Prevalence Of Convergence Excess Among Children In Owerri Municipal Primary School, Owerri, Imo State

ABSTRACT

The research work investigated the Prevalence of convergence excess among primary school children, to find out the number of cases of Convergence excess, the gender in which convergence excess was more prevalent. A hundred subjects ,aged 7-12 years, were selected(50 males and 50 females) from Ikenegbu Primary school Owerri and City Primary School Owerri. The children examined were however randomly selected irrespective of their classes. These subjects were matched in terms of age groups, gender and convergence ability. The convergence ability was determined after series of test like amplitude of accommodation, near point of convergence, near phoria and AC/A ratio were determined. Analysis of the result 73% of the population has Convergence excess while 27% were either orthophoric and convergence insufficient. The chi square test, at 0.05 significance level showed a significant prevalence of convergence excess in children. While using the Z-statistic test at 0.05 significance level and with z calculated as ±1.96,it was observers that there is no significant difference between the Prevalence of Convergence excess in males and females. The study suggests an advocacy programme to raise the awareness of treatment of Convergence excess among primary school children, so as to improve comfortability and academic performance.

CHAPTER ONE

INTRODUCTION

1.1 Background Of The Study

Binocular vision is a vision wherein both eyes aim simultaneously at the same visual target (i.e. the two eyes working in synergy, equally and accurately as coordinated team). Binocular vision is the ability to maintain visual focus on an object with both eyes, creating a single visual image (William Shiel, 2018).

There are some certain situation in which there is difficulty in the synergic working of the eye as a term resulting to a vertical or horizontal misalignment between the line of sight of one eye with the other eye, this condition is known as binocular vision dysfunction.

According to Dr. Cheryl Berger Isrealoff (2013), of Golden City binocluar vision dysfunction isn’t actually an issue with one specific eye instead it’s a mutual disconnect between the two, and how they function with one another.

Convergence is a co-ordinate movement and focus of our two eyes inward. Close work require us to focus both our eyes inward on close object including books, papers, computer screens, etc. convergence skills are learned during our early years.

A problem with the co-ordinate movement of our eyes inward to look at close object is called a convergence problem which is a part of binocular syndrome.

Convergence excess which was first described by Donders in 1864 is a type of binocular syndrome (Convergence problem) or dysfunction in which there is a decompensated esophoria at near with any esophoria for distance being of less amplitude and well compensated. (Evans, 1997). It can also be said to be orthophoria or moderate esophoria for distance and more marked esophoria for near with the cause being excessive AC/A ratio (Tait, 1991).

Although symptoms may be particularly associated with close work and may include frontal headache, a common cause of convergence excess is a high AC/A ratio and /or uncorrected hypermetropia. E.g. the classical Donder’s types squint.

Studies have also revealed that convergence excess (over convergence) can affect an academic output of a child who is engaged in long times test. Due to the symptoms accompanied with convergence excess such as blurred vision, intermittent double vision, headaches, eye strain and fatigue. And according to the American Optometry Association studies indicate that convergence excess have a high rate of occurrence in the pediatric population.

1.1.1  Background Information

Convergence excess occurs when the natural posture of the eyes is closer in than required for near vision tasks. This means the individual, when the look to near vision tasks has a natural tendency to aim the eyes closer in that position they are trying to aim at.

It can be illustrated with someone reading: they eyes should ideally be aimed and focused on the word or the page. In case of convergence excess, the eyes would actually aim and focus at a point closer in the page itself.

Individuals with this often are seen to hold themselves closer to the page or object than is usually necessary.

The signs and symptoms associated with convergence excess are often related to prolonged visually-demanding near centered task such as reading.

This condition can significantly impact in the quality of an individual especially one who is a student or a pupil based on the associated signs and symptoms.

Binocular Vision

The term binocular comes from the Latin ‘bini’ for double and ‘oculus’ for eye.

In biology binocular vision is a type of vision in which an animal having two eyes is able to perceive a single three dimensional image of its surroundings. The neurological researcher Manfred Fahle (1999) has stated six specific advantages of having two eyes rather than just one.

It gives a creature a spare eye in case one is damaged.

