Synoptophore (Major Amblyoscope)

Key points of this examination

• The large amblyoscope (synoptophore) is an instrument with independent optical systems for each eye, used to examine and train binocular vision.
• It allows stepwise evaluation of binocular vision in three stages: simultaneous perception, fusion, and stereopsis.
• It can quantify the subjective angle of strabismus in nine directions: horizontal, vertical, and torsional.
• The main target is children with strabismus aged 3 years and older, and it is standard equipment in specialized strabismus and amblyopia clinics.
• It enables measurement of fusional amplitude and detection of abnormal retinal correspondence and gamma angle abnormalities.
• Vision training proceeds stepwise: simultaneous perception training → fusion training → stereopsis training.
• Cure of strabismus with training alone is limited; it is positioned as an adjunctive therapy before and after surgery.

1. What is the synoptophore (large amblyoscope)?

The large amblyoscope is an instrument used for examining binocular function and vision training in patients with strabismus, amblyopia, and ocular motility disorders. It is also called a synoptophore, with “large amblyoscope” being the official name.

Structure of the device

The large amblyoscope consists of the following components.

• Illumination chamber / slide chamber: The part where the test slides are loaded and the targets are illuminated.
• Reflecting mirror / eyepiece: The optical system that refracts the target light and guides it to each eye.
• Tubes: The structure that integrates the illumination chamber, slide chamber, reflecting mirror, and eyepiece. They exist independently for the left and right eyes.
• Angle display: The scale that displays the direction of the tube in angle (prism diopters or degrees).

The left and right tubes each have independent optical systems and present different targets to the left and right eyes. The examinee undergoes the examination while using both eyes simultaneously, but with different targets presented to each eye (binocular dissociation). Since the targets are presented to each eye as optical distance vision, evaluation can be performed under conditions where the influence of accommodation is suppressed compared to ordinary near vision.

Five categories of examination and training purposes

The functions of the major amblyoscope can be broadly classified into five categories.

1. Quantitative measurement of ocular deviation: Quantifies ocular deviation in nine directions (horizontal, vertical, and torsional).
2. Binocular vision function test: Evaluates the presence and degree of simultaneous perception, fusion, and stereopsis.
3. Fusional amplitude measurement: Quantifies the fusional amplitude.
4. Measurement of cyclodeviation: Quantifies objective and subjective cyclodeviation.
5. Vision training: Trains sensory fusion and stereopsis in a stepwise manner.

The greatest feature of the major amblyoscope is that it can comprehensively perform these functions with a single instrument.

Types of target slides

Multiple sets of two test slides are available, and the appropriate slides are selected according to the examination purpose and the age of the examinee.

Slide type	Example of target	Main examination purpose
Simultaneous perception slide	Lion and cage	Check for simultaneous perception and suppression
Fusion slide	Rabbit (body and tail)	Measurement of fusion range and fusion width
Stereopsis slide	Small-angle disparity target	Assessment of stereopsis function

Simultaneous perception slides use two non-overlapping targets (e.g., lion and cage). If suppression is present, the patient reports that the target in one eye disappears. Fusion slides use targets that are identical in shape but partially different (e.g., rabbit body and tail). When fusion is achieved, the targets merge into a single complete image. Appropriate slide selection directly affects examination accuracy, so it is important to choose slides according to the patient’s age and cooperation level.

2. Indications and Clinical Findings

Indicated Diseases

The major amblyoscope is indicated for the following diseases and conditions.

• Comitant strabismus (esotropia/exotropia): Strabismus where the eye deviation is approximately equal in all directions.
• Incomitant strabismus (ocular muscle palsy/eye movement disorder): Strabismus where the angle of deviation varies with direction.
• Intermittent strabismus: Cases where strabismus appears intermittently.
• Amblyopia: Cases requiring evaluation and training of binocular visual function.

The main subjects for examination are children with strabismus aged 3 years and older. It is standard equipment in strabismus and amblyopia specialty clinics and is one of the basic examination instruments in outpatient care. Since young children need to cooperate by looking into the tube and responding to targets, it is assumed that they have reached a developmental stage where subjective responses can be obtained stably.

Facilities of Use

It is standard equipment in ophthalmology facilities with strabismus and amblyopia specialty clinics. Orthoptists often perform examinations and training, playing a central role in binocular visual function evaluation and orthoptic training using the instrument.

Clinical Findings Obtained from Examination

The major amblyoscope examination allows quantitative evaluation of the following clinical findings.

