CONVERGENSE

Convergence

Convergence is the ability to turn the two eyes inward toward each other to look at a close object. We depend on this visual skill for near-work activities such as desk work at school, working on a smart phone type device, or even in sports when catching a ball.

Convergence requires a coordinated stimulation of some extraocular muscles at the same time others are relaxed. Convergence occurs by stimulation of the medial rectus muscle of both eyes (third cranial [oculomotor] nerve) while simultaneously relaxing the lateral recti (sixth cranial [abducens] nerve).

More than just simple eye movements occur with convergence. When the eyes converge, the focusing (accommodative) system is engaged, and the pupils get slightly smaller. This set of three processes – technically termed convergence, accommodation, and miosis – is known as the near triad.

Divergence

Divergence is the opposite of convergence and is the ability to turn the two eyes outwards to look at a distant object. We depend on this skill for distance activities such as reading the board at school, driving and watching TV.

To diverge, the opposite of the near triad must occur. Now the eyes diverge, accommodation is inhibited, and the pupils slightly dilate.

What if I Have Trouble Converging or Diverging?

Your optometrist may perform a simple eye test. This test is called the near point of convergence (NPC). This test measures the ability of the patient to keep a target single as it moves closer to the nose. Think of this as the distance from your eyes to the point where both eyes are just unable to maintain focus and start to experience double vision. To perform this test, a clinician will use a small penlight or other fixation target and then slowly move the light towards the patient. The patient is to report when the light breaks into two lights (double vision). The penlight is then moved farther away and the patient is asked when the recovery point (resolution of double vision) occurs.

Some patients struggle with convergence or divergence. Convergence insufficiency (CI) is one of the most common disorders of binocular vision; CI affects approximately 5% of children in the United States and may have a serious impact on an individual’s performance in school, choice of jobs, and quality of life. Patients that suffer from CI are unable to keep the eyes converged for an extended period of time. Convergence excess is the opposite of CI. Here a patient over-convergences, which may cause pulling or straining sensations, or an inward eye turn.

Divergence insufficiency is the inability of the eyes to properly and fully diverge. The opposite of divergence insufficiency is divergence excess. Divergence excess is an over-divergence when attempting to look at distant objects.

These conditions that affect convergence or divergence may cause symptoms such as seeing double at near or far while viewing an object. Common symptoms include:

Headaches

Blurred vision

Double vision

Eye strain or fatigue

Sore or watery eyes after a near or far task

Words moving about on a page while reading

Frequenting losing your place while reading

Convergence Insufficiency

Continuing Education Activity

Convergence insufficiency is a binocular vision disorder that results in symptoms such as eyestrain, headaches, and diplopia when looking at a near target. This activity will review the functional components underlying the condition, as well as the prevalence, definition, testing, diagnosis, proper treatment, and implications of the disorder. It will also include the role of the healthcare team in diagnosis and treatment.

Objectives:

Identify the most common symptoms of convergence insufficiency.

Review the eye movements associated with convergence insufficiency.

Outline the diagnostic procedures for convergence insufficiency.

Explain the importance of interprofessional care coordination to enhance outcomes for patients with convergence insufficiency.

Introduction

Convergence insufficiency (CI) is a binocular vision disorder characterized by difficulty maintaining fusion while looking at a near target due to a tendency of the eyes to drift outwards. Commonly associated symptoms of CI include asthenopia (eye strain), diplopia, headaches, blurred vision, movement of print while reading, and difficulty with reading comprehension. Other notable symptoms exhibited after short periods of reading or prolonged near work include sleepiness and the inability to concentrate.

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 comprise 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.

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%.  The prevalence of CI in older age groups, defined by those over the age of 19, was found to be approximately 1 in 6.  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.  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.

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.  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.  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 (2×8) 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.  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.  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.

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

What is it Like to have Convergence Insufficiency?

Reading is a great example of a task that requires normal convergence.

To read, the eyes must rotate inward, the focusing system must engage, and a series of tracking movements are used to move the eyes across a page.

But, if the eyes are unable to maintain a converged position, reading becomes quite difficult! Words may become blurry, doubled, or appear to move around on the page. The end result is often fatigue, headache, and likely avoidance of near work.