It gives a wider field of view. For example humans have a maximum horizontal field of view of approximately 120⁰degrees of which makes up the binocular field of view seen by both eyes) flanked by two uniocular field (seen by only one eye) of approximately 40 degrees.

It can give stereopsis in which binocular disparity (or paratlax) provided by the two eyes different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature.

It allows the angles of the eye’s line of sight relative to each other convergence and those lines relative to a particular object (gaze angle) to be determined from the images in the two eyes. These properties are necessary for the third advantage and they include;

It allows a creature to see more of, all of, an object behind an obstacle. This advantage was pointed out by Leonardo da vinci; who noted that a vertical comuln closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that part of the object might be visible to the right eye.

It gives binocular summation in which the ability to detect faint objects is enhanced.

Other phenomena of binocular vision include utrocular discrimination (the ability to tell which of two eyes has been stimulated by light), eye dominance (the habit of using one eye when aiming something, even if both eyes are open), allelotropia (the averaging of the visual direction of object viewed by each eye when both eyes are open), binocular fusion or singleness of vision (seeing one object with both eyes despite each eye’s having it’s own image of the object, and binocular rivalry (seeing one eye’s image alternating randomly with the other when each eye views images that are so different they cannot be fused).

For efficient single binocular vision to occur, the retinal image for the eyes must be in good focus and of similar size and shape. In addition, the eyes must be capable of aligning themselves in such a manner that the retinal images of a fixated object can easily be placed and maintained on the fovea of the two eyes. For someone to be considered to have normal binocular vision, the eyes must be free of errors of refraction and binocular vision, and all sensory and motor pathways subserving vision are assumed intact.

Binocular Vision Anomalies

Binocular vision anomalies can be considered in terms of two broad categories.

• Anomalies in which binocular vision is maintained but often at the cost of a considerable amount of stress e.g. heterophorias (usually simply called phorias), anomalies of fusional vergence, fixation disparity and anomalies of accommodation.
• Anomalies in which binocular vision is absent e.g. heterotropia, strabismus, or squint.

Binocular Vision Syndrome

Duane and White (2000) described as Borish (1970) noted  a number of binocular vision syndromes of particular interest to optometrists; these syndromes easily lend themselves to graphical representation and can be explained in terms of anomalies of accommodation and convergence. Borish pointed out that these syndromes were originally applied to strabismic conditions rather than phorias but they were modified to some extent by Tait (1991), who described them in terms of accommodation and convergence.

The binocular vision syndromes include;

Basic exophoria, basic esophoria, divergence insufficiency, divergence excess, convergence insufficiency and convergence excess.

Vergence

A vergence is the simultaneous movement of both eyes in opposite directions to obtains or maintain single binocular vision (Cassin, 1990). When a creature with binocular vision looks at an object, the eyes must rotate around a horizontal axis so that the projection of the image is in the centre of the retina in both eyes.

To look at an object closer by, the eyes rotate towards each other i.e. convergence, while for an object farther away they rotate away from each other i.e. divergence.

Exaggerated convergence is called cross eyed viewing (focusing on the nose for example). When looking into the distance, the eyes diverge until parallel, effectively fixating the same point at infinity (or very far away).

Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation, sometimes known as the accommodation – convergence reflex.

As opposed to the 500⁰/s velocity of saccade movement, vergence movements vergence movements are far slowed, around 25⁰/s. The extraocular muscles may have two types of fibre each with its own nerve supply, hence a dual mechanism.

Vergence is also denoted according to its direction horizontal vergence, vertical vergence, and torsional vergence (cyclovergence). And this vergence eye movements results from the activity of the six extraocular muscles. These muscles are innervated by the abducens nerve, trochlear and oculomotor nerve.

1.1.2 Convergence

In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object (Cassin et al., 1990). This is the only eye movement that is not conjugate, but instead adducts the eye.

Convergence is one of three processes an eye does to properly focus an image on the retina. In each eye, the visual axis will point towards the object of interest in order to focus it on the fovea. This action is mediated by the medial rectus muscle which is innervated by cranial nerve III (oculomotor nerve). It is a type of vergence eye movement and is done by extrinsic muscles. Diplopia commonly referred to as double vision, can result if one of the eye’s extrinsic muscles are weaker than the other. This results because the object being seen gets projected to different parts of the eye’s retina, causing the brain to see two images.

