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Скачать презентацию ﻣﺮﻛﺰﺍﻟﺒﺤﺮﻟﻠﻌﻴﻮﻥ The field of vision is
Скачать презентацию ﻣﺮﻛﺰﺍﻟﺒﺤﺮﻟﻠﻌﻴﻮﻥ The field of vision is
ﻣﺮﻛﺰﺍﻟﺒﺤﺮﻟﻠﻌﻴﻮﻥ
The field of vision is defined as the area that is perceived simultaneously by a fixating eye
• Traquair, “an island of vision in the sea of darkness” • Depicts the visual field as a three-dimensional spatial model • The shoreline of the island represents the peripheral limits of the visual field (least sensitive), and the peak correspond to the fovea (greatest sensitivity)
NORMAL VISUAL FIELD LIMITS 60 60 100 X X Fixation 75 X Physiological Blind spot = Diameter of Optic Disc Relation of Disc to Fovea Horizontal: 1. 1 mm, 5. 5 Nasal: 3. 0 mm, 15. 0 Vertical: 1. 5 mm, 7. 5 Inferior: 0. 3 mm, 1. 5 100
Papillomacular bundle Horizontal raphe • The contour of the island of vision relates to both the anatomy of the visual system and the level of retinal adaptation • The highest concentration of cones is in the fovea, which project to their own ganglion cell. This one-to-one ratio between foveal cone and ganglion cell results in maximal resolution in the fovea
Normal Visual Field q Visual field measurement can be affected by: – Patient's age – Size and position of the nose – Orbital structures – Location of eye within the orbit – Color of stimuli – Refractive error – Fixation – Eye movement – Patient cooperation – Ease of operation of the instrument
History of Perimetry • In 1856 Dr. Von Graefe is the first to draw the field using white piece of paper • In 1889 Dr. Bjerrum was using a tangent screen on the back of his clinic door and he described the arcuate scotoma • In 1909 Dr. Ronne developed kinetic isopter perimetry and described the nasal step in glaucoma • In 1945 Dr. Goldmann designed the first cupola perimeter for manual kinetic perimetry • In 1973 the era of automation began with Dr. Fankhuser and his coworkers in Bern, Switzerland • The first standard automated perimeter OCTOPUS 201 - In 1976 OCTOPUS 2000 - In 1980 OCTOPUS 500 - In 1983 OCTOPUS 1 -2 -3 - In 1989 OCTOPUS 101 - In 1993
Knee of Wilbrand`s Meyer`s loop
KINETIC PERIMETRY • In kinetic perimetry, a stimulus is moved from a non seeing area of the visual field to a seeing area • Procedure is repeated with use of the same stimulus along a set of meridians, usually spaced every 15° • Aim is to find points in the visual field of equal retinal sensitivity. By joining these points an isopter is defined • Then luminance and the size of the target is changed to plot other isopters
KINETIC PERIMETRY • In kinetic perimetry, the island of vision is approached horizontally. Isopters can be considered as the outline of horizontal slices of the island of vision • Disadvantages of this technique include the subjectivity, highly dependant on the operator efficiency, time consuming, difficulties with randomising targets and patient cooperation
STATIC PERIMETRY • In static perimetry, the size and location of the test target remain constant • Retinal sensitivity at a specific location is determined by varying only the brightness of the test target • The shape of the island is then defined by repeating the threshold measurement at various locations in the field of vision • This strategy allows for a quantitative measure of the relative density of a defect, more easily than in kinetic perimetry
Terminology Related to Perimetry • Isopter is a line within the visual field which connects points of equal sensitivity or threshold. • Apostilb (asb) is the unit of measurement of luminance (brightness). One apostilb equals to 0. 3183 candela/m², or 0. 1 mililambert. – A healthy patient can perceive a stimulus of 1 abs in the macular area. • Decibels (d. B) is the unit of measurement of neutral density filters. Each decibel equals 1/10 log unit. Thus 10 d. B equals 1 log unit or 10 fold change in intensity. – The decibel scale is not standardized because the maximal luminance varies between instruments. – Data needs to be captured on the same instrument for comparisons.
