Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology for Primary Care 3rd edition_Wright, Farzavandi_2008

Chapter 4

Common Forms

of Strabismus

Comitant Versus Incomitant Strabismus

Strabismus is a misalignment of the eyes and can be classified into 2 basic types: comitant and incomitant. Comitant strabismus means the deviation is the same in all fields of gaze, and even though the eyes are misaligned, they move normally without restriction or muscle paresis. Most childhood strabismus is a comitant strabismus associated with normal muscle function. Incomitant strabismus means there is limited eye movement so the size of the deviation is different in different fields of gaze. Limited eye movement may be caused by a restriction (periocular scarring or tight extraocular muscles) or a muscle paresis (third, fourth, or sixth nerve paresis). Comi tant strabismus usually is not associated with neurologic disease, whereas incomitant strabismus is often secondary to a neurologic disease or muscle pathology. Even so, comitant and incomitant strabismus can be an indica tion of ocular or systemic disease including neurologic disorders; thus early referral of strabismic patients is critical. The presence of strabismus after 2 to 3 months of age requires immediate referral.

Suppression and Diplopia

One might expect children with strabismus to have a difficult time dealing with confusing double vision (diplopia). Actually, if strabismus occurs early (before 4 to 6 years of age), the child will cortically turn off or suppress the image in the deviated eye. This defense mechanism, known as suppression, prevents bothersome double vision. Prolonged suppression of one eye, how ever, can lead to amblyopia of the suppressed eye (see Chapter 2). Acquired strabismus after 6 to 7 years of age, on the other hand, results in diplopia because the more mature visual system cannot suppress the double image. The clinical symptom of diplopia, therefore, is an important clue indicating that the strabismus is acquired. Acquired strabismus, especially incomitant, may indicate a neurologic process. Table 4 1 lists important characteristic red flags indicating that the strabismus may be secondary to a potentially

exodeviations are common in normal neonates, but esodeviations in new borns are rare. Persistent, large angle esotropia that is present after 2 months of age is probably pathologic and usually does not resolve spontaneously.

No consistent inheritance pattern for infantile esotropia has been estab lished, but it does tend to run in families. Some pedigrees appear to be auto somal recessive and others autosomal dominant; however, there is no family history of strabismus in many cases. The variability of inheritance patterns suggests heterogeneity for the phenotype of infantile esotropia. Thus, it is not surprising that the pathogenesis of infantile esotropia is probably mul tifactorial. In some cases, farsightedness (hypermetropia) is causative (see Accommodative Esotropia—common on page 53) and spectacle correction is required. In other cases, infantile esotropia is associated with developmen tal delay, cerebral palsy, or other diseases including Down syndrome.

Patients may show strong fixation preference for one eye, which is an indication of amblyopia, or may alternate fixation. Amblyopia occurs in approximately 50% of children with infantile esotropia. There is often some limitation of abduction in voluntary version testing; however, doll’s head maneuver reveals normal abduction and normal lateral rectus function.

Associated motor anomalies frequently related to infantile esotropia include vertical strabismus (inferior oblique overaction in 70% and dis sociated vertical deviation in 75%). Nystagmus is unusual in children with congenital esotropia, but latent nystagmus is seen in approximately 50%

of cases. Latent nystagmus manifests when one eye is covered. It causes vision to appear poor when doing standard vision testing (testing with one eye covered). Children with latent nystagmus obtain best visual acuity by testing with both eyes open (binocular vision) to avoid inducing latent nystagmus. Interestingly, these associated motor anomalies present late

at age 1 or 2 years, often several months after the esotropia has been surgi cally corrected.

Differential Diagnosis

Differential diagnoses of infantile esotropia include Duane syndrome (discussed later in this chapter), congenital fibrosis of extraocular muscles, congenital sixth nerve palsy, Möbius syndrome associated with sixth nerve paresis, and infantile myasthenia gravis. These disorders are associated with limited abduction and therefore can be differentiated from infantile esotropia where the ductions are full. Esotropia in an infant also may be an

important sign of vision loss. Disorders such as congenital cataracts and reti noblastoma often first present as infantile esotropia.

Treatment of Infantile Esotropia

In most cases, the treatment of infantile esotropia is surgical, usually a reces sion of the medial rectus muscles of both eyes. In patients with farsighted ness (hypermetropia of >+3.00), spectacles should be prescribed first, as spectacles alone may correct the deviation. If amblyopia is present, it should be treated with patching of the dominant eye prior to surgery. This is impor tant because after surgery, parents often feel the problem is solved and may not return for follow up visits. Patching does not correct eye misalignment but improves the vision of the nondominant eye.

