The diagnosis of strabismus means that a patient's eyes are not properly aligned in all cardinal fields of gaze. This is primarily a condition seen in childhood years. When strabismus is suspected a thorough examination should be done by an ophthalmologist trained to evaluate children's eye muscle balances. It is never correct to "wait and see" if the child outgrows the misalignment. Failure on the part of physicians and/ or parents to identify and properly treat strabismus early in life often leads to a lifetime of poor vision and a cosmetically unacceptable ocular alignment.
ANATOMY OF THE EXTRAOCULAR MUSCLES
It is imperative to associate strabismus disorders with the particular muscle or muscles involved. Six extraocular muscles rotate the eye: the medial and lateral recti, the superior and inferior recti and the superior and inferior oblique muscles. The rectus muscles arise from a fibrous anulus which surrounds the optic foramen and abridges part of the superior orbital fissure. These muscles insert themselves into the sclera anteriorly for a distance, ranging from 5.5 mm for the medial rectus to 7.7 mm for the lateral rectus, from their respective radial positions at the limbus. The inferior oblique muscle is the only muscle to arise from the anterior portion of the orbit. It originates from a shallow depression adjoining the anterior portion of the lacrimal fossa. There are many fascial sheaths connected to the muscles and ultimately to the surrounding connective tissue capsule which completely encircles the globe. These attachments are known as check ligaments and if strong, may interfere with muscular function. The third cranial nerve supplies all the extraocular muscles except the lateral rectus which is innervated exclusively and solely by the sixth cranial nerve, and the superior oblique muscle which is innervated exclusively by the fourth cranial nerve. The extraocular muscles arise from associated paraxial mesoderm during the second month of gestation. It is most important that this anatomic development be complete and precise to allow implementation of the fine motor and sensory adjustments that are required throughout life to maintain proper ocular alignment. When this adjustment is interrupted, strabismus occurs.
Of great importance in reviewing strabismus are the possible associated factors. These include the resting position of the eye, muscle development, refraction, fusional status, the patient's neurologic status, family history and most importantly vision.
The resting position of the eye is dependent upon normal development of the bony orbits. Congenital anomalies such as craniofacial dysostosis may interrupt this development and causea mechanical malpositioning of the eye leading to a strabismus. Crouzon's disease is a classic example of such a presentation. These patients demonstrate an exorbitism manifested by shallow and laterally rotated orbits. A subsequent ocular misalignment is frequently found. If proper orbital development is present, the normal basic resting position is then enjoyed as a result of supernuclear impulses balancing the patient's normal desire for convergence and divergence. These normal resting positions are best seen in sleep states; the child showing an inward deviated position; the adult an outward deviated position. The normal child shows anatomically more closely spaced eyes with nearly parallel medial orbital walls; the adult more widely spaced eyes with orbital axes having rotated laterally.
Muscle development is most significant since the overwhelming majority of patients with eye muscle imbalances (strabismus) have completely normal eye muscle structure and function. It is possible, however, to identify patients with abnormal muscular development. These muscles are often atrophic, fibrotic, tendonous or restricted by unduly strong bands. In these instances the patient is not able to maintain normal alignment.
Refraction plays a significant role in strabismus disorders. Approximately 75% of normal newborns are farsighted within certain normal limits ranging from I to 3.5 diopters. Accommodative effort and convergence effort are neurologically linked together. A change in one automatically induces a similar change in the other. Patients who are excessively farsighted are required to exert greater focusing effort to overcome the farsightedness. This in turn induces greater convergence effort and causes the patient to misalign his visual axes inducing a strabismus. Nearsightedness reverses the situation as the child does not have the need to accommodate to see clearly. This leads to a reduced convergence effort and a tendency for the eyes to deviate in an opposite lateral direction.
The patient's associated neurologic status is of extreme importance and any doubts in this regard should lead to a consultation with a pediatric neurologist. Such conditions as seizure disorders (and the use of medications required to control these disorders), cerebral palsy and neuro-degenerative diseases may all lead to or be associated with eye muscle imbalances.
The patient's family history is most important. It is commonly held that approximately 50% of all children with strabismus have a positive family history. Therefore, it behooves the examiner to inquire about family lineage as well as the patient's siblings.