Near Point of Convergence (NPC) is measured by bringing an object to the nose and observing when the patient sees double, or one eye deviates out. Normal NPC values are up to 10cm, any NPC value greater than 10cm is remote, and usually due to high exophoria at near.

In addition, recent studies also proved that tungsten spectral energy influences the convergence which an also show an impact on reading and near visual tasks because of its brightness and miosis. Prolonged reading and working under this lighting can cause convergence disorders and visual fatigue (Male et al., 2017).

Convergence of the eye is measured in terms of the prism diopter. The prism diopter is defined as being equal to a tangent displacement of 1cm.Thus when the eyes converge to the midline at a distance of 50cm and the interpupillary distance is 6cm, the convergence required for each eye is 3(2) = 6D for a total of 12D for both eyes.

Components of Convergence

Maddox in 1893 provided the first description and classification of vergence movements of the eye. Maddox described four components of convergence as tonic convergence, accommodative convergence, fusional (reflex) convergence and proximal (voluntary convergence).

Tonic Convergence

Maddox (1893) stated that if all innervations to the ocular muscles were to cease the anatomical position of rest of the eyes would be one of considerable divergence, and he described tonic convergence as being responsible for moving the eyes from the position of anatomical rest to a more convergence position. This latter position, and it is the position taken by the eyes when no stimuli to fusion are present. Tonic convergence determines to what extent the distant phoria. An excess of tonic convergence results in esophoria, and a deficiency of tonic convergence results in exophoria.

Accommodative Convergence

Accommodative convergence is the convergence that is associated with accommodation. Recall that for a person who has an interpupillary distance of 64mm, the total amount of convergence required at therefore has just the “right amount” of tonic convergence, the amount of accommodative convergence will determine the near phoria.

For example if the amount of accommodative convergence is 15D, it will obviously be enough to supply the entire 15D needed for the 40cm distance with the result that the patient will also be orthophoric at 40cm. in the other hand, if a person who is orthophoric at distance has only 10D of accommodative convergence, he or she will have 5D of exophoria at 40cm; and a person who is orthophoric at distance and has 20D of accommodative convergence will have 5D of esophoria at 40cm.

Fusional Convergence

Moddox (1893) described fusional or reflex, convergence as compensating for any excess or deficit in tonic convergence, identifying retinal disparity as the stimulus. It is convenient to think of fusional convergence as being that component of convergence that keeps the person from seeing double. For example if a person has 5D of exophoria (at either distance or near), 5.00D of positive fusional convergence is needed in order to avoid diplopia; and for a person who has 5D of esophoria, 5D of negative fusional convergence is needed to avoid diplopia. Because fusional convergence is “divergent” for a person who has esophoria (as in the previous example), Morgan (1990) recommended the term fusional vergence rather than fusional convergence.

Proximal Convergence

Maddox (1983) described voluntary convergence now usually called proximal convergence as the convergence that takes place due to the knowledge that an object is nearby. This component has also been referred to as psychic convergence, and as convergence due to the awareness of nearness.

The clinical Measurement of the Convergence Components

The four components of convergence are measured by way of review as follows;

Tonic convergence: It cannot be measured directly but can only be inferred on the basis of the physiological position of rest, or phoria position, during distance fixation.

Accommodate convergence: This may be brought into play by stimulating accommodation, either by the use of a near testing distance or by the use of minus lenses.

Fusional convergence: It may be stimulated by the use of prisms, which include retinal disparity by deviating the rays of light either tempororalward (base-out) or nasalward (base in).

Proximal convergence: It is also incapable of being directly measured. Its existence can be inferred only directly on the basis of other findings.

1.1.3 Convergence Excess

This is also known to be accommodative esophoria. It is a sensori motor anomaly of the binocular vision system, characterized by or tendency for the eyes to over converge at near. The term ‘convergence excess esotropia’ defines an esotropia that is greater at near fixation than distance and was first described by Donders in 1864.