SENSITIVITY VERSUS THRESHOLD • As one ascends the hill of vision toward the fovea, the sensitivity of the retina increases, dimmer targets will become visible. • Therefore, as retinal sensitivity increases, the differential light threshold measured in apostilbs decreases. • In automated perimetry, however, threshold is recorded in the inverted decibel scale, and dimmer targets have higher decibel values. • Therefore, threshold in decibels is directly proportional to retinal sensitivity.
MANUAL PERIMETRY • Goldmann perimeter is the most widely used instrument for manual perimetry. • It is a calibrated bowl projection instrument with a background intensity of 31. 5 apostilbs. • Size and intensity of targets can be varied to plot different isopters kinetically and determine local static thresholds.
GOLDMANN VISUAL FIELD q The stimuli used to plot an isopter are identified by Roman numeral, number, and a letter j Roman numeral represents the size of the target, from Goldmann size 0 to Goldmann size V – Each size increment equals a fourfold increase in area k Number and letter represent the intensity of the stimulus – change of one number represents 5 -d. B change in intensity – change each letter represents 1 -d. B change in intensity
GOLDMANN VISUAL FIELD q Isopters in which the sum of the Roman numeral (size) and number (intensity) are equal can be considered equivalent q The equivalent isopter combination with the smallest target size usually is preferred because detection of isopter edges is more accurate with smaller targets q One usually starts by plotting small targets with dim intensity (I 1 e) and then increasing the intensity of the target until it is maximal before increasing the size of the target The usual progression
GOLDMANN VISUAL FIELD q Once an isopter is plotted, the stimulus used to plot the isopter is used to statically test within the isopter to look for localized defects. In this way, it acts as a suprathreshold stimulus.
AUTOMATED PERIMETRY Ø The introduction of computers and automation heralded a new era in perimetric testing. Ø Static testing can be performed in an objective and standardized fashion with minimal perimetrist bias. Ø A quantitative representation of the visual field can be obtained more rapidly than with manual testing. Ø The computer presents the stimuli in a random fashion. Patients do not know where the next stimulus will appear, so fixation is improved. Also increase the speed of the test by bypassing the problem of local retinal adaptation.
AUTOMATED PERIMETRY • Humphrey Field Analyzer uses a constant target size equal to a Goldmann "III" (4 mm²) and varies the target brightness only. unless otherwise instructed. • The stimulus intensity can reach up to 10, 000 asb in Humphrey and 1000 asb in Octopus. • The background luminance in Humphery is 31. 5 asb and the testing distance 33 cm. while Octopus model uses 4 asb and the testing distance 42. 5 cm
of static and kinetic perimetry to detect shal A. Kinetic evaluation can clearly outline the normal visual field B. Kinetic perimetry may miss shallow scotomas and poorly define the flat slope seen nasally C. The edge of steeply sloped scotomas may be identified easily with kinetic perimetry, but the steepness of the slope may not be appreciated D. & E. Static perimetry readily detects shallow scotomas and can define the slope of both shallow and steep scotomas
In a study of patients with open angle glaucoma, Dr. Ourgaud reported that a defect was found in one third of cases with static perimetry that was missed by kinetic perimetry J Fr Ophthalmol, 1982
GLAUCOMATOUS VISUAL FIELD DEFECTS Any clinically or statistically significant deviation from the normal shape of the hill of vision can be considered a visual field defect. In glaucoma, these defects are either diffuse depressions of the visual field or localized defects that conform to nerve fiber bundle patterns.
DIFFUSE DEPRESSION • Diffuse depression of the visual field results from widespread diffuse loss of nerve fibers of the retina. • It is common in glaucoma but it is non specific sign that can be caused by many etiologies. • By far the most common reason for a diffuse depression is lens opacity. • Other factors include other media opacities, miosis, improper refraction, patient fatigue, inattentiveness or inexperience with the examination, ocular anomalies, and age.
DIFFUSE DEPRESSION • In manual perimetry, is manifested by contraction of the isopters. The isopters retain their normal contour. The most central isopters may disappear entirely as the peak of the island of vision sinks. • In automated perimetry, diffuse depression results in relative defects across the entire visual field.