Vision is, of course, the most important associated finding. Eyes that do not see well commonly drift away from correct alignment. Amblyopia is rather commonly associated with strabismus, being present in as many as 40% to 50% of cases. Therefore it is valuable for the pediatric practitioner to evaluate vision early and frequently. Any discrepancy should be immediately referred for evaluation and thorough treatment. A halfhearted approach at uncovering and treating amblyopia leads not only to a patient who is partially sighted but severely complicates and hinders the success of the strabismus therapy. Additionally, patients with strabismus often show organic conditions associated with reduction in vision. Such diagnoses as cataracts, retinal detachments, retinoblastoma, glaucoma, choreoretinitis and high refractive errors may all be at fault.
The child who develops strabismus and has otherwise normal eyes embarks on a chain of events which, if undiagnosed and untreated, lead to permanent and serious visual loss. Initially, when the two eyes become misaligned, the patient sees two separate images. This visual confusion often leads to a child appearing awkward; being noticed by his parents to stumble; demonstrating past pointing when reaching for objects or exhibiting difficulty in stable and secure locomotion. In addition, he may show at this time a tendency to close one eye in an effort to alleviate the visual confusion. If the problem is not identified and arrested at this level, the patient will begin to suppress the image from one eye. A continuance along this line leads to the development of an amblyopia. By definition this is reduced vision in spite of the patient's best correction. This is "lazy eye": commonly confused by parents to be strabismus. Lazy eye only refers to amblyopia, or visual Joss, and does not describe the patient's ocular alignment. Amblyopia may occur quickly; the younger the patient, the quicker the occurrence and the more profound and permanent the visual loss. Infants may suppress their vision and develop a true amblyopia in as quickly as one week. Amblyopia is discussed in Dr. Stager's article in this issue.
TYPES OF STRABISMUS
FACTORS IN STRABISMUS DIAGNOSIS
Symptoms of strabismus are important in both their early diagnosis and classification. Patients with paralytic strabismus demonstrate an angle of deviation which varies in different fields of gaze because of failure of one of the extraocular muscles to contract properly. The deviation is always present and is in the direction exactly opposite of the muscle involved. On the other hand, the angle of the strabismus is always greatest in the field of action of the involved muscle because the paralytic muscle does not move well and that eye is unable to track consensually with its fellow eye. Double vision is always a symptom, but may not be present if the age of onset was such that the patient developed visual suppression or a compensatory head malposition. Double vision is identifiable by the red glass test. In this test a red plastic lens is held over the right eye (by convention) and the patient is asked to fixate on a white light held in the examiner's hand. The patient will notice two lights, one white and one red. As the examiner moves the white light through the cardinal fields of gaze, the patient's observance of the red and white light will demonstrate that the double vision is greatest in the field of action of the affected muscle. Additionally, when the patient fixes with the eye harboring the paralytic muscle, the angle of the deviation will increase. This phenomenon of primary and secondary deviation is the basis for the rule that secondary deviation is always greater than primary deviation. Abnormal head positioning is a compensatory maneuver on the part of the patient to attempt and possibly achieve fusional status. Until the cause is determined by the pediatric ophthalmologist, this compensatory head move should not be interfered with by parents or school personnel. Rotation of the head on the vertical axis indicates a weakness of the horizontal muscles while head tilting is indicative of the oblique muscle disorders. With time, paralytic and non-paralytic strabismus tend to blend. Again, this sign should impress the examiner of the need for early diagnosis so that correct and precise treatment may be engendered.
Non-paralytic strabismus is present when the angle of deviation is the same in all cardinal fields of gaze. It may vary with distance and near measurements. Thedeviation, however, remains the same regardless of which eye fixes. Hence, the phenomenon of primary versus secondary deviation does not apply. Motion is not reduced and the red glass test fails to reveal a variation in the measurements of the deviation. Because diplopia is usually absent, compensatory head turns and tilts are generally not present. The estimation of these strabismus disorders is easily achieved after visual acuity is determined.
The cover/ uncover test and the alternate cover test should be in the permanent armament of the practitioner as they will allow him to determine if strabismus is present. In the cover/ uncover test, the child is asked to fixate on a distance target at a measured 20 feet. Lesser distances are not considered correct. The fixation target should not be a bright light, but rather a target having accommodative value such as a figure or a Snellen Chart. The examiner covers the patient's right eye while he observes the left eye. If no movement is noted, the left eye is being used properly for fixation. The cover then is removed from the right eye and is placed over the left eye. The observer watches for any secondary shift of the right eye. If neither eye moves, both eyes were then fixing properly on the target and no strabismus is present. If either exposed eye shifts in order to regain proper alignment, then a misalignment was present.
