Convergence Insufficiency (CI) is characterized by a decreased ability to converge the eyes and maintain binocular fusion while focusing on a near target. CI is usually accompanied by a reduced near point of convergence (NPC), decreased convergence amplitudes or an exodeviation (usually > 10 prism diopters) at near. Patients often complain of eye strain when reading, closing one eye when reading, or blurred vision after short periods of near work. The diagnosis of primary CI is based on the patient’s presenting symptoms and the aforementioned clinical signs.1,2,3
The etiology of isolated CI, not related to trauma or neurological disease, has not been completely determined. Primary CI is thought to be caused by an imbalance of vergence eye movements that is either an inborn deficiency or an acquired etiology.2,3
Although the exact disease mechanism that causes CI is unknown, scientists have been able to discover the neurological centers that control convergence eye movements. The midbrain reticular formation is involved with controlling the velocity and amplitude during fusional and accommodative convergence movements. The nucleus raphe interpositus is associated with fast vergence movements, whereas the nucleus reticularis tegmenti pontis is activated during slow vergence movements.4
Although CI can present at almost any age, it is most common in the young adult population.
The incidence of CI in the general population is estimated to be 0.1 to 0.2%2. While exodeviations are only present in 1% of the general population5, CI is present in 11-19% of children with an exodeviation6.
The most common signs of CI include a reduced near point of convergence (NPC), decreased convergence amplitudes and an exodeviation at near.2
An NPC value that is less than 10 cm from the bridge of the nose is considered to be within normal limits. If a patient’s NPC is farther than 10cm from their nose then an evaluation to rule out CI should be performed. The NPC is tested by having the patient focus on a near target while slowly moving the target towards their nose. An exotropia will occur when the patient can no longer maintain fusion. This is referred to as the break point.7
Convergence amplitudes are measured with base out prisms while the patient focuses on a target at near.7 Normal convergence amplitudes are 38 prism diopters at near and 14 prism diopters at distance.8 In general, fusional convergence amplitudes of less than 15 to 20 prism diopters at near are a sign of convergence insufficiency.7,9.
It is important to measure the exodeviation in all positions of gaze to rule out any eye muscle imbalance that could make the exodeviation greater in downgaze. Measurements at near should be done in both primary position and reading position. Superior oblique over action can cause an exodeviation greater in downgaze and can be mistaken for CI.
The treatment of CI was first medically documented by von Graefe in 18553. Since then, orthoptic exercises have been the leading treatment option for this condition.
Patients with convergence insufficiency will have a remote near point of convergence or decreased convergence amplitudes or likely both7. Alternate prism cover testing usually demonstrates a moderate to large exophoria or intermittent exotropia, greater at near than distance.
Symptoms of convergence insufficiency include eye strain, double vision, headaches, blurred vision at near, eye fatigue, tension in and around the eyes, print moving on page, and frequent loss of place when reading.2,7,10
Orthoptic therapy is the primary treatment modality used by most eye care professionals for the treatment of CI. The plasticity of the fusional convergence reflex system allows patients to improve their convergence amplitudes with simple exercises11. There are numerous different types of eye exercises; however, the primary treatment modalities for CI include home-based exercise, in-office exercises, computer vergence exercises or a combination of these. Some studies have suggested that performing both the home-based exercises and the computer program is more effective than either modality performed on its own.10
Conventional convergence exercises
There are two mechanisms used in conventional CI exercises to improve convergence amplitudes.
CI exercises can either utilize voluntary convergence or disparate retinal images to evoke fusional convergence. Voluntary convergence exercises include gradual exercises (pencil push-ups), jump convergence exercises and stereograms.
Gradual Convergence Exercises
Gradual exercises are performed by having the patient focus on a small target (usually an accommodative target) at a remote distance to acquire binocular single vision (BSV) and then slowly move the target toward their nose while maintaining BSV. This exercise requires the patient to be able to recognize physiologic diplopia. If a patient suppresses an image from one of their eyes, then a red filter can be used over one eye as an anti-suppression technique. Caution always needs to be taken when performing anti-suppression exercises to ensure intractable diplopia does not occur.
There are several different types of convergence cards that are used for CI and usually consist of dots or circles. The patient holds the card toward the bridge of their nose while focusing on the most remote dot or circle and progressively moves their eyes to a closer target. For example, ‘dots on a line’ creates an “X” crossing through the fixation dot. If a patient is suppressing then they will only see one line.
A stereogram is a card that consists of two similar images that are separated on the horizontal axis. The patient then converges their eyes to an area in front of the card in order to elicit physiological diplopia. If the patient is successful then a third image will appear in the middle of the two pictures on the card. The middle image is actually a superimposed, combined image of the two pictures.11
Vergence facility exercises
Vergence facility is another type of exercise that has the patient look from a target at near to a target at distance with rapid fixation switches.2
Base out prism exercises
Base out prisms 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 BSV. Some practitioners give a patient a single prism and have them do gradual exercises or near tasks, while other practitioners have the patient use a prism bar and to overcome increasing prism strengths while focusing on a near target.