LOCALIZED NERVE FIBER BUNDLE DEFECTS • Localized visual field defects in glaucoma result from damage to the retinal nerve fiber bundles. • Because of the unique anatomy of the retinal nerve fiber layer, axonal damage causes characteristic patterns of visual field changes. • The most common location of visual field defects occurs within an arcuate area (Bjerrum’s area) extending from blind spot nasally 10 -20 around fixation and terminate at the median raphe.
Nerve Fiber Bundle Defects Ø The superior and inferior poles of the optic nerve head are most vulnerable to glaucomatous damage. j It has been postulated that these areas may be watershed areas at the junction of the vascular supply from adjacent ciliary vessels. k Ultrastructural examination of the lamina cribrosa shows that the pores in the superotemporal and inferotemporal areas are larger. The large pores may make these regions more vulnerable to compression.
PARACENTRAL DEFECTS • Circumscribed paracentral defects are an early sign of localized glaucomatous damage. • The defects may be relative or absolute and frequently found in Bjerrum’s area along the course of the nerve fiber bundle. • With progression paracentral scotomas become deeper and longer and may gradually coalesce forming an arcuate or Bjerrum’s scotoma.
ARCUATE SCOTOMAS • More advanced loss of nerve fiber bundle leads to a scotoma that starts at or near the blind spot, arches around fixation, and terminates abruptly at the nasal horizontal meridian. • In the temporal portion of the field, it is narrow because all of the nerve fiber bundles converge onto the optic nerve. • The scotoma spreads out on the nasal side and may be very wide along the horizontal meridian. Differential Diagnosis of Arcuate Scotomas Glaucoma Branch Vein Occlusion Branch Artery Occlusion Optic Neuritis Ischemic Optic Neuropathy Optic Nerve Drusen Optic Nerve Pit Optic Nerve Cloboma Myelinated NFL
NASAL STEP DEFECTS • A steplike defect along the horizontal meridian results from asymmetric loss of nerve fiber bundles in the superior and inferior hemifields. • Nasal steps frequently occur in association with arcuate or paracentral scotomas, but a nasal step also may occur in isolation. • Nasal step defects may be evident in some isopters but not in others, depending on which nerve fiber bundles are damaged. • Approximately 7% of initial visual field defects are peripheral nasal step defects.
TEMPORAL WEDGE DEFECTS • Damage to nerve fibers on the nasal side of the optic disc may result in temporal wedge-shaped defects. • These defects are much less common than defects in the arcuate distribution. • Occasionally, they are seen as the sole visual field defect. • Temporal wedge defects do not respect the horizontal meridian.
EARLY VISUAL FIELD DEFECTS Werner and Drance found in 35 eyes with previously normal visual fields that the earliest defects were paracentral scotomas with a nasal step (51%), isolated paracentral defects (26%), isolated nasal steps (20%), and sector defects (3%). Hart and Becker found the following initial visual field defects in 98 eyes: nasal steps (54%), paracentral or arcuate scotomas (41%), arcuate blind spot enlargement (30%), isolated arcuate scotomas separated from the blind spot (20%), and temporal defects (3%).
BLIND SPOT CHANGES 1. Enlargement or vertical elongation of the blind spot may occur with early arcuate defect that connects with the blind spot or peripapillary atrophy, which frequently accompanies glaucomatous damage. 2. Baring of the blind spot may be physiologic or pathologic. Physiologic baring of the blind spot is an artifact of kinetic perimetry usually is confined to a single central isopter in the superior visual field. Because inferior retina is less sensitive than the superior retina.