Can Convergence or Divergence Problems be treated?

In short, absolutely. Treatment often focuses on working the ability of the eyes to converge and focus at different distances. Vision therapy uses a series of activities to build up progressively the ability to converge, diverge, and the rapidly alternate between convergence and divergence.

Vision and Learning

According to the American Optometric Association (AOA), up to 80% of a child’s learning in school is through vision. Also, according to a 2015 statement from the College of Optometrists in Vision Development (COVD),

“More than 5 million children in the U.S. have eye coordination and eye focusing disorders which cause them to continue struggling with reading despite the best interventions….Children don’t know how they are supposed to see, so they rarely complain; they show us they have a problem with their behavior.”

Visual Skills for Learning

Most people do not realize how many visual skills are necessary for learning. Unfortunately, the emphasis (including school screenings) is mostly on visual acuity. Visual acuity only gives part of the picture. Other skills such as eye teaming, focusing, and visual tracking are not evaluated in school screenings and often go unevaluated. Eye coordination and eye focusing problems can make learning difficult; they can make the words appear blurry, double, or look like they are moving.

The College of Optometrists in Vision Development (COVD) website outlines at least 17 visual skills necessary for learning. Other resources, such as the book “The Hidden Link Between Vision and Learning: Why Millions of Learning-Disabled Children are Misdiagnosed” by Wendy Rosen, outline more. These visual skills include: Eye Movement Control, Eye Teaming, Vergence, Convergence, Divergence, Focus Accommodation, Depth Perception or Depth Awareness, Peripheral Vision, Visual Motor Integration, Visual Spatial Skills, Visual Memory, Visual Form Recognition, Visualization, and near and far visual acuity.

Reading Skills and Vision

The ability to read requires a series of learned visual and processing skills to be used in tandem. A reader must first and foremost have an understanding of the language of the text (think of how difficult it is to understand what a foreign textbook is trying to convey!). The visual system is then engaged.

A reader must have good visual acuity, which means a clear image falls on the retina of the eyes. First, visual information is passed through the visual pathway to the occipital lobe, where visual processing begins. Both eyes are activated, and the fovea (area of central, clear vision) is directed at the words on a page. The accommodative system of the eye makes an adjustment to help clear the images, and the eyes rotate inward (converge) to point at the same location in space. The images from the right and left eye must be fused together (as double vision when reading would be very debilitating!).

The eyes then begin a series of calculated, short movement across the page–these small jumps are called saccades, and they help the eyes team together to move from one word or group of words across a page and back to the start the next line of text.

The inability to process the words on a page, or if part of the reader’s vision is missing, can have a serious impact on his or her ability to comfortably find the next word or process the meaning of the words.

The complex nature of reading means deficits in any of these areas can impact reading ability. It is estimated that upwards of 16% of the population struggle with reading in some form, and 5% of children have significant problems with reading.

Saccades and Reading

Saccades are the very quick eye movements your eye make when you are reading. They happen over an incredibly short time at very high speeds. Your eyes dart from word to word, quickly stopping (fixating) and then continuing onward.

What are Saccades and Fixations?

Saccades and fixations are types of eye movement. The spatial-temporal sampling of the ability of the human eye, demonstrates the way in which we take in the information we are receiving about the world through our vision. This includes it’s limits. As such, it is useful to study not only the properties of the eye, but it’s movements, and that is where these terminologies are derived from.

Fixations are periods where the eye or eyes are more or less still, and saccades are periods of rapid eye movement between fixation points.

Vergence and Reading

In order to read, it is necessary for the eyes to maintain proper convergence. When they don’t, a person with convergence insufficiency may be able to read without initial problems, but quickly become tired, get double vision, get blurry vision, or have headaches.

During reading, the eyes move more or less in synchrony with the movement of each eye beginning (and ending) in close temporal approximation of each other. While a great deal has been learned about the general characteristics of eye movements during reading, there are important issues that remain somewhat unclear.   The vast majority of the research investigating reading has involved recording the movements of one of the two eyes.