Introduction

Convergence Insufficiency (CI) is a condition in which the patient has a reduced ability in keeping the alignment of the eyes and maintaining binocular fusion on a near object. This inability to sustain convergence causes the patient to use just one eye at a time, or to see double. Commonly, CI is accompanied by a receded near point of convergence (NPC), reduced convergence amplitudes and increased exophoria at near. Consequently, symptomatic patients often complain of eye strain when doing near work such as using smart devices and reading after short periods. The penlight red-green (PLRG) anaglyph procedure is a simple near point of convergence test with good sensitivity to diagnose CI rapidly.

Common symptoms associated with convergence insufficiency

Patients diagnosed with CI often disclose the following symptoms that are experienced during reading or doing close work: eyes feel tired, eyes feel uncomfortable, headaches, feeling sleepy, losing concentration, double vision, “words seem to move, jump, swim or appear to float on the page”, eyes hurt or feel sore, “pulling” feeling around the eyes, words blurring or coming in and out of focus, losing place and having to re-read the same line of words.

Prevalence of convergence insufficiency

Notwithstanding that CI can present at almost any age, it is most prevalent in the young adult population. While exodeviations are only present in 1% of the general population, CI is present in 11-19% of children with an exodeviation.

What are red-green anaglyphs?

The red-green anaglyph (RGA) is a set of glasses used in a variety of eye tests and vision therapy (VT) regimens. The red lens filters out all the green light and the green lens filters out all the red light such that each eye sees a different coloured image from the other. It is renown in amblyopia treatment for increasing the ability of both eyes to work together and also to develop eye-hand co-ordination. The RGA serves an excellent tool for CI diagnosis because the red-green filters act as a suppression check and the penlight/transilluminator creates a non-accommodative stimulus.

Near point of convergence (NPC)

NPC is a procedure that measures the amplitude of convergence which represents the closest distance up to where fusion is maintained. The expected (normal) value for NPC is a break of 3cm ± 4cm and a recovery of 5cm ± 5cm. A break point > 7cm is considered abnormal when using an accommodative target and >10cm when using a non-accommodative target. The NPC needs to be repeated several times because NPC recedes with fatigue in both normal and CI patients. The abnormal NPC recovery point is expected to be approximately 10cm when using an accommodative target and about 15cm when using a non-accommodative target. The RGA is used these days because it fits easily over spectacles or can be used alone as opposed to a red lens that needs to be placed in a trial frame or hand-held, making it cumbersome.

Diagnosis of convergence insufficiency using the penlight/transilluminator and red-green anaglyphs

While the room illumination is dimmed, the patient is asked to put on the red-green glasses or worn over the patient’s own spectacles if the patient generally wears one. The transilluminator or a penlight is presented directly in front of the patient at a distance of approximately 40cm, along the midline and slightly below eye level. The patient is to report on how many lights are seen. If the patient has normal convergence for that distance, the expected response is one light. It should appear as a mixture of the red and green lights. If the patient replies that the light is only one colour (red or green), ask them to blink to break suppression. You can also move around the penlight to encourage the suppressing eye to engage in the task. The patient is next told to immediately report if they see two different coloured lights instead of a single light as the optometrist brings the penlight/transilluminator closer slowly. The patient should report one red and one green light at some point as the transilluminator approaches closer to the nose. This represents the convergence break point. The optometrist will also notice ocular divergence near this break point. The distance from this point to the lateral canthus is measured. Thereafter, the penlight/transilluminator is moved slowly away from the patient’s nose until the patient reports seeing one light again, this is the recovery point. This procedure is repeated thrice and only the convergence break and recovery points for the third series of measurements are recorded. The reason why the procedure is done in three trials is because the NPC recedes over time due to fatigue, particularly when the patient has convergence insufficiency.

To confirm CI, corroborating findings include: exophoria at near is  ≥ 4 ∆  greater than the far heterophoria, insufficient positive fusional vergence at near(≤ 12 ∆ base-out to blur or ≤ 15 ∆ base-out to break) and fails Sheard’s criterion (base-out to blur is less than twice the near exophoria).

Treatment of convergence insufficiency

Vision therapy (VT) is the primary treatment modality used by eye care practitioners for the treatment of CI. Convergence insufficiency exercises are designed to: develop the kinesthetic awareness of converging and diverging; develop the ability to voluntarily converge and normalise the near point of convergence. There are numerous types of eye exercises; however, the common treatment modalities for CI include home-based exercise, in-office exercises, computer vergence exercises or a combination of these.  There are two mechanisms used in conventional CI exercises to improve convergence amplitudes where CI exercises can either utilise voluntary convergence or disparate retinal images to evoke fusional convergence. Voluntary convergence exercises include gradual exercises (“pencil push-ups”), Brock string, jump convergence exercises and stereograms. These procedures elicit physiological diplopia to treat CI. Computer orthoptics uses random dot stereograms to form pictures that require bi-foveal fixation to stimulate the vergence system. Patient must maintain single binocular vision as random dot stereograms are disparated to create base-out or base-in vergence demand.

Base-out prism exercises can also be used to stimulate the converge reflex. The base-out prism induces crossed diplopia and the patient must converge to overcome the prism strength and obtain binocular single vision (BSV).

Base-in prism glasses are prescribed if conventional CI exercises are unsuccessful or not practical. The tentative prism is derived from the following equation:

P = 2/3 H – 1/3C

Where, P = tentative prism value, H = near heterophoria, C = compensating fusional vergence

The practitioner will usually prescribe the least amount of prism necessary to achieve comfortable BSV at near reducing CI symptoms. The goal of the treatment for CI is not only to eliminate symptoms but also to improve the patient’s near point of convergence and positive fusional vergence at near measurements. Occasionally patients will require additional treatment strategies such as anti-suppression.

Clinical pearls

• The PLRG is a simple, quick test that is sensitive to CI derived from evidence-based principles to assist patients experiencing vision difficulties with near-centred activities.
• Clinically, it is important to differentiate true convergence insufficiency from pseudo-convergence insufficiency. True CI is a convergence abnormality while a pseudo-CI is an accommodative dysfunction. Treatment for CI includes vision therapy and prism at near whereas pseudo requires an addition for near. Please note, an addition will exacerbate the symptoms of a CI patient.
• CI is associated with weakened binocular vision system and in certain traumatic brain injuries.
• A diagnosis of CI is made when: exophoria at near is at least 4∆ greater than the far heterophoria, a receded near point of convergence, and insufficient positive fusional convergence at near (ie, failing Sheard’s criterion where positive fusional vergence is less than twice the near phoria or minimum positive fusional vergence of 15∆  base-out to break).
• To confirm that the break (diplopia) is indeed reached during PLRG near point of convergence, the patient should notice that the red light is seen to the left (crossed) of the green light.
• Base-in prisms are favoured by presbyopic patients and young patients who are not amenable to VT.

Conclusion

The red-green anaglyph serves a useful application in convergence insufficiency management. The diagnosis of convergence insufficiency is established on the patient’s presenting symptoms and accompanied signs. The red-green anaglyph provides a NPC measurement without an accommodative stimulus. CI affects the patient’s vision at near and interferes with the comfort of close working distance and may have a serious impact on an individual’s performance in school, choice of jobs, and quality of life. 

References

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