or she asks you to judge which combination of lenses gives you the sharpest vision. During manual Goldmann visual field testing, you look at a testing screen on which small spots of light come into your field of view.

You press a button each time you see the light spot and your response is mapped by a technician. Your doctor evaluates this map, which helps identify defects in your field of vision. Your visual field is the full extent of what you can see to the sides without moving your eyes.

The visual field test determines whether you have difficulty seeing anywhere in your overall field of vision. Types of visual field tests include:. Using your responses to one or more of these tests, your eye doctor determines the fullness of your field of vision. If you aren't able to see in certain areas, noting the pattern of your visual field loss can help your eye doctor diagnose your eye condition.

You could have poor color vision without realizing it. If you have difficulty distinguishing certain colors, your eye doctor might screen your vision for a color deficiency. To do this, your doctor shows you several multicolored dot-pattern tests. If you have no color deficiency, you'll be able to pick out numbers and shapes from within the dot patterns.

If you do have a color deficiency, you'll find it difficult to see certain patterns within the dots. For most people, color blindness that's present at birth congenital is red-green, meaning you can't distinguish those colors. Most people who develop color blindness as a result of disease, such as glaucoma or optic nerve disease, can't distinguish blue-yellow. Your eye doctor may use a microscope called a slit lamp to examine the front of your eye. The microscope focuses an intense narrow line of light on your eye.

The slit lamp provides a magnified, 3D view of the eye and allows your doctor to detect any small abnormalities.

Used with special lenses held close to the eye, the slit lamp also provides detailed views of the back of the eye. A slit lamp is a microscope that magnifies and illuminates the front of your eye with an intense line of light. Your doctor uses this device to examine the eyelids, lashes, cornea, iris, lens and fluid chamber between your cornea and iris.

Your doctor may use a dye, most commonly fluorescein flooh-RES-een , to color the film of tears over your eye. This helps reveal damaged cells on the front of your eye. Your tears wash the dye from the surface of your eye fairly quickly. As you lie down, recline in a chair or sit up, your doctor examines the inside of your eye with the aid of a special lens and a bright light.

This allows your doctor to see details of the retina and other structures in your eye in three dimensions. This examination — sometimes called ophthalmoscopy or funduscopy — allows your doctor to evaluate the back of your eye, including the retina, the optic disk and the retinal blood vessels that nourish the retina.

Having your pupils dilated with eyedrops before the exam keeps your pupils from getting smaller when your doctor shines light into the eye. After administering eyedrops and giving them time to work, your eye doctor may use one or more of these techniques to view the back of your eye:. This test measures fluid pressure in your eye. The test involves using a slit lamp equipped with forehead and chin supports and a tiny, flat-tipped cone that gently comes into contact with your cornea.

The test measures the amount of force needed to temporarily flatten a part of your cornea. Tonometry measures the fluid pressure inside your eye intraocular pressure. This is one test that helps your eye doctor detect glaucoma, a disease that damages the optic nerve.

Applanation tonometry. This test measures the amount of force needed to temporarily flatten a part of your cornea. You'll be given eyedrops with fluorescein, the same dye used in a regular slit-lamp examination. You'll also receive eyedrops containing an anesthetic. Using the slit lamp, your doctor moves the tonometer to touch your cornea and determine the eye pressure. Because your eye is numbed, the test doesn't hurt.

If your eye pressure is higher than average or your optic nerve looks unusual, your doctor might use a pachymeter, which uses sound waves to measure the thickness of your cornea.