There are six muscles attached to each eye called extra ocular muscles (i.e. the lateral rectus muscle, medical rectus muscle, superior oblique muscle, inferior oblique muscle, superior rectus muscle and lateral rectus muscle) that are responsible for moving the eyes in tandem in all directions(Anordi, 1998). When they work as they should, in perfect synchrony, they help provide effortless coordination of the eyes at all distances and ranges of vision, all day long.

We need to be able to converge our eyes, while eyes looking straight ahead are not normally converged, when we reach something up close, our eyes must convergence somewhat so we don’t have double vision. But when we converge too much it leads to a condition known as convergence excess.

Convergence excess is the term used to describe an eye muscle imbalance, which causes a tendency for the eyes to want to aim and where the object is located in space more inwardly than needed.

Figure1.1  A diagram depicting convergence excess and it’s associates.

Usually, this mismatch between where the eyes aim and where the object is located in space has more an effect on vision up close as opposed to looking father away. When a convergence excess exist, it requires the muscle coordination system to work harder to maintain alignment of the eyes. Sometimes, the extraocular muscles are taxed beyond their power to maintain single binocular vision and can result in episodes of diplopia (double vision) or even to appoint where using both eyes is not possible. In cases like these, people will either see double most of the time, the brain wants to reject this and learns to suppress at least the central vision from one eye. This can affect only one eye all the time, or it can alternative back and forth between the two the medical term for this condition is strabismus esotropia. It is a type of strabismus where one eye turns inwards, rather than outward. In milder forms of convergence excess is refers to as esophonia.

Symptoms of esophoria may include blurred vision at distance, near or both; headaches; intermittent diplopia; ocular fatigue, a drawing or pulling sensation around the eyes; excess blinking and head tilting. Dizziness a tendency for motion sickness and an avoidance of near tasks such as reading, writing or computer use are also reported. And obviously academic performance will suffer when children are esophoric.

This is typical high AC/ A ratio case that has symptoms of eyestrain/fatigue, headaches, double vision, blurry vision, and loss of place when reading, difficulty reading and concentrating, avoidance of near work, poor sports performance. It can also present with less common symptoms of dizziness or motion sickness and general fatigue.

Convergence excess have also been defined and considered in different ways by different people. Most people consider that for convergence excess to be significant, the different between near and distance fixation should be greater than 8 prism diopters (8PD) and that this difference remains after full hypermetropic correction with single focus lenses. Some reserve the definition for patients who are orthophoric (or have well controlled esophoria) for far distance fixation is particular in North American as it is many of these patients who are appropriate for bifocal therapy.

An acceptable definition of convergence excess esotropia would therefore be a convergence squint which is more, than 8prism diopter (8PD) greater for near fixation than distance fixation after full hypermetric correction.

In some patients the AC/A ratio is normal {non-accommodative convergence excess esotropia) and Costenbader (2000) in his research described a group of patients with a low AC/A ratio who had a remote near point of accommodation. Their poor accommodation requires than to exert excessive accommodative effort resulting in near esotropia. This uncommon condition has been called hypoaccommodative convergence excess esotropia.

1.1.4 Symptoms Of Convergence Excess

The symptoms of convergence excess include; but are not limited to the following:

Eyestrain/fatigue, headaches, double vision, blurry vision, loss of place when reading, difficulty reading and concentrating, avoidance of near work and poor sports performance.

Less common symptoms include;

Dizziness or motion sickness, general fatigue and abnormal postural adaptation/abnormal working distance.

1.1.5 Diagnostic Factors Of Convergence Excess

In addition to greater esophoria at near than at distance, convergence is characterized by one or more of the following diagnostic or clinical findings;

High AC/A ration

Low negative or excessive positive fusional vergence ranges

Reduced positive relative accommodation (PRA)

Eso-fixation disparity with higher than normal associated phoria

Inadequate binocular accommodative facility

More esophoria at near than far.

