10.1 Sensory Fusion Testing: Worth 4-Dot, Maddox Rod & Bagolini Lenses
Key Takeaways
- The Worth 4-Dot target has four lights — one red, two green and one white — viewed through a red filter over the right eye and a green filter over the left.
- Seeing four lights means fusion, five means diplopia, and two or three means suppression.
- The Worth 4-Dot is more dissociating at distance, so a small suppression scotoma may show at distance but fuse at near.
- The Maddox rod converts a point light into a line perpendicular to the cylinders' axis and fully dissociates the eyes.
- Bagolini striated lenses are the least dissociating sensory test and best reveal anomalous retinal correspondence.
The dissociation gradient
Sensory tests differ in how strongly they break fusion. That single idea explains nearly every apparently contradictory result.
| Test | Dissociation | What it detects best |
|---|---|---|
| Bagolini striated lenses | Least — near-normal viewing conditions | Anomalous retinal correspondence, subtle suppression under natural conditions |
| Worth 4-Dot | Moderate | Peripheral fusion, suppression, diplopia |
| Red filter / red glass test | Moderate to strong | Diplopia and its direction |
| Maddox rod | Strong — the images are utterly dissimilar | Phorias, vertical and cyclo deviations |
| Prism dissociation | Complete | Heterophoria measurement, binocular balance |
A patient who fuses on Bagolini, suppresses on Worth 4-Dot at distance and shows a large phoria on Maddox rod is not giving inconsistent answers — they have peripheral fusion that breaks progressively as the test dissociates harder.
The Worth 4-Dot test
Apparatus. A flashlight or wall-mounted box showing four lights: one red (top), two green (lateral), one white (bottom). The patient wears goggles with a red filter over the right eye and a green filter over the left — the standard convention. The red filter passes only the red light; the green filter passes only the green lights. The white light is visible to both eyes.
Procedure. Test at 33 cm (near) and at 6 metres or further (distance), with habitual correction, in a dimly lit room. Ask the patient simply: "How many lights do you see, and what colours?"
Interpretation.
| Response | Meaning |
|---|---|
| 4 lights (1 red, 2 green, 1 that may alternate or look mixed) | Fusion |
| 5 lights (2 red, 3 green) | Diplopia — uncrossed (red to the right of green) suggests esodeviation; crossed suggests exodeviation |
| 2 red lights | Left eye suppression (only the red filter's eye is seeing) |
| 3 green lights | Right eye suppression |
| Alternating 2 red then 3 green | Alternating suppression |
The distance-near difference is the diagnostic point. At near, the lights subtend a large angle and stimulate a wide area of peripheral retina, so a small central suppression scotoma is bypassed and the patient fuses. At distance, the lights subtend a small angle falling within the scotoma, so suppression appears. Fusion at near with suppression at distance therefore indicates a small central suppression scotoma, typical of microtropia and small-angle strabismus.
Record both distances: "W4D: fuses at 33 cm, 3 green at 6 m — right suppression at distance."
The Maddox rod
A Maddox rod is a set of parallel high-power cylinders mounted in a holder, conventionally red. Looking at a point light source through it, the eye sees a streak of light perpendicular to the axis of the cylinders. Because one eye sees a point of light and the other sees a red streak, the images are too dissimilar to fuse, so the eyes dissociate completely and any phoria becomes manifest.
Setting up. Room dimmed, a single point light source at 6 metres for distance or 33 cm for near, habitual correction in place, Maddox rod over one eye (conventionally the right).
Measuring a horizontal deviation.
- Orient the cylinders horizontally — this produces a vertical streak.
- Ask where the streak is relative to the light.
- Streak on the same side as the eye wearing the rod (rod over the right eye, streak seen to the right) = uncrossed = esophoria.
- Streak on the opposite side (rod over the right eye, streak seen to the left) = crossed = exophoria.
- Add horizontal prism — base out for eso, base in for exo — until the streak passes through the light. Record the prism power.
Measuring a vertical deviation.
- Orient the cylinders vertically — this produces a horizontal streak.
- Streak below the light with the rod over the right eye = right hyperphoria (the right eye is deviated up, so its image falls below).
- Streak above the light = right hypophoria.
- Neutralise with vertical prism until the streak passes through the light.
The governing optical rule is that the image displaces opposite to the direction of the eye's deviation, because the deviated eye's fovea is no longer aimed at the target.
The double Maddox rod for cyclodeviation. Two Maddox rods, conventionally red over one eye and white over the other, both oriented vertically to produce two horizontal streaks, usually with a vertical prism to separate them. If a streak is tilted, the eye is cyclorotated. The patient rotates the rod until the streaks are parallel, and the number of degrees of rotation is the cyclodeviation. Excyclotorsion of 10 degrees or more in a bilateral superior oblique palsy is a classic finding.
Bagolini striated lenses
Bagolini lenses are plano lenses with very fine striations that convert a point light into a faint streak while leaving the rest of the visual field essentially normal. Because the patient can still see the surroundings, fusion is barely disturbed — this is the least dissociating sensory test.
With the striations set at 45 degrees in one eye and 135 degrees in the other, the patient sees two oblique streaks crossing at the light.
| Response | Meaning |
|---|---|
| Symmetrical cross through the light | Normal fusion (or anomalous retinal correspondence in a manifest strabismus) |
| One streak has a gap in its centre | Central suppression scotoma |
| One streak absent | Complete suppression |
| Two streaks that do not cross at the light, in a patient with a manifest deviation | Normal retinal correspondence with diplopia |
The key contrast: a patient with a manifest strabismus who sees a symmetrical cross through the light on Bagolini but shows the deviation on cover testing has anomalous retinal correspondence — the brain has re-mapped the deviated eye's fovea. That combination is why Bagolini is used when a strabismic patient reports no diplopia despite a measurable deviation.
The 4 prism dioptre base-out test
For suspected microtropia (a deviation under 8 to 10 prism dioptres with a central suppression scotoma), place a 4Δ base-out prism over one eye and watch the movement. A normal eye makes a small refixation movement followed by a fusional recovery; an eye with a central scotoma makes no movement when the prism is placed over it, because the image moves within the scotoma. It is a delicate observation and is best done at the slit lamp or with good lighting.
A patient wearing Worth 4-Dot goggles with red over the right eye reports seeing three green lights. What does this indicate?
A patient fuses on the Worth 4-Dot at 33 cm but sees only two red lights at 6 metres. What does this pattern indicate?
A Maddox rod with horizontally oriented cylinders is placed over the right eye. The patient sees the vertical red streak to the right of the light. What is the deviation?
Which sensory test is least dissociating and therefore best at revealing anomalous retinal correspondence?
Two vertically oriented Maddox rods are used with a vertical separating prism, and one horizontal streak appears tilted. The patient rotates the rod 12 degrees to make the streaks parallel. What has been measured?