• Subjective angle of strabismus: The subjective ocular deviation in horizontal, vertical, and torsional directions is measured in degrees.
• Fusional amplitude: The range of angles over which fusion can be maintained is quantified.
• Presence of simultaneous perception, fusion, and stereopsis: Each stage of binocular visual function is assessed individually.
• Retinal correspondence: Determines whether normal retinal correspondence or anomalous retinal correspondence is present.
• Angle gamma: The angular deviation between the visual axis and the fovea is evaluated. An abnormal angle gamma may cause apparent strabismus in appearance.
• Objective angle of strabismus: The deviation of eye position measured objectively. Comparison with the subjective angle allows diagnosis of anomalous retinal correspondence.

Q At what age can a major amblyoscope examination be performed?

A

The main target is children aged 3 years and older. Since the child must look into the tubes and respond to targets, it may be difficult for younger children with immature language and comprehension skills. For children under 3 years, other tests (alternate cover test, Hirschberg method, Krimsky method, etc.) are preferred.

3. Principles and Methods of Examination

Principle of Binocular Separation

The major amblyoscope inserts different target slides into the left and right tubes, presenting separate targets to each eye as optical distance vision. Unlike ordinary near vision, the optical system of the tubes artificially creates a distance vision state, allowing examination under conditions that minimize accommodation.

However, note that proximal convergence (convergence due to physical proximity) and accommodative convergence (convergence linked to accommodation) may be slightly induced by the act of looking into the instrument. The obtained angle of strabismus must be interpreted with these interventions in mind.

Subjective and Objective Examinations

Two types of examination are possible with the major amblyoscope.

• Objective examination: The examiner observes the subject’s eyes and calculates the objective angle of strabismus from the fixation position. Since the subject’s conscious cooperation is not required, it can be performed on young children or patients with communication difficulties.
• Subjective examination: The subject reports the position where the targets align (subjective eye position), and the subjective angle of strabismus is calculated from the tube angle.

Quantification of Eye Position in 9 Directions

Eye position is measured along three axes: horizontal deviation (esotropia, exotropia), vertical deviation (hypertropia, hypotropia), and torsional deviation (intorsion, extorsion). This allows creation of a 9-direction eye position map, useful for identifying paretic muscles in incomitant strabismus (ocular muscle palsy) and evaluating postoperative progress.

Examination Procedure

1. The subject sits facing the instrument and places both eyes against the eyepieces of each tube.
2. Adjust the height and distance (interpupillary distance) of the tubes to fit the patient’s face.
3. Insert the target test slide into the slide chamber.
4. Instruct the patient to look at the target; for subjective tests, give instructions such as “Move the two images until they overlap.”
5. Read the angle scale on the tubes and record the angle of strabismus and fusion range.

Each test item (simultaneous perception, fusion range, cyclodeviation, retinal correspondence) is performed sequentially within the same session. The typical order is simultaneous perception → fusion → stereopsis.

Tips for slide selection

Choosing the appropriate slide according to the patient’s age and examination purpose directly affects test accuracy. For young patients, using slides with familiar targets such as animals helps maintain attention. If responses are difficult to obtain, be flexible, such as changing the test order.

4. Examination Techniques and Detailed Evaluation Items

Techniques for each test item

Simultaneous perception test: Use simultaneous perception slides to check whether the patient can recognize targets from both eyes simultaneously. If suppression is present, the patient may report that the target from one eye is not visible (disappears).

Subjective ocular alignment measurement: Use fusion slides and record the tube angle at which the patient reports that the two targets “overlap into one.” Measure horizontal, vertical, and cyclodeviations. A large difference from the objective angle of strabismus suggests anomalous retinal correspondence.

Fusion range measurement: Use fusion slides and gradually move the tubes inward and outward to find the angle at which fusion breaks. Calculate the fusional amplitude from the limits of convergence (+) and divergence (−). Narrowing of the fusional amplitude indicates binocular vision dysfunction. In children with intermittent exotropia, convergence fusional amplitude has been reported to be reduced compared to normal children¹. Note that fusional amplitudes measured with the synoptophore and prism bar may not match (especially on the convergence side, where the synoptophore tends to show larger values), and it is not recommended to use them interchangeably².

Stereopsis test: Use stereopsis slides to check whether the patient can perceive depth. Stereopsis evaluation with the major amblyoscope has the advantage of adjusting the angular difference between the targets presented to the left and right eyes.

