Ординатура / Офтальмология / Английские материалы / Oxford American Handbook of Ophthalmology_Tsai, Denniston, Murray_2011

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Diagnostic tests and their interpretation

Visual field testing: general 48 Goldmann perimetry 51

Automated perimetry: performance and interpretation (1) 53 Automated perimetry: performance and interpretation (2) 54 Automated perimetry: protocols 56

Ophthalmic ultrasonography (1) 57 Ophthalmic ultrasonography (2) 58 Fluorescein angiography (FA) 60 Indocyanine green angiography (ICGA) 63 OCT, HRT, and SLP 65

Corneal imaging techniques 68 Electrodiagnostic tests (1) 71 Electrodiagnostic tests (2) 73

Ophthalmic radiology: X-ray, DCG, and CT 75 Ophthalmic radiology: MRI and MRA 77

48 CHAPTER 2 Diagnostic tests and their interpretation

Visual field testing: general

The visual field is often regarded as “an island of vision surrounded by a sea of darkness” (Traquair’s analogy). It is a three-dimensional hill: the peak of the hill is the fovea and at ground level it extends approximately 50° superiorly, 60° nasally, 70° inferiorly, and 90° temporally (Table 2.1).

Indications

Visual field testing aids in diagnosis and in monitoring certain ophthalmic (e.g., glaucoma) and neurological diseases.

Definitions

•A scotoma is an area of visual loss or depression surrounded by an area of normal or less depressed vision. An absolute scotoma

represents a total loss of vision, where no light can be perceived. A relative scotoma is an area of partial visual loss, where bright lights or larger targets are seen, whereas smaller and dimmer ones cannot be seen.

•Homonymous is when the defects are in the corresponding region of the visual field in both eyes. For example, in a right homonymous

hemianopia, there is a defect to the right of the midline in both visual fields.

•Congruousness describes the degree to which the field defects match between the two eyes. Generally, the more congruous the field defect the more posterior along the visual pathway the lesion is located.

•Isopter is a threshold line joining points of equal sensitivity on a visual field chart.

Caution

Interpretation problems affecting all visual fields can include ptosis or dermatochalasis, miosis, media opacities such as cataracts, incorrect positioning at the machine, poor attention to or incomprehension of the test, tremor, inadequate retinal adaptation, or refractive status (overcorrection by 1 diopter will cause a reduction in sensitivity of 3.6 dB). To compare serial visual fields, background luminance, stimulus size, intensity, and exposure times need to be standardized. Significant changes noted on visual field testing should be confirmed with repeat testing.

Confrontational visual fields (p. 33)

This is a simple qualitative method for gross detection of defects in the peripheral visual field. The use of hat pins (white and red) enables more subtle defects to be plotted. Results should be recorded the way the patient sees them; however, there can be interexaminer variability.

Amsler grid (p. 24)

This is used to assess the central 10° of the visual field. The test is easy to perform and the grid is portable. It is used to detect central and paracentral scotomas. Held at a testing distance of 1 foot, each square subtends 1 degree of visual field.

50 CHAPTER 2 Diagnostic tests and their interpretation

Kinetic perimetry

This involves presenting a moving stimulus of known luminance from a non-seeing area to a seeing area. The target is then presented at various points around the clock and marked when recognized; these points are then joined, producing a line of equal threshold sensitivity, which is named the isopter.

Tangent screen

The tangent screen (Bjerrum screen) is not commonly used in clinical practice.

Indication

It is used for examining the central 30° of visual field at 6.5 feet.

Method

The patient sits 6.5 feet from the screen, wearing a corrective lens for distance, if required. The nontested eye is occluded in turn. The patient fixates on a central spot and informs the operator when he/she sees the target. White or red disc targets are used, either 1 or 2 mm in diameter.

Results

The results are plotted on charts as the patient sees them. The target size and color is the nominator (1 mm white target = 1w), and the denominator is the distance (mm) of the patient from the chart (e.g., 1w/2000).

Goldmann perimetry

This is the most common type of kinetic perimetry in clinical practice (p. 51.)

Static perimetry

Most automated perimetry is based on static on–off stimuli of variable luminance presented throughout the potential field (p. 54).

GOLDMANN PERIMETRY 51

Goldmann perimetry

•It is usually kinetic (but static perimetry is used for the central field).

•Skilled operators are required.

•It is useful for patients who need significant supervision to produce a reliable visual field.

Method

The machine should be calibrated at the start of each session.

Distance and near add with wide aperture lenses are used during testing (to prevent ring scotoma). Aphakic eyes should, where possible, be corrected with contact lenses.

Seat patient with chin on the chin rest and forehead against rest. Occlude the nontest eye; ask patient to fix gaze on central target and to press the buzzer whenever he/she sees the light stimulus.

From the opposite side of the Goldmann, the examiner directs the stimulus to map out the patient’s field of vision to successive stimuli (isopters). The examiner should move the stimulus slowly and steadily from unseen to seen, i.e., inward for periphery and outward for mapping the blind spot/central scotomas. To move the stimulus arm from one side to the other, it must be swung around the bottom of the chart.

