Single field analysis of Humphrey visual field (HVF) 24-2, Swedish interactive threshold algorithm- standard, printout of the right eye (RE) of a one eyed patient with a medium size disc, 0.7: 1 cup disc ratio and inferio-temporal notch and corresponding retinal nerve fiber layer defect in the left eye [Fig. 1], shows reliable HVF, superior arcuate scotoma, single depressed point in the inferior hemifield and the blind spot plotted on the left or temporal field of vision in the raw data and gray scale plot Fig. 2a, arrow], instead of the right side. Total deviation plot (TDP) and pattern deviation plot (PDP) shows blind spot to be located on the right or nasal side of field of vision [Fig. 2b]. At first glance, if the HVF is seen in isolation without considering the clinical findings, it may appear to be situs inverses of the disc as the blind spot for the “right” eye is mapped on the left side of the visual field. However, there was no situs inverses of the optic disc clinically as the eye tested was left eye [Fig. 1] and it was found that though the technician had tested the patient's left eye (LE), but the eye to be tested was incorrectly entered as RE (which is seen in the printout). A give away to this was that, the threshold values of raw data and its conversion to gray scale map (derived from the patient's actual threshold information obtained), the blind spot is correctly depicted on the left side of field of vision in the HVF printout. However, as the numerical TDP is derived after comparing patient's raw data with the statistical package (STATPAC) of HVF software, it shows 2 blank points in the presumed location of blind spot on the nasal side of field of vision and as PDP is derived from TDP, it also shows blank points in same location. HVF software calculated the values in numerical TDP and PDP in the patient's actual blind spot location on temporal side in post processing and considered the threshold values in the nasal side as blind spot location and in the printout, it was seen as blank points.
This case illustrates one of the technician-related mistake in patient data entry and misleading pitfalls in automated static perimetry. At the beginning of the test, all the patient-related information should be entered correctly by the perimetrist, as it will appear the same in the HVF printout. Also, the HVF should never be interpreted in isolation and should always be correlated clinically. This case emphasizes the significance of data entry by the perimetrist and importance of thorough checking of each and every data in the HVF printout by the clinician to avoid misdiagnosis.
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