Ординатура / Офтальмология / Английские материалы / Strabismus Surgery and Its Complications_Coats, Olitsky_2007

rectus muscle. A dual insertion is said to be present in almost 11% of inferior oblique muscles [25]. Failure to recognize a dual insertion can result in minimal and/or unpredictable effects on ocular alignment following surgery. The capsule of the inferior oblique muscle is relatively thick and fine attachments between its capsule and that of the lateral rectus muscle are generally present near the insertion of the inferior oblique muscle (>Fig. 1.21). A large orbital fat pad in the inferotemporal quadrant of the orbit can be easily disturbed during surgery on the inferior oblique muscle. This usually occurs during the isolation and dissection of the belly of the inferior oblique muscle, and can result in intraoperative bleeding and intrusion of orbital fat into the operative field, both of which can hinder the visualization necessary to complete surgery and can result in the development of a restrictive strabismus postoperatively.

The inferior oblique muscle is usually identified surgically as it courses across the inferotemporal quadrant, approximately 15 mm from the limbus. Unlike the other extraocular muscles, it is not surgically identified in its resting position on the globe, but rather is retracted inferiorly and identified as it courses within Tenon’s capsule (>Fig. 1.22). The muscle is isolated by passing a hook posterior to the belly of the muscle and retracting the muscle anteriorly. It is during this process, that the surgeon is most likely to encounter surrounding orbital fat and may encounter a vortex vein located in the inferotemporal quadrant near the lateral border of the inferior rectus muscle. These complications can be minimized by attention to careful surgical technique as reviewed in Chap. 11. We recommend that the surgeon directly visualizes the posterior border of the inferior oblique muscle and the nearby vortex vein prior to attempting to isolate the muscle on a hook.

The effective insertion of the inferior oblique muscle is not at the medial orbital wall where the anatomical insertion is lo-

cated, but instead is at the neurovascular bundle which enters the inferior oblique muscle near the temporal border of the inferior rectus muscle [20]. The neurovascular bundle is usually not visualized or disturbed during standard surgery on the inferior oblique muscle. An exception is denervation and extirpation of the inferior oblique muscle, which requires transection of the neurovascular bundle (Chap. 11). This bundle is generally best palpated with a hemostat, a maneuver that facilitates its visual identification.

References

1.Paysse EA, Khokhar A, McCreery KM, Morris MC, Coats DK (2002) Up-slanting palpebral fissures and oblique astigmatism associated with A-pattern strabismus and overdepression in adduction in spina bifida. J AAPOS 6:354–659

2.Coats DK, Paysse EA, Stager DR (2000) Surgical management of V-pattern strabismus and oblique dysfunction in craniofacial dysostosis. J AAPOS 4:338–342

3.Parks MM (1958) Isolated cyclovertical muscle palsy. AMA Arch Ophthalmol 60:1027–1035

4.Wilson ME, Hoxie J (1993) Facial asymmetry in superior oblique muscle palsy. J Pediatr Ophthalmol Strabismus 30:315–318

5.Paysee EA, Coats DK, Plager DA (1995) Facial asymmetry and tendon laxity in superior oblique palsy. J Pediatr Ophthalmol Strabismus 32:158–161

6.Velez FG, Clark RA, Demer JL (2000) Facial asymmetry in superior oblique muscle palsy and pulley heterotopy. J AAPOS 4:233–239

7.Helveston EM, Krach D, Plager DA, Ellis FD (1992) A new classification of superior oblique palsy based on congenital variations in the tendon. Ophthalmology 99:1609–1615

8.Bagolini B, Campos EC, Chiesi C (1982) Plagiocephaly causing superior oblique deficiency and ocular torticollis. A new clinical entity. Arch Ophthalmol 100:1093–1096

9.Apple DJ, Rabb MF (1985) Ocular pathology. Clinical applications and self-assessment, 3rd edn. Mosby, St. Louis, Mo.