End-stage defects • Only a small central island a temporal island of vision remain. • The temporal island is more resistant than the central island
Visual field changes in Normal. Tension Glaucoma 1. Greater depth 2. Steeper slops 3. Closer to fixation
Important Points q Both central visual acuity and field of vision may improve if the IOP is reduced in early stages of the disease q In most patients with glaucoma, clinically recognizable disc changes precede detectable field loss q With standard manual perimetric techniques as many as 35% of fibers may gone in an eye with normal field q 20% loss of cells, especially large gangelion cells in the central 30 of the retina, correlates with a 5 -d. B sensitivity loss
The following table indicates the threshold tests and the points tested Threshold Test Extent of Visual Field/Number of Points 10 -2 10 degrees/68 point grid 24 -2 24 degrees/54 point grid 30 -2 30 degrees/76 point grid 60 -4 30 to 60 degrees/60 points Nasal Step 50 degrees/14 points The following table indicates screening tests and the points tested Screening Test Extent of Visual Field/Number of Points Central 40 30 degrees/40 points Central 76 30 degrees/76 points Central Armaly 30 degrees/84 points Peripheral 60 30 to 60 degrees/60 points Nasal step 50 degrees/14 points Armaly full field 50 degrees/98 points Full Field 81 55 degrees/81 points Full Field 120 55 degrees/120 points
Commonly used programs for glaucoma. Ø The Octopus program 32 and the Humphrey program 30 -2 are tests of the central 30° with 6° of separation between locations. Ø The Humphrey program 24 -2 eliminates the most peripheral ring of test locations from program 30 -2 because it provides the least reliable data, except in the nasal step region, so testing time can be shortened.
DIFFERENTIAL LIGHT THRESHOLD • Static computerized perimetry measures retinal sensitivity at predetermined locations in the visual field. • These perimeters measure the ability of the eye to detect a difference in contrast between a test target and the background luminance. • Threshold is defined as the dimmest target perceived by the patient at a given discrete point; psychophysicists define the term as the ability to perceive a stimulus 50% of the time. frequency-of-seeing curve
THRESHOLD PROGRAMS 3. Strategy q Full threshold A staircase, or bracketing, strategy is used to estimate threshold at each test point. Most commonly, a 4 -2 algorithm is employed. • Testing starts with a suprathreshold stimulus. The intensity of the stimulus is decreased in 4 -db steps until the stimulus is no longer seen ( threshold is crossed ). Threshold is crossed a second time by increasing the stimulus intensity in 2 -db steps until it is seen again.
The 4 -2 bracketing strategy • Octopus perimeter estimates threshold as the average of the last seen and unseen stimulus intensities. • Humphrey perimeter uses the intensity of the last seen stimulus as threshold. • Full threshold is rarely indicated, since newer thresholding algorithms are equally as valid and much faster.
How can test time be minimized? • The closer the initial stimulus is to the actual threshold, the faster the test will be. Humphrey and Octopus use a "region growing" technique to determine the starting level for each point. • The test begins with measuring the threshold at one spot in each quadrant of the central field. This then determines their reference hill of vision after correcting for age and general responsiveness of the patient. Adjacent locations are tested with appropriate starting thresholds.
OTHER THRESHOLD PROGRAMS q FASTPAC – Was most commonly used strategy – Use 3 d. B step and only cross threshold once – Save 25% of test time (Humphreys). – Measurement are statistically identical to the standard strategy – Trade off ST fluctuation over estimated
q Swedish Interactive Thresholding Algorithm (SITA) – SITA utilises an alternative strategy to the bracketing method. – Fast with similar accuracy and reproduciblity – It is available as either SITA standard or SITA fast. – Use computer intelligence by calculating expected thresholds and begin testing close to the actual threshold value. – Two likelihood functions are calculated for each test location, one based on the assumption that the test location is glaucomatous and the other based on the assumption that the location is normal. The likelihood functions are updated as the examination progresses. The updating is informed by a combination of patient responses and internal models of normality and glaucoma.
SCREENING PROGRAMS • First the four primary points in each quadrant are thresholded to calculate theoretical hill of vision. • Targets are then presented 6 d. B brighter than theoretical hill of vision. Failure to detect the stimulus after it is presented for the second time will result in different strategies as the following: Two Zone Points are presented the 6 d. B above theoretical hill of vision level. If the point is not seen it is tested for a second time Printouts display circles for seen stimuli and solid squares for unseen stimuli. Three-zone Points that are not detected after being presented twice 6 d. B above theoretical hill of vision level, are retested at the brightest level which is 10 000 asb. If target is seen a circle is displayed, "x‘ on the printout for a relative defect or a solid block if the target is not seen. Quantify defects The points missed twice at the 6 d. B brighter than theoretical hill of vision level, are thresholded to quantify the depth of the defect at that location. Printouts display circles for seen stimuli, and numbers for defects.