Timing of Surgery for Infantile Esotropia and Prognosis

The standard approach has been to operate between 6 months and 2 years of age. This is based primarily on a study by Ing that showed peripheral fusion can be achieved if the eyes are aligned before 2 years of age. However, peripheral fusion is subnormal binocular vision, and approximately half of the children with congenital esotropia will require multiple surgeries. Poor binocular fusion after surgery is the rule because even brief periods of stra bismus during the early period of visual development result in permanent loss of binocularity. Crawford and von Noorden showed that even 3 weeks of prism induced esotropia in infant monkeys resulted in irreversible loss of binocular cortical cells. This author has shown that excellent fusion with high grade stereopsis and good alignment can be obtained when surgery

is performed prior to 6 months of age (Wright 1994). Surgery should be considered as early as 3 months of age if the following criteria are met: large angle esotropia (40 prism diopters or more), constant or increasing deviation documented by 2 visits 1 month apart, and the infant is a good anesthesia risk. Spontaneous resolution of the esotropia can occur; however, it is rare if the previously stated criteria are met (Wright 2002).

Pseudoesotropia

Pseudoesotropia is a common condition that also needs to be distinguished from infantile esotropia. With pseudoesotropia, the infant has a wide nasal bridge and prominent epicanthal folds, giving the appearance of esotropia; however, the eyes are aligned (orthotropic) (Figure 4 2). It is important to

Figure 4 2.

Three-year-old with pseudoesotropia. Patient was referred with a question of right esotropia.

document proper eye alignment using the Hirschberg corneal light reflex test. It can be difficult to convince parents that the eyes are truly straight, so it may be helpful to show the parents that the light reflex is well centered in each eye. Follow up is important in patients with pseudoesotropia, as a small percentage will develop a true esotropia.

Accommodative Esotropia—common

•Associated with farsightedness.

•Acquired between 3 months and 5 years of age.

•Treatment is to prescribe spectacles; however, many will require spectacles

AND eye muscle surgery.

•Relatively good prognosis for stereo acuity and binocular fusion with early surgery.

Children who are farsighted (hypermetropic) must increase focusing

effort (accommodate) to see clearly. Because accommodation is linked to convergence, increased accommodation will result in increased convergence of the eyes. The increased accommodation and convergence results in eso tropia, thus the term accommodative esotropia. Hypermetropic spectacle correction is useful in these cases to decrease the need to accommodate,

thereby reducing convergence and correcting the esotropia (Figure 4 3). The presence of a combined mechanism of accommodative esotropia and a basic esotropia is termed partially accommodative esotropia. Patients with this diagnosis improve with hypermetropic spectacle correction, but still require surgery for the residual esotropia (Figure 4 4).

Clinical Features

Accommodative esotropia is acquired and can present any time between infancy (infantile accommodative esotropia) to late childhood, but most often presents between 12 months to 5 years of age. Initially, the deviation is small and intermittent. The esotropia is seen mostly at near fixation or when the child is tired, and may be manifested by the child squinting or closing

Figure 4 3.

An 8-year-old child with accommodative esotropia. A, Note the large-angle esotropia associated with accommodation for farsightedness. Patient is squinting, as she is forcibly accommodating to see clearly. Increased accommodation results in increased convergence. B, Patient is given hypermetropic spectacle correction that focuses the retinal image without increased accommodation and convergence. With spectacle correction, accommodation and convergence diminish, thereby allowing the eyes to straighten.

Figure 4 4.

A 3-year-old with partially accommodative esotropia. Patient is wearing full hypermetropic correction of +3.00 diopter sphere in both eyes; however, a residual esotropia persists.

one eye. Over time, sometimes after only a few weeks, the deviation may increase to become constant and amblyopia may develop.

Differential Diagnosis

The differential diagnosis of acquired esotropia includes any neurologic cause of a sixth nerve palsy including intracranial tumor, hydrocephalus, mastoiditis, post viral sixth nerve palsy, Arnold Chiari malformation, and myasthenia gravis.