10.Oh SY, Clark RA, Velez F, Rosenbaum AL, Demer JL (2002) Incomitant strabismus associated with instability of rectus pulleys. Invest Ophthalmol Vis Sci 43:2169–2178

11.Roth A, Muhlendyck H, De Gottrau P (2002) [The function of Tenon’s capsule revisited.] J Fr Ophtalmol 25:968–976

12.Clark RA, Rosenbaum AL, Demer JL (1999) Magnetic resonance imaging after surgical transposition defines the anteroposterior location of the rectus muscle pulleys. J AAPOS 3:9–14

13.Demer JL, Miller JM, Poukens V (1996) Surgical implications of the rectus extraocular muscle pulleys. J Pediatr Ophthalmol Strabismus 33:208–218

14.Demer JL, Miller JM, Poukens V, Vinters HV, Glasgow BJ (1995) Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Vis Sci 36:1125–1136

15.Demer JL, Oh SY, Poukens V (2000) Evidence for active control of rectus extraocular muscle pulleys. Invest Ophthalmol Vis Sci 41:1280–1290

16.Demer JL (2006) Current concepts of mechanical and neural factors in ocular motility. Curr Opin Neurol 19:4–13

Chapter 1

17.Sevel D (1986) The origins and insertions of the extraocular muscles: development, histologic features, and clinical significance. Trans Am Ophthalmol Soc 84:488–526

18.Apt L (1980) An anatomical reevaluation of rectus muscle insertions. Trans Am Ophthalmol Soc 78:365–375

19.Scobee RC (1948) Anatomic factors in the etiology of strabismus. Am J Ophthalmol 31:781

20.Stager DR, Weakley DR Jr., Stager D (1992) Anterior transposition of the inferior oblique. Anatomic assessment of the neurovascular bundle. Arch Ophthalmol 110:360–362

21.Helveston EM (1993) Surgical management of strabismus. An atlas of strabismus surgery, 4th edn. Mosby, St. Louis, Mo.

22.Helveston EM, Alcorn DM, Ellis FD (1988) Inferior oblique inclusion after lateral rectus surgery. Graefes Arch Clin Exp Ophthalmol 226:102–105

23.Wright KW (1991) Superior oblique silicone expander for Brown syndrome and superior oblique overaction. J Pediatr Ophthalmol Strabismus 28:101–107

24.Stager DR Jr., Wang X, Stager DR Sr., Beauchamp GR, Felius J (2004) Nasal myectomy of the inferior oblique muscles for recurrent elevation in adduction. J AAPOS 8:462–465

25.Deangelis DD, Kraft SP (2001) The double-bellied inferior oblique muscle: clinical correlates. J AAPOS 5:76–81

26.Duke-Elder S (1973) Ocular motility and strabismus. In: Duke-Elder S (ed) System of ophthalmology. Mosby, St. Louis, Mo., p 8

Kinematics is the science concerned with movements of the parts of the body. All movement potentially possible for the globe can be broken down into a combination of one or more of six elements. These include the three translatory movements and three rotary movements. Translatory movements of the globe may be left or right, up or down, and anterior or posterior. The center of the globe must shift during translatory movements. Rotary movements of the globe can occur around a vertical, a horizontal, and an anteroposterior axis. The center of the globe does not shift position during a pure rotary movement. While technically possible for a globe to undergo all of these movements, the translatory movements are small and can be ignored when discussing the basic physiology of ocular movements.

The primary and secondary actions of each of the extraocular muscles refer to major and minor actions of individual extraocular muscles when the muscles act on the globe from the primary position. There is a fixed muscle plane for each of the extraocular muscles that passes through the center of rotation of the globe and runs along the direction of the muscle from its origin (or functional origin) to its insertion into the sclera, when the muscles are operating on the globe while it is in the primary position. In other positions of gaze, these muscle planes change and movements of the globe that result from muscle contraction in other gaze positions are altered compared to movements from the primary position. For example, the superior oblique muscle functions primarily to produce incyclorotation from the primary position, but depression becomes its primary function when the globe is adducted. The functions of each of the extraocular muscles from the primary position can readily be determined by analysis of the muscle planes relative to the globe with reference to three major axes of rotation as described below.