Once the peripheral isopters are plotted, the central area is examined for scotoma. The examiner should monitor patient fixation via the viewing telescope. The central 20° with an extension to the nasal 30° is appropriate for picking up early glaucomatous scotomas. The vertical meridian is particularly explored in suspected chiasmal and postchiasmal disease.

Results

Isopters are contours of visual sensitivity. Common isopters plotted are as follows (see also Fig. 2.1):

•I-4e (0.25 mm2, 1000 asb stimulus).

•I-2e (0.25 mm2, 100 asb stimulus).

•II-4e (1.0 mm2, 1000 asb stimulus).

•IV-4e if smaller targets are not seen (16 mm2, 1000 asb stimulus).

The physiological blind spot should also be mapped.

Interpretation

The target sizes are indicated by Roman numerals (0–V), representing the size of the target in square millimeters, each successive number being equivalent to a 4-fold increase in area.

The intensity of the light is represented by an Arabic numeral (1–4), each successive number being 3.15 times brighter (0.5 log unit steps). It is measured in apostilbs (asb).

A lower-case letter indicates additional minor filters, progressing from a, the darkest, to e, the brightest. Each progressive letter is an increase of 0.1 log unit.

52 CHAPTER 2 Diagnostic tests and their interpretation

I-2e

II-2e

III-4e

Figure 2.1 Normal Goldmann visual field of the right eye.

Calibrating the Goldmann perimeter

Setup

•Insert standard test paper, verifying alignment.

•Lock stylus (at 70°on right-hand side), using knob on the pointer arm.

Stimulus calibration

•All levers should be to the right (i.e., V-4e).

•Turn stimulus (or test) light to permanently on.

•Move the white flag (photometer screen; located on left-hand side of machine) to the up position.

•Adjust the stimulus rheostat (knob furthest from examiner on lefthand side) until the light meter reads 1000 asb. If it does not reach 1000 asb, the bulb may need to be rotated or changed.

Background calibration

•Return white flag to down position.

•Set levers to V-1e (stimulus intensity of 32.5 asb).

•Adjust background illumination to match this stimulus intensity. This is achieved by adjusting the lampshade while looking through the notch on one side of the hemisphere to the photometer screen opposite.

•The photometer can be removed and the pointer handle unlocked.

AUTOMATED PERIMETRY: PERFORMANCE & INTERPRETATION 53

Automated perimetry: performance and interpretation (1)

These machines are usually configured to test static perimetry. The stimulus in this case is stationary but changes its intensity until the sensitivity of the eye at that point is found. It is measured at preselected locations in the visual field. Program selection includes the central 30°, 24°, 10°, or full field.

Suprathreshold tests are quickest to perform and are screening tests. They calculate the threshold adjusted for age by testing a few predefined spots with a 4- to 6-dB step. They may miss subtle variations in a scotoma’s contour, as they do not go on to map defects. They should not be used to monitor glaucoma.

Threshold testing steps of 4 dB are used until a visual defect is detected, at which point it is retested in 2-dB steps. This is the gold standard for monitoring glaucoma and requires patient cooperation and concentration; there is a subject learning curve seen in the first few tests.

Humphrey perimetry

•Sensitive and reproducible, but difficult to perform.

•Fixation monitoring (by tracking gaze and retesting the blind spot).

Method of Humphrey visual field (HVF)

The machine automatically calibrates itself on start-up. Selection of programs includes the following:

•Threshold (full threshold or Swedish interactive threshold algorithm [SITA] central 30–2, 24–2, 10–2).

•Suprathreshold testing (screening central 76 point, full-field 120 point, and Esterman).

•Colored stimuli can also be used.

Interpretation of Humphrey perimetry

When analyzing the results of automated perimetry, consider the following:

•Reliability indices (Table 2.2).

•Absolute retinal thresholds.

•Comparison to age-matched controls.

•Overall performance indices (global indices).

Table 2.2 Reliability indices (subject reliability)

54 CHAPTER 2 Diagnostic tests and their interpretation

Automated perimetry: performance and interpretation (2)

Interpretation of Humphrey perimetry (cont.)

Table 2.3 Typical graphical results from automated perimetry (Fig. 2.2)

Table 2.4 Global indices (a summary of the results as a single number used to monitor change)

Probability values (p)

These values indicate the significance of the defect <5%, <2%, <1%, and <0.5%. The lower the p value, the greater its clinical significance and the less the likelihood of the defect having occurred by chance.

Glaucoma progression analysis

Software available for Humphrey perimetry applies Early Manifest Glaucoma Trial criteria for progression to HVF data. Two visual fields are selected as a baseline for comparison for later exams. Symbols denote points that are worse than baseline on one, two, or three subsequent exams.

<5%

<2%

<1%

<0.5%

Figure 2.2 Typical graphical results from automated perimetry of the right eye of a patient with glaucoma, demonstrating nasal step and developing superior arcuate field defect.