The alternate cover test allows the examiner to uncover latent or intermittent deviations. The testing situation is similar to that described except the examiner alternately covers first the right, then the left eye, closely watching for any motion in the eye which is just being uncovered. If there is no such motion, the patient is Orthophorie and strabismus does not exist. As a general rule, an estimation can be made as to the degree of deviation by noting the position of a light reflex in each eye. A reflex which is not centered is abnormal. If the reflex is at the pupillary border, approximately 15° of deviation exists, lfoverthe middle portion of the iris, approximately 30° of deviation exists. If to the extreme edge of the limbus, greater than 50° of deviation are present, this patient should be seen quickly by an ophthalmologist trained to measure and treat such disorders.
Many clinical types of strabismus exist. The etiology of this broad -based disorder is complex. We have previously discussed visual, structural and physiologic influences which relate to strabismus. A classification of strabismus is important and is easily recorded by the examiner as Pseudostrabismus, latent strabismus or a true manifest strabismus. Basically, the examiner is interested in recording which way the eye turns and how often, the time of onset, any recordable etiologic factors, and any prior treatment for ocular and systemic diseases. The term phoria indicates a transient deviation, one in which there is a tendency for the eyes to be misaligned. This misalignment is held in check through the patient's normal fusion mechanism. The eyes often become misaligned as a result of fatigue, illness, emotional peaks, the use of certain medications having a somnolent side effect and other stressful causes. The term tropia describes a constant manifest deviation present under all visual conditions. The prefix eso defines a convergent strabismus, while exo indicates a divergent strabismus. Hyper indicates an eye which is elevated, while hypo indicates an eye that is depressed.
Pseudostrabismus is an apparent but not real deviation. These infants have been classified as having a broad, flat nasal bridge with prominent epicanthal skin fold. Because the nasal sclera is covered the iris seems to have shifted nasally giving the impression of anesotropia. This condition is present in up to 30% of normal newborns. It is not associated with visual loss and requires no treatment. Patients showing intermittent ocular deviations referred to as non-dysjunctive eye movements do truly deviate from the basic position of orthophoria. However, the deviation is erratic, not constant, present for only a few seconds at a time and is generally speaking, outgrown by the age of four months.
A latent strabismus represents the previously discussed phorias. These deviations are present only after fusional impulses have been interrupted and the normal ability of the patient to hold the eyes in check is removed. These conditions are very common and may be seen in up to 80% of normal individuals to a small degree. Larger phoric deviations may indeed produce symptoms of eye strain such as somnolence, blurred vision, diplopia and headaches. School-aged children may commonly manifest reading difficulties, difficulty in adjusting synchronization of focusing and convergence from distance to near such as is required from looking from a blackboard to a book and returning to the blackboard. The etiology of phorias is poorly understood as is the etiology of strabismus in general. They seemingly have a relation to the previously stated causes such as refractive errors, muscle development, resting position of the eyes, the fusional and neurologic status and of course to vision. Phorias require treatment only when symptoms are present. Eye exercises originally done as orthoptics have fallen into disfavor because of a failure to elicit effective and long-term results. The only exception to this is the patient with an exophoria demonstrating a small tendency to manifest an ocular misalignment away from the midline. These patients may be aided with convergent reinforcement exercises. These are easily demonstrated to the patient and the parents and should be done in a home setting. Eye exercises other than this, visual retraining programs and other such non-scientific endeavors have never been shown to be of value, and indeed, may harm the patient by masking his symptoms or preventing his being evaluated properly by a pediatric ophthalmologist. In rare instances eye muscle surgery may be required to correct a portion of a large and symptomatic phoria. The term tropia describes the true manifest, constant deviation. These deviations are present at all times and in all fields of gaze whether the patient is allowed to use their fusional mechanism or not. They may be congenital, a term used by the ophthalmologist to describe an early onset, generally less than six months of age. They may be acquired; most commonly seen in the 2 ½ to 3 ½ year age range. Or they may be intermittent. Phorias are also divided according to the direction of deviation.