The surgeon need only understand the relative positions of the effective origin and insertion in relation to these axes to understand the functions of each of the extraocular muscles acting on the globe in the primary position and in other gaze positions. In the discussion that follows, several assumptions are made. The eyes are considered to be fixating on a distant target and oriented in the primary position at the initiation of the movement, unless otherwise stated. Movements are assumed to occur around a fixed center of rotation of the globe and the muscles pairs in each eye are considered to have identical muscle planes relative to the major axes of rotation of the globes. It should be noted, however, that small translatory

movements of the globe do occur and the muscle planes of the muscle pairs are slightly askew. While these assumptions are not technically accurate, they are sufficient to serve as a basis of reference for discussion of ocular movements.

2.1Axes of Ocular Rotation and Listing’s Plane

From a practical standpoint, the center of rotation of the eye is stable, and small translatory movements of the globe can be ignored when considering the physiology of normal eye movements. While technically small changes in the center of rotation of the eye can and do occur, we will consider each eye to have only three axes of rotation, all passing through this “center of rotation” (>Fig. 2.1). The three major reference axes of the

Fig. 2.1. Reference axes of rotation of the globe and Listing’s plane. Note that the x-axis and the z-axis are in Listing’s plane and that the y-axis is perpendicular to Listing’s plane

eye were designated the y-axis, x-axis, and z-axis, respectively, by Fick in 1854. The y-axis is the anteroposterior axis and coincides with the line of fixation. The median plane of the globe is oriented along the y-axis. The two remaining axes of rotation are perpendicular to the y-axis. The horizontal axis is known as the x-axis and the vertical axis is known as the z-axis. The horizontal and vertical axes of rotation are assumed to lie in Listing’s plane. Listing’s plane is a fixed plane in the orbit that passes through the center of rotation of the globe containing these two major axes of rotation. Listing’s plane is considered to pass through the equator of the globe.

2.2 Duction Movements

Duction movements are excursions of an individual globe. Excursions around the vertical axis (z-axis) represent horizontal eye movements and include adduction, a nasal movement, and abduction, a temporal movement. Rotations around the horizontal axis (x-axis) represent vertical eye movements. Elevation or sursumduction represents an upward movement of the eye while depression or deorsumduction represents downward movement of the eye. Combinations of horizontal and vertical excursions produce movement of the globe into oblique positions of gaze. The axes of rotation of oblique movements also lie in the equatorial or Listing’s plane.

Ocular excursions also occur around the y-axis, though they are more difficult to appreciate clinically. Rotations of an eye around the y-axis are known as cycloductions. Rotation of the 12 o’clock meridian nasally represents incycloduction, ­while rotation of the 12 o’clock meridian temporally is referred to as excycloduction.

2.3 Version Movements

Version movements are simultaneous movements of the two eyes in the same direction. Version movements function to expand the field of view and to direct the fovea of each eye to the object of attention. Versions can be both voluntary, stimulated by the desire of the subject to redirect his/her attention to a new object of regard, or can be involuntary. Examples of involuntary versions include reflex movements of the eyes stimulated by reaction to auditory or visual stimuli and reflex movements such as those stimulated by the vestibular system.

2.4 Vergence Movements

Chapter 2

converge if the fovea of each eye is to maintain fixation on the object of interest. Vertical vergence and cyclovergence also occur, typically as involuntary movements to spontaneously correct vertical heterophorias and cyclophorias, respectively.

2.5 Basic Laws Governing Eye Movements

The eyes function together as a pair based on several fundamental laws of ocular motility. The key laws of ocular motility that are most important clinically include Sherrington’s law of reciprocal innervation and Herring’s law of equal innervation. Contraction of an eye muscle produces movement of the globe. The muscle that produced the movement is known as the ago­ nist. The antagonist produces a movement in the opposite direction. For example, the medial rectus muscle adducts the globe while the lateral rectus muscle abducts the globe. Thus the two muscles are antagonists relative to each other.

