Measurement of Stereopsis forms an important part of the clinical assessment of patients with disorders of ocular motility. The introduction of a real depth distance stereoacuity test (FD2) was evaluated in clinical practice and to what extent the introduction affected clinical management. Seventy-three patients under evaluation before and following the introduction of the test were included. Combined thresholds were measured at near using the Frisby and TNO test and at distance using the FD2. Fifty healthy controls were included. Forty-five patients demonstrated Stereopsis using the FD2 and 23 of these had a change in their management based in part on their responses using the FD2. Patients with evidence of Stereopsis using the FD2 were significantly more likely to have change in their management than expected from the whole sample (
Stereopsis is routinely measured at near as part of the clinical assessment of patients with disorders of ocular motility and strabismus. Recently the FD2 [
Stereopsis is usually reserved for the impression of depth arising from binocular disparity [
Until the introduction of distance stereoacuity tests, assessment of binocular alignment when viewing distant objects relied upon the absence of a detectable heterotropia and the response to Bagolini striated lenses or the Worth 4 Dot test. Neither test, however, provides sufficient information to demonstrate the present of stereopsis. The FD2 is a real object or depth rather than simulated stereoacuity test. Although it is, therefore, not prone to disassociative stimuli, because it is a real object test, it is prone to monocular cues and limited in its range.
The use of the FD2 to evaluate the success of exotropia surgery has been evaluated [
A retrospective case note review of 73 consecutive adult patients with disorders of ocular motility who, had been under review before and following the introduction of the FD2 were included. They were divided into 4 groups according to whether their depth discrimination could be measured for; both near and distance, only at one distance, or not at either distance. Patients in each group were then assessed according to whether there had been a change in their management and whether this was based in part on the information provided by the FD2. A prospective study was undertaken on 50 healthy individuals to determine the monocular thresholds and normative data of the stereoacuity tests that had been used.
Ocular deviations were measured (using alternate prism and cover test) in prism dioptres (rounded to nearest 2PD for deviations up to and including 20PD, and the nearest 5PD for deviations greater than 20PD). Monocular near and distance best corrected visual acuity (BCVA) was measured at 6 m using Snellen chart and at 0.33 m, using a near reading type.
Stereoacuity tests were used according to the manufacturer’s instructions. Depth discrimination thresholds were measured at near (between 0.3 m and 0.8 m) using either the TNO or Frisby stereoacuity tests in the normal reading position (i.e., on downgaze) and at distance (6 m) using the FD2. The range of thresholds for the Frisby for each of the 3 plates can only be varied by changing the viewing distance, and the measured thresholds were at the distances specified between 0.3 m and 0.8 m by the manufacturer. The FD2 comprises of a box containing four back illuminated shapes mounted on rods, in a transparent support frame. Each of the shapes can be moved so that one of them is set nearer than the other three shapes. The rods are marked, so that disparity can be set between 5 and 50 seconds of arc in steps of 5 seconds. The FD2 was positioned at the correct measured height for each subject’s eye level, and patients were asked not to move their head during testing. The subjects were required to identify three out of four correct choices at a given disparity until the patient reached threshold. In the control group thresholds of depth discrimination were measured with both eyes open (combined threshold, (CT)) and then with one eye occluded (monocular threshold, (MT)) for both the near Frisby and FD2 at 6 m, 3 m, and 1 m. The binocular threshold (BT) was calculated, according to the function, BT = (1/CT-1/MT
Chi-squared test was used to test for a difference between the patient groups. The logarithm of the depth discrimination threshold (combined, monocular, and binocular) was used for the analysis. For comparison between the stereoacuity tests in the control group a repeated measures model was used.
50 healthy subjects (mean age 38 years (12)) were included. Using the FD2, MT was not measurable at 6 m. At 3 m MT was measurable in 10% and at 1 m in 60% of subjects (Table
Depth discrimination thresholds of control subjects using the near stereoacuity tests, Frisby, TNO, and the FD2 at 1 m, 3 m, and 6 m. Combined (CT), monocular (MT), and binocular (BT) depth discrimination thresholds. Mean, standard deviation (SD) and median in logarithm of arc seconds. Range of test and subjects in arc seconds. Not measurable (NM).