Measurement of cyclodeviation: Subjectively, have the patient adjust the cyclotarget to the position where it appears “straight”; objectively, evaluate together with fundus photographs or ophthalmoscopic findings. This provides important information for diagnosing ocular muscle palsy.

Retinal correspondence test: Compare the subjective and objective angles of strabismus. If they match, it is normal retinal correspondence; if there is a difference, it is judged as anomalous retinal correspondence (ARC).

Summary of information that can be evaluated with the major amblyoscope

Evaluation item	Slide used	Clinical significance
Presence of simultaneous perception	Simultaneous perception slide	Presence and degree of suppression
Subjective angle of strabismus	Fusion slide	Reference for surgical amount
Fusion range	Fusion slide	Strength of binocular vision
Stereopsis	Stereopsis slide	Assessment of highest-level binocular vision
Cyclodeviation	Cyclorotational slide	Diagnosis of superior oblique palsy, etc.
Retinal correspondence	Fusion/simultaneous perception slide	Presence or absence of ARC

5. Vision Training with the Major Amblyoscope

Concept of Vision Training

Vision training is a treatment that coordinates sensory and motor training to achieve good eye alignment and binocular vision. The major amblyoscope is used for both stepwise assessment and training of binocular visual function. In young children, using the instrument and obtaining subjective responses are difficult, so training is realistically performed after school age.

Indications for Training

The main indications for vision training with the major amblyoscope are as follows:

• Intermittent exotropia after school age: Performed as suppression removal, convergence training, and fusion training before and after surgical treatment.
• Strabismus with amblyopia: When evaluating and training binocular visual function in parallel with amblyopia treatment.

In young children (generally before school age), cooperation with the examination is difficult to obtain, making training implementation challenging. The start of training is typically considered after school age, when stable subjective responses can be obtained.

Management of Young Children

In children under 3 years of age, cooperation with examination and training may be difficult. If subjective responses cannot be obtained, do not force continuation; instead, combine other examination methods for objective evaluation.

Three Stages of Training

Vision training using a major amblyoscope proceeds in the following three stages. Each stage is carried out sequentially, and progression to the next stage occurs only after mastery of the previous stage is confirmed.

Stage 1: Simultaneous Perception Training

For patients with suppression (a condition where the brain unconsciously eliminates the image from the squinting eye), training is first conducted to enable simultaneous recognition of targets in both eyes. Using simultaneous perception slides, techniques such as flashing and brightness adjustment are employed to break suppression and promote simultaneous binocular recognition. Acquiring simultaneous perception is a prerequisite for fusion and stereopsis training. It has been reported that the technique of gradually changing the illumination of the synoptophore between the left and right eyes can be applied to quantify suppression depth with higher sensitivity than the Worth 4-dot test ³.

Stage 2: Fusion Training

After simultaneous perception is achieved, the goal is to expand the fusion range. Using fusion slides, the patient repeatedly practices superimposing the targets into one. The ranges of convergence (positive fusion) and divergence (negative fusion) are gradually expanded to strengthen stable single binocular vision.

Stage 3: Stereopsis Training

After fusion is stabilized, the aim is to acquire fine stereopsis. Using stereopsis slides, exercises are performed to elicit depth perception from the disparity between left and right targets. Acquisition of stereopsis is an indicator that binocular visual function has reached a practical level.

Combination of Outpatient and Home Training

Training using a major amblyoscope is generally conducted on an outpatient basis. Since the major amblyoscope cannot be used at home, it is recommended to combine it with home convergence training (such as reading exercises and pencil push-ups) to maintain continuous training effects.

Limitations and Role of Vision Training

Cases where strabismus is completely cured by vision training alone are limited. For intermittent exotropia after school age, it is effective as an adjunctive therapy before and after surgical treatment, and is used in combination with preoperative binocular visual function evaluation and postoperative fusion reinforcement. It is important to periodically reassess training effects with a major amblyoscope and adjust the training program. It has been reported that combining binocular vision training with refractive correction and prism therapy improves ocular alignment rate and binocular visual function in children with esotropia and amblyopia compared to monotherapy ⁴. In amblyopia treatment, improvement in fusion function has been shown to correlate with visual acuity improvement ⁵.

• Preoperative role: Release of suppression, preparation for postoperative binocular vision by expanding fusion range
• Postoperative role: Stabilization of ocular alignment through fusion training, promotion of stereopsis recovery
• Cases not requiring surgery: Training support for intermittent exotropia with small deviation angle, when observation is maintained while preserving binocular vision.