Computer based convergence exercises
In recent years a new treatment method for CI has emerged. A computer based orthoptic program known as Computer Vergence System (CVS) is used by many eye care professionals. The program uses random dot stereograms to form pictures that require bi-foveal fixation to stimulate the vergence system. The program gradually increases the amount of vergence required to appreciate the stereogram picture and can monitor progression online.
In-Office vision therapy
Eye care professionals will some times prescribe both in-office and home-based CI exercises. Occasionally patients will require additional treatment strategies such as anti-suppression or extra time and assistance with the exercises and will require in-office treatment.
It is important to note that some of the treatment strategies used by behavioral optometrists are known as vision therapy and are not medically proven and are not supported by the American Academy of Ophthalmology, American Academy of Pediatric Ophthalmology and Strabismus or the America Academy of Pediatrics.12
Base – In prism Glasses
Base-In prism glasses are sometimes prescribed if conventional CI exercises are unsuccessful. The practitioner will usually prescribe the least amount of prism necessary to achieve comfortable BSV at near. Interestingly, a study by the CITT group found that base-in prism glasses were no more effective than plano placebo reading glasses in children13. However, additional studies have suggested that base-in prism glasses are effective at reducing CI symptoms in presbyopic patients.10
Randomized Clinical Trial Treatment
In 2011 Sheiman, Gwiazda, and Li, in association with the Cochrane Collaboration, published a study detailing non-surgical interventions for convergence insufficiency. They did a system wide database analysis to review all abstracts related to non-surgical treatment methods for CI. The objective of the study was to review all randomized controlled trials (RCT) and analyze their methodology, risk of bias and outcome data. Out of 529 abstracts reviewed, only 6 studies met their criteria for RCTs. Four of the six studies were judged to below risk for bias, while the other two studies were deemed high risk for bias. All four studies determined to be low risk for bias were performed by the Convergence Insufficiency Treatment Trial (CITT) study group.10
In 2005, the CITT study group published 3 studies related to non-surgical treatment for convergence insufficiency13. In one study 72 children with symptomatic CI were randomly assigned to wear either base-in prism reading glasses or placebo reading glasses for all near tasks. The authors found no significant improvement in symptoms or convergence ability after 6 weeks of using base-in prism glasses as compared to the placebo group.13
The 3 remaining studies outlined in the Cochrane review focused on the effectiveness of home-based, office-based and computer convergence exercises10,14. The most recent study in 2008 randomly assigned 221 children to receive either home-based pencil push-ups, home based computer therapy with additional pencil push ups, office-based therapy with home reinforcement or office based placebo treatment9,10. The authors report that children randomized to the office-based treatment group had the most significant improvement in convergence amplitudes, NPC and a lower symptom survey score. Several eye care professionals have reported flaws in the methodology of this study. Reported flaws in the study include unequal treatment dosing and a misrepresentation of home-based exercises. Patients in the office-based treatment group were prescribed significantly more treatment time than any other randomized group with additional 60 minutes of treatment time per week. Another controversial limitation of the study was the use of “pencil pushups” for home-based convergence exercises. Many practitioners feel this is a misrepresentation of conventional orthoptic therapy. Most eye care professionals that prescribe home-based exercises include a variety of exercises and use accommodative targets.15
Symptom severity is one of the primary criteria used to diagnose and monitor patients with CI. It is important to include a placebo group when investigating the efficacy of a treatment modality, especially when symptom surveys are used as a success criterion. Excellent results have been obtained with all three types of CI exercises; however spontaneous resolution of symptoms has also been reported. Moreover, clinical studies with placebo groups have also shown that patients in the placebo group tend to show an improvement in symptoms, although to a much less degree.
As a last option, some surgeons will perform a small medial rectus resection of one or both medial rectus muscles. This is somewhat controversial and some authors have shown poor results with this surgery.7 Prior to surgery it is important to determine divergence amplitudes at distance and test for post operative diplopia at distance. Patients should be fully advised of the risks of diplopia at distance after surgery.
Medical Follow up
The follow-up schedule for CI exercises is dependant on the treatment prescribed and is determined by the eye care professional. The follow up schedule is usually based on the severity of the patient’s signs and symptoms. Some patients may require a follow assessment once a month, while other patients may require follow-up every two weeks.
In general, home-based and computer exercises have a longer duration between follow up appointments and tend to be significantly less expensive than in-office exercises.
Idiopathic convergence insufficiency responds very well to convergence exercises and has a very high reported success rate. Published success rates vary between 70 to 80% depending on the patient population and study size9,10,15,16.
Convergence insufficiency is also associated with several neurological disorders. Neuro-degenerative diseases affecting the basal ganglia, such as Parkinsons Disease, Progressive supranuclear palsy (PSP) and Huntington’s Chorea have a higher incidence of convergence insufficiency3,17,18. Lesions in the pretectum and posterior commissure can cause dorsal midbrain (Parinaud’s) syndrome which is also known to have a higher incidence of CI. CI can also be associated with head trauma, myasthenia gravis, thyroid ophthalmopathy, chemical or pharmacological agents, or ischemia.2,3,17,19
Contributing Author: Laura Kirkeby CO
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