Test | Mean CT (SD) Median (Log seconds) | Range of CT (seconds) | Measurable range of stereoacuity test (seconds) | Mean BT (SD) Median (Log seconds) | Mean MT (SD) Median (Log seconds) |
---|---|---|---|---|---|
TNO | 1.75 (040) 1.78 | 15–1980 | 15–1980 | 1.75 (0.40) 1.78 | NM |
Frisby | 1.74 (0.34) 1.83 | 20–170 | 20–600 | 1.74 (0.34) 1.83 | NM |
FD2 6M | 1.14 (0.28) 1.18 | 5–50 | 5–50 | 1.14 (0.28) 1.18 | NM |
FD2 3M | 1.45 (0.23) 1.30 | 20–160 | 140–200 | 1.47 (0.25) 1.30 | 2.34 (0.2) 2.30 |
FD2 1M | 2.28 (0.39) 2.26 | 182–558 | 952–2042 | 2.32 (0.10) 2.31 | 3.13 (0.15) 3.14 |
73 adult patients were included (mean age 58.3 years (19.5)). There were 39 patients whose depth discrimination thresholds could be measured at both distances (FD2 and near test positive), and 6 patients whose depth discrimination could not be measured with either test (FD2 and near test negative). There were 22 patients whose depth discrimination thresholds could be measured at near only, and 6 patients whose depth discrimination could be measured at distance only. Nine of the patients who had incongruous results with the near and FD2 tests had a change in management. Overall 23 patients had a change in management, the FD2 contributing in part to this decision (Table
Patients who had a change in clinical management following introduction of FD2. Patients 68–73 had evidence of stereopsis using the FD2 but not with near tests, patients 20–22, stereopsis with the near tests but not FD2 and patients 48–61 evidence of stereopsis with both the FD2 and near tests, respectively. Patient 21 was excluded from further analysis as the change was due to retinal detachment. Summary of the change in management is included in the right-most column. Diagnosis is included where established. Where no specific diagnosis established, extraocular muscle deviation recorded. Prism dioptres (pd), intermittent heterotropia (T), esophoria (EP), esotropia (ET), hyper or hypotropia (HT), hyper or hypophoria (HP), base-out (BO), and base-in (BI) prism, lateral rectus (LR), superior rectus (SR), superior oblique (SO), medial rectus (MR), inferior rectus (IR), right (R) and left (L), underaction (u/a), thyroid eye disease (TED).
Patient | Symptom | Deviation | Diagnosis/lesion | FD2(sec) | Near(sec) | Change in management | |
Distance | Near | ||||||
68 | Diplopia on depression | 3pd HP, | 4pd HT, 1 | (L)IVn paresis | 50 with prism | nil | Prism accepted over entire varifocal |
69 | Intermittent diplopia | 3pd HT | 2pd ET, 2pd HT | Bilateral IR u/a. Pineal germinoma | 25 with prism | nil | Given weakest prism possible to achieve distance stereo. |
70 | Diplopia | 10pd ET, 5pd HT | 16pd ET, 6pd HT cyclotorsion | LR and (L) SR and (R) SO u/a | 25 with prism | nil | BO prism achieved dist stereo, but not for near |
71 | Increasing diplopia on downgaze | 12pd E(T) | 8pd E(T), 14pd ET on downgaze | Bilat VIn paresis. Acoustic neuroma | 50 with prism | nil | Increased BO prism for distance to achieve distance stereo. |
72 | Diplopia | 7pd HT | 7pd HT, 7pd XT cyclotorsion | (L) IVn paresis | 30 with prism | nil | Vertical prism accepted for distance. |
73 | Reversal of diplopia at near with prism | 4pd HT | 2pd HT cyclotorsion | (R) SR, (L) SO u/a | 50 with prism | nil | Vertical prism accepted only for distance. |
20 | Intermittent diplopia | 10pd ET, 4pd H(T) | 2pd EP | Bilateral VIn (L) SR u/a | nil with prism | 120 with prism | No diplopia with reduced prism. FD2 negative with deviation corrected |
21 | Intermittent diplopia | 8pd XP | 25pd XP | Decompensated XP Retinal detachment | nil | 240 | FD2 positive previously. Referred to vitreo retinal team. |
22 | Intermittent diplopia | 4pd EP(T) | 2pd EP | TED. Proptosis | nil | 55 | Orbital decompression. FD2 positive previously. |
48 | Diplopia on depression | 7pd HP | 7pd H(T) | IR u/a | 30 | 300 with prism | Prism only on reading segment |
49 | Intermittent diplopia | 6pd E(T) | 2pd EP | (L)VIn | 50 with prism | 150 | Prism over distance segment |
50 | Intermittent diplopia | 2pd H(T) | 10pd XP | TED | 30 with prism | 85 | Prism over distance segment |
51 | Asymptomatic | 2–4 pd E(T) | 6pd XP | (R)VIn | 50 with prism | 480 | Binocular with prism over distance segment |
52 | Intermittent diplopia | 8pd E(T) | 4pd EP | Bilat VIn | 50 with prism | 60 | Increased prism to achieve distance stereoacuity |
53 | Resolving diplopia | 6pd EP | 2pd EP | (L)VIn resolving | 15 | 55 | Binocular without prism |
54 | Intermittent diplopia for near objects | 12pd XP | 35pd X(T) | Decompensating XP | 50 without prism | 600 with 10pd BI prism | Prism removed from distance segment |