READING FIELD PRINTOUT 1. Name, ID and Age Ensure that this data is accurate. The correct age is essential as the patient is compared to age matched normals. 2. Type of Test This indicates whether the test was a threshold or screening test. • Screening tests are a fast effective method to detect suspect areas in the visual field and indicate the need for further evaluation • Threshold tests determine the sensitivity at various points in the visual field and detect early changes in retinal sensitivity. 4. Pupil Diameter – – While large pupils do not affect the results significantly, miotic pupils can induce a defect. The pupil should be at least 3 mm to avoid false defects. Automated pupil size measurement (Humphry)
5. Glasses Used – The proper near add refraction, as determined by the patient's age and the diameter of the perimeter's cupola, must be used. – This lens must be positioned properly to prevent artifactual defects caused by the rim of the lens. – Use Trial lenses only for central tests (within 30 ), or the central part of a full field test. For Peripheral test > 30 degrees, remove the lenses. – Uncorrected refractive errors cause defocusing of the test target and apparent depression of retinal sensitivity. Each diopter of uncorrected refraction causes a 1. 26 -db depression of retinal sensitivity
ASSESSING RELIABILITY (Reliability Indices) Ø Fixation losses • Fixation is central to the validity of a visual field. • The following strategies are employed to ensure adequate fixation: – Video monitoring of the eye or Gaze tracking (Octopus) – The manual method which requires constant supervision of the patient during the test (Goldmann) – Heijl-Krakau Technique in which fixation during the examination is periodically monitored by presenting stimuli in blind spot (Humphry) • If fixation losses exceed 20% indicative of poor fixation or that the blind spot was not correctly mapped out then XX will be printed next to the numbers
ASSESSING RELIABILITY (Reliability Indices) Ø False positives Catch Trials • By withholding of a stimulus projection (only sound is presented) and patient is still responding. The patient responds to the sound clue alone • The scores are flagged with XX if errors exceed 33% of the trials • High score suggests a ‘trigger happy’ patient Ø False negatives Catch Trials • Failure to respond to a stimulus 9 d. B brighter than previously seen at same location • The scores are flagged with XX if errors exceed 33% of the trials • High score indicates inattention, or advanced field loss
q The numeric data Expresses the patient's test responses in decibels. The STATPAC software analyses this info and gives it age adjusted significance and it is then that this information is really relevant and worth drawing conclusions from. q The Grayscale The grayscale is a colour scheme of the visual loss. It is useful to provide an overview of the visual field loss but cannot be relied on by the clinician to make a definitive diagnosis of the extent of the visual field loss. It is useful for the patients to understand the extent of the visual field loss and the risks that they face.
Deviations q Humphery Field Analyzer's statistical package (STATPAC) uses a model based on test results of patients with normal fields, retinal sensitivity, and pupil size for each different age group. It compares the patient's test results against this model to determine how their threshold results, for each tested point, compares or falls outside the normal population model. 1. Total deviation (Comparisons in Octopus) – Upper numerical display shows difference (d. B) between patient’s results and age-matched normal – These negative values become diagnostic when they reach (-5) or greater and more so if there are several grouped together. – Lower graphic display shows these differences as grey scale ie. the defect depth
Deviations 2. Pattern deviation (Corrected Comparisons in Octopus) – Similar to total deviation except the STATPAC correct total it for diffuse effects eg. cataract, miotic pupils or incorrect testing lens – Display any superimposed pattern of localized loss (eg. subtle glaucoma changes) that is hidden under a generalized depression 3. Probability plot – Indicate the degree of abnormality – The darker the symbol in the probability plot the more significant the deviation from normal – P<1% means that this deviation happens in less than 1% of the normal population
