Convergence Insufficiency

Etiology

Vergence eye movements (either convergence or divergence) are a summation of components in response to a stimulus. Vergence implies the movement of both eyes in conjunction with one another. In patients with convergence insufficiency, the etiology is presumed to be an innervational difference in these components that results in the limited capacity to converge with near demands. These components, often described as Maddox components of vergence, include tonic, proximal, fusional, and accommodative vergences.

Tonic vergence describes the angle of vergence in the absence of a stimulus. The physiological position determined by tonic vergence often differs from the fusional vergence demand, and this difference is what constitutes an individual's phoria. A phoria manifests during a disruption of binocular vision, such as monocular viewing, and is an error of binocular alignment. Phorias are elicited in monocular viewing since, under binocular viewing, the other vergence components compensate for a difference in vergence demand.

Another component is proximal vergence, which refers to vergence stimulated by a perceived distance or depth. Proximal vergence results in binocular fixation composed of large gaze shifts and comprises the majority of binocular shifts in fixation. Upon the completion of binocular shifts from proximal vergence, fusional vergence takes over to maintain vergence. Fusional, or disparity vergence, is the third component and utilizes feedback from retinal image disparity to maintain ocular alignment accurately through small eye movements. Accommodative convergence, the final component, describes the coupling relationship between stimulated convergence and the response to ensure clarity by the accommodative system.

For CI, treatment options are tailored towards improving the tonic and proximal vergence components through procedural therapies, as they are adaptable with exercises. The contributions of fusional and accommodative vergences were found to be untrainable.[2]

Epidemiology

The prevalence of convergence insufficiency in school-age children documented in the literature ranges from 2% to 13%, with the most common figure cited in literature around 5%.[3] The prevalence of CI in older age groups, defined by those over the age of 19, was found to be approximately 1 in 6.[4] Limited literature exists to compare these findings at this time. The lack of literature for CI in presbyopes is attributed to the effect presbyopia has on the accommodative convergence of the convergence system, as well as the natural tendency for individuals to become more exophoric with age.[5] There does not appear to be a correlation between gender and CI, although familial predisposition has yet to be examined.

History and Physical

A definite diagnosis of CI requires specific physical evaluation criterion including:

• Exophoria (XP) greater at near than far by at least 4 prism diopters
• Receded Near Point of Convergence (NPC) of 6 cm or more
• Insufficient Positive Fusional Vergence (PFV) to meet Sheard's Criterion
• Symptomatic according to the Convergence Insufficiency Symptom Survey (CISS)

Phorias, the natural ocular alignment, are best measured by performing an alternating cover test and neutralizing eye movements to determine the magnitude. Exophoria describes temporally deviated ocular alignment, whereas esophoria describes nasally deviated alignment. 

NPC measures the magnitude of convergence by following a target in towards the nose. The magnitude is measured when the patient subjectively reports diplopia or when the examiner objectively observes an eye drift outwards. This number is generally recorded in cm and is labeled as the "break" point. A normative value for this measurement is approximately 5 cm break from the nasal bridge. Three acceptable targets can be used to measure the breakpoint, including 1) an accommodative target, 2) a penlight, and 3) a penlight with red/green glasses.[6]

PFV measures the magnitude of convergence by placing base out prism in front of the patient in increasing strengths until the patient reports blur then diplopia (or break). The strength of prism is then decreased until fusion (or recovery) is reported. It is recorded as Blur/Break/Recovery. PFV may be measured with either rotary prism in a phoropter or with a prism bar. Measurement repeatability between the two methods demonstrates the trend of higher breakpoints when measuring with a phoropter and higher recovery points when measuring with a prism bar; however, the two methods have been found to be repeatable within 6 prism diopters.[7] Sheard's criterion is a formula used to determine if a patient is likely to experience discomfort. It is calculated by measuring vergence ranges and states that the fusional reserve must be twice the phoric demand.[8] For example, two patients are both 8 prism diopters exophoric (phoric demand=8). The first patient has PFV ranges of 14/20/16 (reserve=14) while the second has 18/24/20 (reserve=18). The first patient is more likely to be symptomatic than the second because the reserve for the first patient is less than 16 (2x8) while the reserve for the second patient is more than 16.