Muscles which move the globe in the same direction are know as synergists. For example, the superior rectus muscle and the inferior oblique muscle both elevate the globe and therefore are synergists with respect to elevation. They are antagonists, however, with respect to cyclorotations of the globe. The inferior oblique muscle produces excyclorotation, while the superior rectus muscle produces incyclorotation.

Muscles in each eye, which move the two eyes in the same direction, are also synergistic and are known as yoke muscles (>Fig. 2.2). The medial rectus muscle of the right eye and the lateral rectus of the left eye both function to move the eyes to the left (levoversion), and thus are yoke muscles. Muscle pairs in the two eyes are also yoked in this fashion. For example, the inferior oblique muscle and superior rectus muscle (both of which elevate the eyes) of one eye are yoked with these same elevators of the opposite eye.

Vergence movements of the eyes are simultaneous movements of the eyes in opposite directions. Vergence movements are utilized to maintain fixation of the object of regard on the fovea of each eye as the object changes distance from the eyes, such as visually tracking an object that is moving toward the eyes from a distant point. In the latter example, the two eyes must

Fig. 2.2. Yoke muscles primarily responsible for movement of the eye into the six diagnostic positions of gaze. (LIO Left inferior oblique, LIR left inferior rectus, LLR left lateral rectus, LMR left medial rectus, LSO left superior oblique, LSR left superior rectus, RIO right inferior oblique, RIR right inferior rectus, RLR right lateral rectus, RMR right medial rectus, RSR right superior rectus)

2.6Sherrington’s Law of Reciprocal Innervation

The concept of reciprocal innervation of pairs of extraocular muscles was first conceived by Descartes in the 1600s. The physiologic basis for reciprocal innervation was demonstrated by Sherrington in 1894 [1]. Sherrington’s law of reciprocal innervation simply states that when an impulse to contract is received by the agonist muscle in an eye, the antagonist muscle receives an inhibitory impulse. It is not clear if the inhibitory impulse is merely absence of innervation or active inhibition. Thus, normal movement of the globe always involves contraction of one or more extraocular muscles and relaxation of the respective antagonist(s).

2.7 Herring’s Law of Equal Innervation

Under normal circumstances, innervation to the extraocular muscles in the two eyes must occur in parallel. In other words, neural stimulation to perform eye movements is always integrated. Thus yoked muscles in the two eyes always receive equal innervation under normal circumstances. This basic law of ocular motility is known as Herrings’s law of equal innervation. The law applies to both voluntary and involuntary eye movements, which are always coordinated and the law applies to both versions (simultaneous eye movements in the same direction) and vergences (simultaneous eye movements in opposite directions). Herring’s law of equal innervation is hardwired in that a single fiber tract from the cortical centers is

directed to the cranial nerve nuclei involved in eye movements and impulses are distributed to appropriate muscle pairs in each eye by further hardwiring between these various cranial nerve nuclei. Herring’s law is the explanation for the fact that patients with paralytic or restrictive strabismus have a primary and a secondary deviation (Chap. 4).

2.8 Donders’ Law

Because the eye has 3 degrees of freedom for rotary movements around the major axes of rotation, in theory there are an infinite number of possible coordinates for designating the position of the globe in any eccentric position of gaze. Fortunately, movements around the y-axis (anteroposterior axis) are very restricted. Restriction of movement around the y-axis is important for spatial orientation and greatly simplifies both the study of ocular motility and the treatment of strabismus. Donders [2] determined that there was a constant and defined amount of torsion (x-axis and z-axis coordinates relative to the y-axis) present for each gaze position away from the primary position and this principle is known as Donders’ law.