Treatment of Accommodative Esotropia

Immediate referral is important in patients with acquired esotropia to provide early treatment, establish binocular vision, and rule out ocular or neurologic disease. The treatment of accommodative esotropia is based on prescribing full hypermetropic correction as soon as possible via spectacles or contact lenses. Patients who are corrected to proper alignment with glasses for distance viewing but still cross at near can be corrected at near by prescribing bifocal glasses. If spectacles do not correct the esotropia, surgery is needed in addition to spectacles. In most cases, these children require hypermetropic spectacles postoperatively to maintain good alignment.

Another less common form of treatment is to prescribe miotic drops such as Phospholine Iodide (echothiophate iodide) to both eyes. Miot ics block cholinesterase, enabling acetylcholine to last longer. This reduces

the accommodative effort needed to focus and clear the retinal image. Less accommodative effort results in less accommodative convergence, therefore

decreasing the esotropia. Miotics can occasionally be successful in treating patients with relatively small angle accommodative esotropia, but hyper metropic spectacle correction is the treatment of choice. In addition, miot ics have significant side effects including iris pupillary cysts, lens opacities, retinal detachment, and angle closure glaucoma. Topical Phospholine Iodide also has a systemic effect, lowering cholinesterase activity in the blood for several weeks. Patients on miotics who are undergoing general anesthesia should avoid succinylcholine or be off miotics for at least 4 to 6 weeks before having succinylcholine anesthesia. Miotics such as Phospholine Iodide prolong the effect of succinylcholine, which may result in prolonged respira tory paralysis postoperatively. Because of the side effects and relatively poor results, miotics are rarely used.

Prognosis for Binocular Fusion

Unlike children with congenital esotropia, patients with acquired accom modative esotropia have had straight eyes during early visual development; thus, they retain relatively good fusion potential. The earlier the eyes are straightened, the better the chances for recovering fusion. Establishing bin ocular is an important reason to immediately refer patients with acquired esotropia for treatment. This author has personal experience with infantile accommodative esotropia because one of his 5 children developed a small, variable angle esotropia at 2½ months of age with a refractive error of +5.00 diopter sphere OU. Early prescription of glasses at 3 months of age and early surgery at 6 months of age resulted in excellent eye alignment and high grade stereopsis, both of which are still present after 18 years.

Sensory Esotropia—uncommon

Loss of vision may cause an eye to drift. Sensory esotropia is an esodevia tion caused by unilateral blindness. The general teaching has been that if the visual loss occurs prior to 2 years of age, patients develop esotropia. This is only a general rule because there are many exceptions, and the presence of an esotropia is not a good marker for the onset of blindness. The treat ment of sensory esotropia is to perform strabismus surgery on the blind eye, avoiding surgery on the eye with good vision. Indication for surgery is cos metic appearance.

Intermittent Exotropia–common

•Most commonly occurring between ages 2 to 8 years.

•Intermittent strabismus: the exotropia occurs when the child is tired or daydreaming.

•Patient may squint one eye.

•Good stereopsis and binocular fusion are present when the eyes are aligned.

•Treatment is usually strabismus surgery if the deviation is poorly controlled.

A phoria is a tendency for the eyes to drift apart, but alignment is

maintained by binocular fusion. A tropia is a manifest deviation of the eyes (eyes not aligned) without binocular fusion. Small exophorias (the eye tends to drift out but binocular fusion controls the eye alignment) are common in the general population and the small deviation is easily con trolled. Large exophorias, however, may be difficult to control. Intermittent exotropia is an exodeviation that is controlled part of the time by fusional convergence, but intermittently becomes manifest (Figure 4 5). This is, by

far, the most common type of exodeviation. The pathogenesis of intermittent exotropia is unknown.

Clinical Manifestations of Intermittent Exotropia

Intermittent exodeviation usually occurs between 2 and 8 years of age, but may present any time between infancy and adulthood. Initially, an exotropia may only be seen when the patient is fatigued or ill. Covering an eye will manifest the exotropia (Chapter 3, Figure 3 8). Thus, this is a form of stra bismus best detected by the cover test rather than the corneal light reflex test. Symptoms include blurred vision, asthenopia (vague visual discom fort such as eyestrain or brow ache), visual fatigue, and photophobia with squinting. The photophobia and squinting is thought to be a mechanism for eliminating diplopia or visual confusion. The natural history of intermittent exotropia is variable. Approximately 70% will show an increasing frequency of the exotropia and progressive loss of fusion, 20% will stay the same, and a very small percentage will improve over time.

During the exophoric phase, patients have bifoveal fusion with excellent stereo acuity. When exotropia (manifest strabismus) is present, most patients demonstrate suppression. Occasionally, patients with late onset intermittent