q Glaucoma Hemifield Test – GHT is based on the fact that glaucoma usually causes asymmetric field loss and not a generalised global depression. – GHT evaluates five zones in the superior field and compares these zones to their mirror image zones in the inferior field. Then prints one of three messages below the graytone format: » GHT within normal limits Outside Normal limits Borderline – The test is not available with tests using Fastpac q Defect (Bebie) Curve in Octopus
GLOBAL INDICES 1. Mean deviation (Mean defect in Octopus) – Reflects deviation of patient’s overall field from normal • It is simply the average (Octopus) or the weighted average (HFA) • • • of the deviation values for all locations tested. p values are < 5%, < 2%, < 1% and < 0. 5% The lower the p value the greater the significance The mean deviation is most sensitive to diffuse changes and is less sensitive to small localized scotomas. 2. Pattern standard deviation (Loss variance in Octopus) – measurement of the degree to which the shape of the patient's field departs from the age-matched normals reference field. • Represent the local non-uniformity of the visual field • low PSD indicates a smooth hill of vision or if the damage is more or • high PSD indicates an irregular hill or presence of scotoma less even
GLOBAL INDICES 3. Short-term fluctuation – – The threshold is measured twice at 10 pre-selected points. A fluctuation value is then determined by using the difference between the first and the second readings. – 2 d. B or less indicates reliable field – 4. Represent intra test variability and measure the consistency of responses > 3 d. B indicates either poor patient compliance or a sign of glaucomatous field loss and flagged with p values, eg. P< 0. 01 Corrected pattern standard deviation CPSD (Corrected Lloss variance in Octopus) – Measurement of how much the total shape of the patient's hill of vision deviates from the shape of the "NORMAL" hill of vision for the patient's age, after being corrected for intra-test variability (short-term fluctuation) – It is increased when localized defects are present
PSD – SF = CPSD
MD = 1. 9 CPSD =. 7 CATARACT NORMAL MD = 1. 9 MD = 11. 9 CPSD = 2. 6 GLAUCOMA + CATARACT
INTEREYE COMPARISONS • The difference in the mean sensitivity between a patient's two eyes is less than 1 db 95% of the time and less than 1. 4 db 99% of the time. • Intereye differences greater than these values are suspicious if they are unexplained by non glaucomatous factors, such as unilateral cataract or miosis.
Clover leafe field
Glaucoma (1) Humphrey Central 24 -2 Threshold Test
Glaucoma (2)
Glaucoma (3)
Glaucoma
Incomplete Left Superior Quadrantonopia (Temporal lobe Syx)
Neuro (1)
Neuro (2)
Neuro (3) Bilateral Optic Neuropathy
Pseudotumour cerebri
Retinal Toxicity secondary to Plaquenil
Post portum CVA 2 nd VF largely resolved
Conclusion • Visual field measurement is a critical component in the armament against potentially blinding diseases. • Visual field measurement has undergone an evolution from the mechanical to the automated measurement process, resulting in greater accuracy, ease of use and greater depth of analysis. • • Other psychophysical methods for testing the visual field for damage are now being explored. These methods include contrast sensitivity, acuity perimetry, and color perimetry.
References • Cavallerano A. A. When & When not to Do Perimetry. Guide for Interpretation of Visual Fields. Dicon 1995. • Harrington D. O. The Visual Fields. Saint Louis: C. V. Mosby Company, 1976 pp 1 -4. • Haley MR. (ed). The Field Analyser Primer. San Leandro, California: Allergan Humphrey, 1987. • Townsend JC, Selvin GJ, Griffin JR, Comer GW. Visual Fields. Clinical Case Presentations. Boston: Butterworth-Heinemann, 1991 pp 3 -37. • Lalle P. A. Visual Fields. In Fingeret M & Lewis TL, eds. Primary Care of the Glaucomas. Norwalk, Connecticut: Appleton and Lange; 1993: 159 -196. • Humphrey Field Analyser: Users Guide. Allergan Humphrey, 1994. • William TD. Quantitative Perimetry. In Eskeridge BJ, Amos JF & Bartlett JD, eds. Clinical Procedures in Optometry. Philadelphia: JB Lippincott, 1991 pp 447 -461. • Melton R & Randall T. How to Interpret the Visual Field Printout. A Supplement to Review of Optometry. June 1998 pp 12 A-13 A. • Viswanathan AC. Visual Field analysis in Glaucoma. In www. city. ac. uk/optometry. • Melton R & Randall T. Interpreting a Visual Field Printout. In 4 th Annual Guide to Therapeutic Drugs (supplement). Optometric Management May 1995 pp 52 -56. • Flammer J. The concept of visual field indices. Graefes Arch Clin Exp Ophthalmology 1986 224 389 -395.
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