2.9 Cardinal and Diagnostic Positions of Gaze

There are nine cardinal positions of the gaze for distance fixation (>Fig. 2.3). The primary position can be considered the position when the eyes are looking straight ahead and the body and the head are erect. Pure rotations around the z-axis and

Fig. 2.3. Cardinal positions of gaze. The central position is known as the primary position. The secondary positions are up, down, right, and left and occur around the x-axis and z-axis. The tertiary positions are

up right, up left, down right, and down left and occur around oblique axes of rotation in Listing’s plane

x-axis move the eye into positions designated as the secondary positions of gaze: adduction, abduction, elevation, and depression. The oblique positions represent tertiary positions of gaze and occur around oblique axes of rotation in Listing’s plane. These nine cardinal positions of gaze are used to analyze incomitant strabismus and play in important role in diagnosis and in surgical planning.

There are six diagnostic positions of gaze (>Fig. 2.2). The diagnostic positions of gaze are useful clinically to evaluate the action of individual extraocular muscles in each eye. In each of the diagnostic positions of gaze, function of a single extraocular muscle in each eye is emphasized. Thus, abnormalities of eye movements into a given diagnostic position of gaze are usually attributed, in large part, to abnormalities in the function of a single extraocular muscle.

2.10 Actions of Individual Muscles

For purposes of discussion, the individual actions of each extraocular muscle are reviewed in isolation. In reality, extraocular muscles never act in isolation. For example, contraction of the medial rectus muscle is primarily responsible for adduction, but both the superior rectus muscle and the inferior rectus muscle also make a small contribution to adduction [3]. Additionally, traditional descriptions of the individual actions of extraocular muscles are based on simple models of orbital anatomy and function, not taking into account the effect of the muscle pulleys on altering both the paths of the rectus muscles in the orbit and in altering muscle function (Chap. 1). Simply put, the actions of the extraocular muscles and their impact on changes in the position of the globe are much more complex than traditionally taught and reviewed here [4]. Nevertheless, for clinical purposes, this simplified description of the functions of the extraocular muscles is satisfactory for both understanding and managing the vast majority of patients with strabismus.

2.10.1 Horizontal Rectus Muscles

The actions of the horizontal rectus muscles are relatively simple if their function is evaluated only from the standpoint of eye movement with contraction of these muscles from the primary position. The muscle plane of the horizontal rectus muscles corresponds with horizontal plane of the eye, incorporating the x-axis and y-axis. When the eye is in the primary position, the horizontal rectus muscles produce isolated rotation around the z-axis. Thus the lateral rectus muscle is a pure abductor and the medial rectus muscle is a pure adductor (>Fig. 2.4).

However, the actions of the horizontal rectus muscles become more complex when the eyes are not rotating purely along the horizontal plane. For example, if the eye is both abducting and depressing simultaneously, the medial rectus muscle also produces some degree of depression. Alterations

Fig. 2.4. The muscle plane of the medial rectus muscle is perpendicular to its axis of rotation, resulting in pure adduction from the primary position

in the function of the horizontal rectus muscles with movement of the globe out of the horizontal plane of rotation may help to explain certain aspects of ocular movement disorders that occur in patients with strabismus, such as over elevation with adduction, which might occur with contraction of a medial rectus that has been displaced upward.

2.10.2 Vertical Rectus Muscles

The actions of the vertical rectus muscles are more complex than those of the horizontal rectus muscles. The actions are more complex because the vertical rectus muscle planes do not coincide with either of the major axes of rotation of the globe. The axis of rotation of the superior rectus muscle is obliquely oriented between the x-axis and the y-axis of rotation (>Fig. 2.5). Though it is not technically accurate, for clinical purposes, the muscle planes of the superior and inferior rectus muscles can be considered to coincide so that there actions fully compliment each other [5].

When the eye is in the primary position, the muscle plane of the superior and inferior rectus muscles forms an angle of approximately 23° with the y-axis (>Fig. 2.5). The insertions of the rectus muscles are located more lateral than their origins, so that the planes of the vertical rectus muscles diverge from each other in the two eyes. Careful study of Fig. 2.5 allows deduction of the primary and other actions of the vertical rectus muscle in moving an eye from the primary position.