Esotropia is the most common type of ocular misalignment. Congenital esotropia is associated with several findings. Besides the early age of onset, most patients show good visual acuity in each eye and the ability to alternately fixate with each eye voluntarily. They are, however, unable to use the two eyes in a coordinated fashion. If this condition is left untreated, the patient remains a functionally monocular individual throughout life. The angle of deviation of the eyes in congenital esotropia is usually large, commonly ranging to 50 diopters or greater. Cross fixation is present. This is the ability of the patient to alternately fix with one or the other eye into opposite fields of gaze without making any effort to rotate the two eyes consensually. It is important in this disorder to determine that the patient does not manifest other ocular disorders. Important among these are sixth nerve palsy, Duane's syndrome and Brown's syndrome. A true Duane's syndrome will show a narrowing of the palpebral fissure and possibly a retraction of the globe will be seen on adduction. Brown's syndrome will demonstrate an inability to elevate the eye in adduction. The patient with a true sixth nerve palsy will fail to demonstrate proper ocular rotations to horizontal doll's head rotation. Patients with congenital esotropia normally show proper refractions which are bilaterally equal and within the range of normal hyperopia. The treatment for congenital esotropia is always surgical. This treatment should not be withheld as current pediatric ophthalmic surgery techniques and excellent pediatric anesthetic techniques reduce the patient's risk from procedure or anesthetic to a fraction of l%. Failure to identify and surgically correct the congenital esotropia in the early age reduces the patient's chances for a successful and permanent correction. No longer is it viewed proper for the patient to be "older" for the surgery to be done and it is generally currently held that the proper time for corrective surgical intervention is approximately six months of age. Esotropias are also classified as accommodative or non-accommodative. Accommodative esotropias are commonly seen between two and four years of age and are associated with higher degrees of farsightedness ranging above two diopters. The greater the hyperopic error, the more likely the patient is to develop an accommodative esotropia. Once this breakdown occurs the patient is faced with a difficult situation. They will experience either blurred single binocular vision or a clear image brought on with accommodative effort, the latter engendering greater convergence and diplopia. The patient may show a consistent degree of deviation for both distance and near fixation in which case his convergence to accommodation ratio is normal or nearly so. They may show a progressively increasing angle of deviation as the object of regard is brought from distance to near fixation (a high convergence to accommodation ratio). Patients with accommodative esotropias are treated with spectacles to correct the excessive hyperopia. This relaxes the patient's needs to overfocus and hence his need to over-converge. Those patients with increased convergence to accommodation ratios, in addition, require greater spectacle correction at near (ie, bifocals). Both of these corrections are effective, allowing the patient to see clearly and function binocularly. In addition, they are usually temporary and outgrown as the patient ages.
Non-accommodative esotropias, of which congenital esotropias are a part, may come on at any age. The deviation is present at all times and is not associated with accommodation. Surgical correction is required. Accommodative and non-accommodative deviations may coexist; the patient requiring both glasses and surgery to correct.
Exotropias may be classified as intermittent or constant. Those which are intermittent show a strong hereditary tendency often being observed between the ages of one and four years. Initially, the deviation varies between tropia and phoria. Such elements as fatigue, illness, visual inattention and bright sunlight, in addition to those previously mentioned, tend to cause the patient an increase in tropia. History may reveal the patient experiencing diplopia as manifested by closure of one eye when exposed to bright sunlight or with prolonged visual effort. The deviation is often greater for distance gaze than near vision; the patient using his fusion and near point of convergence to help overcome deviation on near gaze. Refractive errors are usually small. However, myopia is associated with exophoric and tropic deviations commonly and should be fully corrected. Suppression of vision only occurs when the strabismus is worsening, the angle of deviation great or when the deviation becomes truly manifest. Treatment consists of correction of any associated amblyopia and underlying refractive errors. Intermittent exotropias commonly require surgical correction. If left untreated, in time they generally will decompensate to a constant manifest strabismus of more complicated presentation.
Constant exotropic deviations may be, as their esotropic counterparts, congenital or acquired. Congenital oxotropla is inherited and rare. Its treatment is surgical and the expected long-term result is excellent. Generally speaking, exotropic deviations which are constant do not produce diplopia and do not vary with the physical status of the patient, being present at all times. The deviant eye is commonly amblyopic and the angle of deviation greater at distance then at near, depending to a large degree upon the patient's convergence to accommodation ratio. Treatment of any consistent exotropia requires full correction of any myopia. Any residual deviation requires eye muscle surgery, the surgery being performed to allow the opportunity for binocular visual performance. At no time is the surgery considered solely cosmetic.