1.1.6 Management And Treatment Of Convergence Excess

Optical Aids (Lenses/Prisms)

Lenses normally used for the management of convergence excess are bifocal lenses. In the use of bifocals, the full cycloplegic correction is prescribed for the distance segment. For any patient who appears to have a partially accommodative esotropia, the refraction is rechecked using atropine 1% drops for 3 days before repeat refraction. It is worth remembering that “convergence excess” esotropia may be erroneously diagnosed in patients with under corrected hypermetropia. Bifocals can be prescribed in one of two ways; either as +3.00 add for all patients Pratt Johnsonb (1995) or more commonly, the smallest amount of add which controls the near deviation, up to a power of +3.5Ds.

There is a recorded difficulty in the practicality of bifocal wear in childhood even alert, intelligent, and motivated 45 years old sometimes find it difficult to cope with their first pair of bifocals. The position of the bifocal segment needs to be high, preferably bisecting the pupil in the primary position. A flat top design is best and the glasses need to be fitted securely to prevent slippage, requirements which are often impossible to achieve lastingly in children since the nose offers little support for these bulky glasses. And this bifocal is only effective at one third of a metre leaving intermediate and closer distances inappropriately corrected.

The aim of treatment with bifocal is to allow the development of fusional amplitudes. Once the patient has been demonstrated to have stable alignment and fusion with amplitudes for a period of a year or so, the strength of the bifocal may be progressively decreased and discontinued as soon as practically possible.

Experience has shown that the provision of added plus power at near is an effective method of managing convergence excess. The alternative approach of use of prism for correction and training is also available. A base-out prism power is prescribed for correction for close work while a base-in prism power is prescribed for training to increase the amplitude of negative fusional vergence. However, experience has shown that this form of training often is not effective.

Surgical Approach

Although the surgical management of patients with esotropia that is significantly greater at near than distance represents a challenging problem.  The surgical management are being considered between two types of patients, those with binocular vision maintained at distance fixation with or without microtropia and those that have manifest esotropia at distance (which may be intermittent with a larger constant esotropia at near when fixing on a accommodative target.

The surgical approach is aimed at alleviating the patient symptoms of (diplopia, asthenopia, cosmetically unacceptable esotropia), eliminating the need for bifocal glasses, maintaining or improving sensory binocularity and reducing the near deviation to a level that enables control by the patients motor fusion reserves without detrimentally altering the distance angle.

The surgical approach involves commonly on the bilateral medical rectus recessions based on the manifest distance deviation alone, arguing that larger recessions may lead to over corrections. West and Repka retrospectively compared the outcome of surgery personnel for the distance or the near distance disparity esotropia.

The concept of an augmented bilateral medial rectus recession surgery for near/distance disparity esotropia was introduced in the 1970s and was widely accepted.

Vision Therapy/Exercise

Vision therapy is aimed at the following:

• To normalize associated deficiencies in ocular motor control and accommodation.
• To normalize the accommodative convergence relationship.
• To normalize fusional vergence ranges and facilities.
• To reduce or eliminate suppression.
• To normalize depth judgement and or stereopsis.
• To interrogate binocular function with information processing.

The vision therapy involved here include:

Pencil spreading

This involves the use of two pencils with erasers suspending it at an eye level, aligning the two pencils with an object such as clock and trying to keep the middle pencil seen after some procedures have been followed, single even if it looks like a wider pencil, as you spread apart the two pencils until a limit beyond which he will not be able to spread the pencil.

This exercise is performed 3-5 times a day or whenever eye strain or work related headache develops. It generally strengthens the lateral rectus muscles.

Thumb spreading

This involves the two thumbs, the patient is to look between the two thumbs relaxing the eyes and imagining that he is viewing an object at far. The two thumbs are to become four thumbs. The more the patient is able to relax the eye the more the two middle thumbs will move towards each other. Once the patient is able to fuse or superimpose the two middle thumbs and see the single thumb in the middle the patient looks at the middle thumb for a few seconds. The patient is now using the lateral rectus muscles to diverge the eyes and to fuse the thumbs into one.

Factors that affect Management of Convergence Excess and duration of Treatment

Severity of the symptoms and the diagnostic factors including onset and derivation of the problem.

Implications on the patients general health and associated visual condition.

The extent of visual demands placed on the patient.