55 | Blurred vision. | 0 with spectacles | 4pd XP to variable ET | Intermittent accommodative spasm | 15 with spectacles | 60 | Accepted spectacles to achieve binocularity. |
56 | Intermittent diplopia for distant objects | 2–4pd HT | 2pd HP | (R) SR u/a | 15 with prism | 60 | Binocular only with prism |
57 | Diplopia (monocular and binocular) | 2pd EP | 6pd X(T) | MR u/a Polyopia/correctopia | 15 with and without prism | 110 | Removed prism for distance |
58 | Diplopia at near | Decompensating XP | 40 | 1980 without, 85 with prism | Separate reading glasses with prism incorporated | ||
59 | Intermittent diplopia | Bilateral VIn | 50 with prism | 120 | Not aware of diplopia but binocular only with prism | ||
60 | Diplopia | Partial IIIn | 30 with prism | 150 | Binocular with prisms. For strabismus surgery. | ||
61 | Intermittent diplopia and aesthenopia | 10XP | 0 | Decompensating XP | 20 | 55 | Distance exercises and consideration for surgery |
Six patients (68–73 Table
Patients 20 and 22 (Table
Patients 48 to 61 (Table
Overall, patients who had evidence of depth discrimination stereopsis using the FD2 were significantly more likely to have some modifications to their clinical management than expected from the whole patient sample (
The most commonly used stereo tests in clinical practice are usually viewed at near. In addition, many adult patients wear bifocals or varifocals requiring that the near stereo tests be placed in their normal reading position. Distance stereoacuity using the FD2 is measured at eye level. One therefore cannot assume that because an individual has stereoacuity at near or in a reading position, that they will also have stereoacuity at distance in the primary position. For example, in this study some patients achieved 25 seconds using the FD2 but only achieved 120 seconds using the Frisby at 0.4 m. It is unclear whether this difference arose because of changes in ocular position or differences in the inherent monocular and binocular cue content in each test. This is apparent in the relatively poor correlation between near tests and FD2.
Importantly we found that for the patients in this study, the demonstration of stereopsis at distance contributed to a change in their management. Although it is difficult to be sure of the true contribution of the FD2 to the change in management given that a change in condition could have occurred before and after introduction of FD2, this is unlikely as most of the patients’ ocular motility was relatively stable. Similarly whilst it is possible that a similar proportion of patients may have had a change in their management in the absence of the results of the FD2, this is also unlikely, given that the management decisions were influenced by the results of the FD2.
The relatively poor association between the TNO, Frisby, and FD2 for the healthy control where the maximum association was 40% and for patient group, where there was no significant association, is likely to reflect the differences in the monocular and binocular cue content of these tests. For example, although vertical disparity, accommodation, and vergence are potentially important binocular cues for near tests, they may play less of a role in distance stereoacuity tests [
The targets used in the near stereoacuity tests have minimum and maximum thresholds of 15 seconds and 1980 seconds (TNO) and 20 seconds and 600 seconds (Frisby). In contrast the FD2 at 6 m has a minimum and maximum threshold of 5 seconds and 50 seconds. This together with the noncontinuous or stepwise increments in all of these stereoacuity tests means that there is little overlap between the near tests and the FD2.
This limitation may account for those patients who had thresholds greater than 50 seconds with near tests but who had no demonstrable stereopsis using the FD2 at 6 m. Shortening of the viewing distance to 3 m extends the maximum threshold with the FD2, but also leads to an increase in monocular cues [
Other distance stereoacuity tests have been introduced such as the Mentor Binocular Video Acuity Tester (BVAT) [
The introduction into clinical practice of the FD2 distance stereotest was found to contribute to clinical decision making which otherwise may not have occurred using only near stereoacuity tests. The FD2 is however limited by its range at 6 meters and using the test at shorter distances necessitates measurement of MT with calculation of the BT as described above. Despite these limitations measurement of stereoacuity at distance is an important aspect in the evaluation of patients with disorders of ocular motility.