Vertical deviations as previously alluded to are less common, although often quite devastating to the young patient. It is physiologically proper to refer to the deviation in the non-fixing eye. Hence, if the eye deviates upward it is termed a hypertropia; downward, a hypotropia. The vertically acting muscles may include not only the recti, but indeed, the oblique muscles. Any prolonged and manifest deviation of the latter, often leads to a vertical misalignment. Inferior oblique overacting muscles commonly follow esotropias and are more common than superior oblique disorders. In the childhood population, the inferior oblique muscle is frequently encountered as improperly functioning (ie, overacting). These patients may demonstrate a greater deviation of exotropias in upward gaze or of estropias in downward gaze. In addition, a head tilt to the opposite shoulder is common.
Patterned deviation, such as A or V patterns, have been thought to be associated with oblique muscle disorders or possibly as isolated entities. The letters are chosen to describe the position of the eyes in upward, horizontal and downward positions. They are relatively uncommon phenomenon and do not directly relate to vision or treatment.
Treatment should initially concern itself with the diagnosis of any associated amblyopia. If this is present, it must be corrected fully and completely to the best possible level. It is most important to communicate to parents the physician's genuine interest and concern in solving this difficult and at times, long-term problem. Failure to correct an underlying amblyopia will endanger the overall successful treatment of the strabismus. Primary treatment should include the use of glasses where required and occlusion therapy to the preferred, or fixing, eye. Often both glasses and occlusion therapy are required.
Glasses are necessary to correct any abnormal refractive error. It is most important that the patient with strabismus see an equally clear image with each eye. Failure to do so will allow the patient preferential fixation and interfere with the strabismus correction. Glasses play a more direct role in patients with accommodative strabismus as mentioned previously. Glasses are not routinely prescribed for patients with strabismus disorders and should not be prescribed as a "trial." This latter, often done to mollify anxious patients, in actuality delays the patient from receiving proper and necessary treatment. Occlusion therapy is done exclusively for the correction of amblyopia. The parents should never be told that the eye will be patched in order to bring about proper alignment. This is never the aim of occlusive therapy and while it is most important to correct amblyopia prior to any correction of strabismus, this treatment is entirely separate from that which will be brought to bear on the strabismus disorder.
Medications have not proven to be effective and indeed, may show harmful complications. Such antiaccommodative measures as phospholide iodide have been shown to be associated with cataracts and severe iris cysts. Their use is at best only for short-term diagnostic purposes and offers no specific advantage over other nonaccommodative measures such as the use of glasses when required.
Orthoptics means straight eyes and as mentioned previously, is currently thought to have value in treating primarily the small angle intermittent exotropic patients. Visual motor retraining programs have never been shown to have any scientific basis or clinical value and are held as having no therapeutic value. Surgery when properly used is an effective tool in correcting the strabismus disorders not requiring glasses. Newer surgical techniques allow the patient to enter the hospital and, following surgery, leave the same day. More restrictive measures such as bedrest and bilateral eye patches are generally not required. Pediatric anesthesia and the more current pediatric anesthetic techniques allow for a safe, well-controlled, uneventful hospital stay. It is important to discuss these modalities of treatment with parents, to answer their questions in a straightforward fashion so that their fears and misconceptions may be laid to rest and the patient treated in an atmosphere of positive cooperation.
In summary, patients with strabismus should be viewed as having a serious ocular disorder requiring early diagnosis and evaluation and should be encouraged to have proper therapy so that the patient may be returned to a more normal ocular status at the earliest possi ble age. The ultimate goal of the pediatric ophthalmologist is always good vision and proper alignment. Neither alone is acceptable. It behooves the pediatric practitioner to actively diagnose strabismus disorders at the earliest possible age so that full and proper treatment may be brought to bear, thus allowing the patient to enjoy binocular vision function throughout his lifetime.
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3. Newell FW: Ophthalmology Principles and Practices, ed 4. St Louis, CV Mosby Co, 1979.
4. Parks MM: Ocular Motility and Strabismus. New York, Harper & Row Pubs Inc. 1975.
5. Burian HM: Exodeviations and their classifications, diagnosis and treatment. Am J Ophthalmol 1966; 62:1 161.
6. Coogan DG: Neurology of the Extraocular Muscles, ed 2. Springfield. Illinois. Charles C Thomas Inc. 1956.
7. Von Noorden GK: Classification of amblyopia. Am J Ophthalmol 1967: 63:238.
TYPES OF STRABISMUS
FACTORS IN STRABISMUS DIAGNOSIS