Patient compliance must be considered.

Previous or prior intervention.

1.2  Statement Of The Problem

According to Chung (2009) convergence excess which can be seen as esophoria is a risk factor in myopia development in children.

Studies have also shown that esophoria result in larger accommodative lag, resulting in retinal blur and also to an extent due to the symptoms of convergence excess causes poor academic performance in children.

This calls for a research on the comparative investigation on the prevalence of convergence excess in children and also need for  timely management of the condition.

1.3 Aim And Objectives Of The Research

1.3.1 Aim

To determine the statistical population of Primary school children with convergence excess in Owerri municipal L.G.A.

1.3.2  Objectives Of The Research

– To ascertain the prevalence of convergence excess among children.

– To ascertain the gender specific of prevalence of the convergence excess.

– To ascertain the age specific of prevalence of the convergence excess.

• Research Questions

– What is the prevalence of convergence excess among children?

– How does convergence excess result to development of Myopia?

– What effect does convergence excess have on the academic output of school children?

– What is the gender specific of prevalence of convergence excess?

1.5     Research Hypothesis

(H₀):  There is no significant prevalence of convergence excess in children.

(Hi): There is a significant prevalence of convergence excess in children.

(H₀):  There is no significant difference between prevalence of convergence excess among female and males.

(Hi): There   is   a   significant   difference  between  prevalence   of convergence excess among females and males.

1.6      Significance Of Study

This study would enable us ascertain the prevalence of convergence excess in children between the age of 7-11 years. it would enable us to know the gender specific prevalence of convergence excess.

This study would also enable us to qualitatively ascertain how convergence excess can result to development of myopia, as much as controlling and managing the prevalence of convergence excess in children.

The patients will benefit from the effective and efficient management of their cases based on the information derived from the study.

1.7 Scope Of Study

This study is to be carried out to establish the prevalence of convergence excess amongst children (male and female) in Owerri municipal Primary Schools, Owerri, Imo State. It encompasses the following:

• An in-depth knowledge of convergence excess.
• Understanding of children behavioral pattern and being able to detect a malingering one.
• Myopia and its possible causes with precision to convergence excess.
• Possible ways in which the convergence excess symptoms can be alleviated.

1.8 Definition Of Terms

Prevalence: Frequency or range of occurrence and incidence of a condition.

Accommodation: The eye’s automatic fine focusing facility, it is the alteration in the convex shape of the crystalline lens of the eye by a contraction of the ciliary muscles thereby adjusting its dioptric power and one attains a distinct maximally sharp retinal image of distant object.

Esophoria: This is the inward deviation of the visual axis of the eye when binocular vision is interrupted.

Exophoria:  This is the turning (deviation) of the eyes visual axis outwards, when binocular vision is interrupted.

Hypermetropia: This is a condition of the eye in which with accommodation relaxed parallel rays of light focus behind the retina.

Myopia: This is a condition of the eye in which with accommodation relaxed parallel rays of light focus in front of the retina.

Therapy: A medical treatment that involves attempted remediation of a health problem, usually following a diagnosis. As a rule each therapy has indications and contraindications.

Orthophoria: This is a condition of complete balance between the alignment of the two eyes, such that perfect alignment is maintained even when one eye is covered.

Accomodative convergence per accommodation AC/A ratio

The ratio of accommodative convergence of accommodation i.e. amount of convergence in prism dioptre per dioptre change in accommodation.

Visual axis: This is the line joining the fixation point (0), nodal point (N), and the fovea (f) of the two eyes.

Asthenopia: It is in other words called eye strain, it is an eye condition that manifest through nonspecific symptoms such as fatigue in or around the eyes, blurred vision, headache, and occasional double vision.

Prism: A transparent optical element with flat, polished surfaces that refract light. At least two of the flat surfaces must have an angle between them, which depend on the application.

Positive fusional reserve: The amount of positive convergence which can be exerted while the accommodation remains unchanged. It is measured clinically using base-out prisms.

Negative Fusional reserve: The amount of negative convergence which can be exerted while accommodation remains unchanged. It is measured clinically using base-in prisms.

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Category: Project

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Chapters: 1-5

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