The CISS is a survey designed to quantify a patient's severity of symptoms based on 15 questions associated with CI.[9] Questions are read aloud, and responses are selected from 5 possible answers, which are graded from 0 to 4 based on the frequency of symptoms (never, infrequent, sometimes, fairly often, or always) and are tallied for a total score. A score of 16 or more is determined to be symptomatic and is specific for patients with CI.[10][11] One factor in the administration of the CISS is to consider the near demand the patient regards for their symptoms, as emphasizing reading over other near related tasks has been suggested to overestimate near related symptoms.[12]

Evaluation

A diagnosis of convergence insufficiency can be determined with a standard binocular vision examination. A definite diagnosis, as discussed previously, is determined with a cover test to determine one's phoria measurement, NPC, and PFV in either free space or behind a phoropter.

Treatment / Management

Office-based vergence therapy (OBVT) with home reinforcement is considered the first-line treatment option for symptomatic CI in children. OBVT has demonstrated to be significantly more effective in improving both symptoms and clinical signs than other accepted treatment options, including home-based pencil push-up therapy, home-based computer vergence therapy, base-in prism glasses, or surgery.[13] Clinical trials on the effectiveness of OBVT demonstrated success in approximately 75% of patients and were the only treatment option more effective than a placebo treatment.[14](A1)

OBVT, as documented in the literature, follows a weekly, hour-long office visit practicing 4 to 5 different procedures with a trained therapist. Procedures commonly used in OBVT include Brock strings, barrel cards, vectograms, Life-Saver free-space fusion cards, and eccentric circles free-space fusion cards. After the visit, a handout is generally provided to the patient with instructions for home reinforcement procedures. Typical home reinforcement includes multiple procedures to be performed at home for 15 minutes per day, 5 times per week.[11][15] OBVT has demonstrated significantly better (lower) mean scores on CISS, improved NPC, improved stereo acuity, and significantly improved PFV ranges compared to baseline measurements.[16][17][18] Although OBVT has not been shown to treat learning disabilities, it has been suggested that relieving clinical symptoms may help to reduce barriers to improved academic performance.[19](A1)

Differential Diagnosis

Differential diagnoses to consider include:

• Exotropia (XT): Exotropia is a form of strabismus in which either eye misaligns outward. Tropias, including XT, are clinically diagnosed by performing a unilateral cover (or cover-uncover) test, whereas phorias are determined using the alternating cover (or cross cover) test. During a unilateral cover test, the examiner covers one of the patient's eyes while observing the uncovered eye. A deviation of the uncovered eye indicates a tropia. The movement will be inward in the case of exotropia. 

• Basic exophoria: When both distance and near are measured as exophoric, the diagnosis is basic exophoria. In the case of CI, the near phoria is greater than the distance.

• Acquired exotropia

• Diplopia

•  Dorsal midbrain syndrome

• Internuclear ophthalmoplegia

• Myasthenia gravis

• Oculomotor nerve palsy

• Thyroid eye disease

Pertinent Studies and Ongoing Trials

The Convergence Insufficiency Treatment Trial (CITT) published two randomized clinical studies through a collaborative multicenter effort and was the first large-scale placebo-controlled, randomized clinical trial to study the various treatment options of convergence insufficiency. The first landmark study demonstrated that computer exercises, combined with OBVT, were more effective than pencil pushups or computer exercises alone for CI in children, ages 9 to 18 years old.[13]  

The second CITT landmark study found comparable results for young adults, ages 19 to 30 years old. Vision therapy/orthoptics was compared head to head to pencil pushups and placebo and were the only treatment that produced clinically significant improvements in the NPC and PFV.[20] A Cochrane Review suggested that OBVT was more effective than home-based vergence exercises or home-based computer vision therapy/orthoptics for children.[21]

Prognosis

There is a good prognosis for patients diagnosed with convergence insufficiency, with one study showing 73% improvement in CI symptoms and measured convergence following in-office and home-based therapy after 12 weeks.[13] Most patients remained symptom-free following one year after discontinuing treatment.[22] Limitations for the prognosis are dependent on access to offices that can provide these therapy services and the cost of treatment. In patients with convergence insufficiency after concussion or trauma, there may be a prolonged recovery period.[23]

Complications

Undiagnosed and, therefore, under-treatment of convergence insufficiency does not pose a significant health risk; however, symptoms characteristic of CI discussed previously will persist.