Q Can strabismus be cured by training with the major amblyoscope alone?

A

Only limited cases of strabismus are cured by vision training alone. In intermittent exotropia after school age, it is positioned as an adjunctive therapy before and after surgery. It is important to confirm the results of training through reassessment with the major amblyoscope and consider the indication for surgery as needed.

6. Development of binocular vision and significance of testing

Developmental stages of binocular vision

Binocular vision develops stepwise through visual experience after birth. The developmental sequence is as follows.

1. Simultaneous perception: The first binocular function acquired. The stage where images from both eyes are recognized simultaneously in the brain.
2. Fusion: The function of integrating images from both eyes into a single image. Includes sensory fusion and motor fusion.
3. Stereopsis: The highest-order binocular function that perceives depth from binocular disparity (slight differences between the images in the left and right eyes).

This developmental sequence corresponds to the three stages of binocular vision evaluated with the major amblyoscope (simultaneous perception → fusion → stereopsis). In patients with strabismus or amblyopia, any stage may be impaired during development, and identifying which stage is affected directly determines the treatment strategy.

Position of the major amblyoscope: Comparison with other tests

The major amblyoscope is characterized by its ability to quantify each binocular function stepwise under conditions of binocular separation. In contrast, near stereopsis tests (such as the Titmus stereo test and Randot stereo test) measure stereopsis under natural binocular conditions (using polarized glasses), and they are complementary.

Comparison item	Major amblyoscope	Near stereopsis test (Titmus, etc.)
Measurement conditions	Binocular dissociation, simulated distance conditions	Natural near binocular conditions
Measurable functions	Simultaneous perception, fusion, stereopsis, eye position	Mainly stereopsis
Quantification	High (quantified in angles and degrees)	Medium (displayed in seconds of arc)
Application to training	Possible	Not possible

By combining the evaluation results of binocular visual function obtained with the major amblyoscope and the results of near stereopsis tests, a more accurate overall picture of the patient’s binocular visual function can be obtained. The involvement of proximity and accommodative convergence is a limitation of the major amblyoscope, and caution is needed in interpreting the results. Stepwise evaluation of binocular visual function provides important information for determining the indication for strabismus surgery, surgical technique selection, and postoperative management.

Q What is the difference between simultaneous perception, fusion, and stereopsis?

A

Simultaneous perception is the ability to recognize images from both eyes at the same time, fusion is the ability to superimpose them into one, and stereopsis is the ability to perceive depth from the slight disparity between the left and right images. Development proceeds in this order (simultaneous perception → fusion → stereopsis). The major amblyoscope can evaluate and train these individually.

Q In what facilities can a major amblyoscope be used?

A

It can be performed at ophthalmology facilities with a strabismus and amblyopia specialty outpatient clinic. At facilities with orthoptists, not only testing but also orthoptic training (simultaneous perception, fusion, and stereopsis training) can be provided.

References

Footnotes

1. Fu T, Wang J, Levin M, Su Q, Li D, Li J. Fusional vergence detected by prism bar and synoptophore in chinese childhood intermittent exotropia. J Ophthalmol. 2015;2015:987048. PMID: 25954512. doi:10.1155/2015/987048 ↩

2. Haque S, Toor S, Buckley D. Are Horizontal Fusional Vergences Comparable When Measured Using a Prism Bar and Synoptophore? Br Ir Orthopt J. 2024;20(1):85-93. PMID: 38525409. PMCID: PMC10959145. doi:10.22599/bioj.326 ↩

3. Plaumann MD, Roberts KL, Wei W, Han C, Ooi TL. Refining Clinical Quantification of Depth of Suppression in Amblyopia through Synoptophore Measurement. Life (Basel). 2023;13(9):1900. PMID: 37763304. PMCID: PMC10532546. doi:10.3390/life13091900 ↩

4. Liang J, Pang S, Yan L, Zhu J. Efficacy of binocular vision training and Fresnel press-on prism on children with esotropia and amblyopia. Int Ophthalmol. 2023;43(2):583-588. PMID: 35945412. doi:10.1007/s10792-022-02462-8 ↩

5. Lv Z, Tao Z, Hu G, Deng H. Significance of binocular fusion in enhancing visual acuity during amblyopia treatment. Transl Pediatr. 2024;13(10):1767-1776. PMID: 39524389. PMCID: PMC11543132. doi:10.21037/tp-24-125 ↩