Documentación sobre Solvencia

There are eight parts to the single field printout (Fig. 9.5). Each has to be examined serially before drawing a conclusion.

First assess the reproducibility ( Zone-1 ) of the concerned fields (Consistency). At the onset, check the printed information at the top of the page, to ensure listing of the correct patient, the type of test done (30-2, 24-2, 10-2), eye in question and date of birth (the software package statistically compares the patients response with age corrected normal population). The recorded visual acuity, refraction and pupil size are important parameters as they all can affect the data. When pupils are miotic, or smaller than 2.5 mm, dilatation is required so as to prevent generalized depression from occurring. The decision to dilate patients with large pupils rests with the clinician, but consistency for all visual fields must be maintained.

Next scans the reliability indices (Zone-2). Fixation losses are noted as the ratio of the number of times the patient responded when he saw a target placed in the blind spot against the total number of times fixation was tested. In automated

Fig. 9.5: The Humphrey single field printout is divided into eight zones. Each must be reviewed sequentially A

perimetry fixation is assessed and monitored by i. Sensors,

ii. Closed circuit TV monitors iii. Heijl-Krakau method.

Sensors are used to detect minute shift in eye

position. They are highly sensitive to slight movement in eye position but are expensive, too sensitive, such that insignificant physiological fixation shifts induced by respiration, systole and involuntary head movements get registered as fixation losses.

Closed circuit TV monitor displays the image

taken by an infrared camera. This allows the examiner to view the patient’s eye and judge and assist in fixation quality. Advantages of this system are continuous monitoring of fixation throughout the test with no extra time spent in monitoring fixation per se (Blind Spot Projection Technique). However, continuous video monitoring is expensive, prone to hardware failure and there exists a potential for the machine to disregard fixation losses in patients with fairly good but not excellent fixation.

Heijl-Krakau method: In this method, the

machine assumes or plots the blind spot at the beginning of the test and then retests after every eight to twelve stimuli by projecting a supra- threshold stimuli in the blind spot. A positive response indicates fixation loss. This, however, does not work well when significant field loss is adjacent to or involving the blind spot.

When fixation losses are more than 20%, it is bracketed (XX) and is indicative of questionable reliability. However, not all fixation losses are due to unsteady gaze. A “pseudo-loss” of fixation is seen when there is an improper location of the blind spot, or when the initial blind spot is present near the edge of a scotoma, so even though it is presented throughout the test, it is occasionally visible. Also, a head tilt or change in head position occurring during the test will lead to a faulty blind spot location. Finally a

patient who is continually responding even when a light is not flashed will have a number of fixation losses. For these reasons the fixation loss score is not considered in isolation, but rather compared to the other reliability scores.

False positives (FP) result when the patient

responds to the audible click of the perimeter with no stimulus projected (trigger happy). It is also expressed as a ratio of the number of times the patient responds to a pause in the testing sequence without presentation of the target against the total numbers of pauses. It is the single most significant reliability indicator. Bracketing occurs when FP’s are 33% but often 15-20% rate can also destroy the credibility of a field. A high rate can also occur due to a poor understanding of the test requirements by the patient. A high FP ratio , will be accompanied by a high positive mean defect, white areas on the gray scale indication of very high threshold levels (white scotomas), a high number of fixation losses and a message of abnormally high sensitivity on GHT.

False negatives (FN) are expressed as a ratio,

and occur when the patient does not respond when a point previously thresholded is retested with a brighter stimulus. High FN ratio occurs when the patient tires as in the later part of the examination, when he changes his internal criterion on whether or not he sees a point or when the edge points of a scotoma are tested. A 33% FN ratio is considered excessive and makes the test suspect. However, the presence of a scotoma and a high number of FN, with all other reliability measures being normal, is indicative of a reliable field.

Foveal threshold measures over 30 dB for a

visual acuity of 6/12 or better. A normal foveal value and a poorly recorded acuity indicates need for a refraction or mild amblyopia. Likewise a good visual acuity and a depressed foveal value suggest early damage.

The Gray scale (Zone-3) is a rough indicator

of the extent of field damage, but can be misleading. Each point on the gray scale is represented by a symbol of varying darkness which corresponds to the threshold level at that point. These are not indicative of disease. A normal elderly patient will have a darker gray scale than a younger patient because of reduced sensitivity in aging eyes. Additionally, there are a fewer points tested in the periphery, each of which occupies a larger space on the gray scale. For these reasons, the gray scale should not be the sole criterion for assessing the visual field. The Total deviation plot (Zone-4) is created by subtracting the actual raw data from the expected value for age matched controls, at each point. This depending on whether the patient did better or worse than expected is expressed as a positive or negative number. The correspon- ding probability symbols seen below the data indicate the statistical probability of finding such a point in normal subjects. These probability symbols increase in significance from a set of 4 dots to a black box, p<5%, <2%, <1% and 0.5%. The presence of a black box indicating that a few normal subjects will have that score, it does not necessarily correspond to an absolute defect. Many points with p<0.5% are relative defects their actual threshold is available from the raw data.

The Pattern deviation plot (Zone-5) based on further calculations, is derived from the total deviation data and the overall depression of the visual field. It highlights focal changes which are concealed within diffuse changes, after making adjustment for the height of the hill of vision. Whereas the statistical significance, expressed as probability symbols, is measured for each point, the total deviation and pattern deviation probability maps are analyzed by taking the entire field into account and identifying how clusters of affected points occur, the number of points involved, their density and location.

The Pattern and Total Deviation need to be compared and a difference if present should be explained. Corneal opacity, cataract and small pupil are the usual causes.

Raw data / numeric data (Zone-6): It is the

actual threshold score for each thresholded point. Areas flagged in the Pattern and Total Deviation plot should be inspected carefully for confirmatory signs like double thresholded points of abnormal or foci of high local fluctua- tion. This should be followed by a geographic survey of the entire numeric data.

Global indices (Zone-7) are presented in the

lower right hand corner of the printout and include:

Mean deviation (MD): It is the weighted score of

all the points on the total deviation plot. It takes into account both the severity of loss and amount of field affected. A positive MD indicates that the patient scored better than expected for his age, a negative number indicates that the score was worse than expected.

Pattern standard deviation (PSD): It measures

the extent to which the damaged points vary from the expected hill of vision (localized loss).

Short term fluctuation (SF): Though listed under

global indices it is a good indicator of intra test reliability. It measures the variation at each point on repeated thresholding in the same test. A SF from a patient with poor reliability scores is high, further indicating a poor test taker.

Corrected pattern standard deviation (CPSD): It

is calculated with the help of SF to adjust the PSD. It is a more accurate indicator of the extent of damage.

Glaucoma Hemifield test (Zone-8) is a

sophisticated analysis of 5 geometric point clusters in the superior and the inferior arcuate regions whose probability maps are compared with one another. It is very sensitive and specific at detecting asymmetry between these regions as well as symmetric deviations from normal data. The GHT can be within normal limits,

outside normal limits, borderline sensitivity, generalized reduction or abnormally high sensitivity (Fig. 9.6).