The major action of the superior rectus muscle from the primary position is to produce elevation of the globe. Because of the relationship between the muscle plane of the superior rectus muscle and the cardinal axes of rotation of the globe, contraction of the superior rectus muscle also produces adduction and a small amount of incycloduction (>Table 2.1). If the globe is abducted approximately 23°, the muscle plane of the superior rectus muscle roughly coincides with the y-axis of the globe. At this point, the superior rectus muscle technically becomes a pure elevator, hence the vertical action of the superior rectus muscle is maximal when the globe is abducted. Alternatively, when the eye is abducted, incycloduction , the tertiary action of the superior rectus muscle, predominates. The superior rectus muscle can never produce pure incycloduction, however, because the eye cannot be adducted 67° from the primary position, the point at which the superior rectus muscle would theoretically produce almost pure incycloduction. Thus, while incycloduction is maximal when the eyes adducted, a significant degree of elevation function persists.

The function of the inferior rectus muscle is comparable to that just described for the superior rectus muscle, with obvious modifications. From the primary position, the inferior rectus muscle functions primarily as a depressor. It also produces excycloduction and a small amount of adduction (>Table 2.1). The depressor function of the inferior rectus muscle is maximal when the globe is in abduction and, for practical purposes, the inferior rectus muscle acts as a pure depressor when the globe is abducted approximately 23° from the primary position, where the muscle plane is parallel with the y-axis. Like the superior rectus muscle, the cycloduction function of the inferior rectus muscle is maximal when the globe is adducted (>Fig. 2.5).

2.10.3 The Oblique Muscles

For practical purposes, we will consider that the muscle planes of the superior oblique tendon and inferior oblique muscle coincide, a reasonable assumption based on available evidence [5]. Because the path of the superior oblique muscle/tendon is diverted acutely at the trochlea, the trochlea is the functional origin of the superior oblique muscle. The muscle plane and functions of the superior oblique muscle are thus all related to the superior oblique tendon. Like the vertical rectus muscles, the muscle planes of the oblique muscles do not coincide with any of the three major axes of rotation of the globe. Thus, like the vertical rectus muscles, the actions of the oblique muscles are complex [6].

When the eye is in the primary position, the axis of rotation of the superior oblique tendon is oriented obliquely between the y-axis and the x-axis. The muscle plane of the superior oblique tendon forms an angle of approximately 54° with the y-axis or median plane of the eye (>Fig. 2.6). The major action produced on a globe that is in the primary position during isolated contraction of the superior oblique muscle is incycloduction. Because of the relationship between the muscle plane of the superior oblique tendon and the major axes of rotation of the globe, contraction of the superior oblique muscle also produces depression and a small amount of abduction (>Table 2.1).

If the globe is adducted, the angle between the median plane (y-axis) of the eye and the muscle plane is reduced, enhancing the depressor function of the superior oblique muscle. If the eye could be adducted 54°, the superior oblique muscle would act as a pure depressor. With abduction of the eye, the angle between the muscle plane and the median plane of the globe increases, enhancing the cycloduction function of the superior oblique muscle. When the eye is abducted approximately 36°, the superior oblique muscle action is primarily one of pure incycloduction (>Fig. 2.6).

The functions of the inferior oblique muscle are comparable to those just described for the superior oblique muscle, with obvious modifications. From the primary position, the inferior oblique muscle functions primarily to produce excy-

Fig. 2.5. Relationship of the muscle plane of the vertical rectus muscles to the horizontal (x-axis) and vertical (z-axis) axes of rotation

cloduction. It also produces elevation and a small amount of abduction (>Table 2.1). The elevator functions of the inferior oblique muscle are enhanced in adduction while its function of excycloduction is enhanced in abduction. The inferior oblique muscle would function essentially as a pure elevator if the eye could be adducted approximately 51° and its function of excycloduction greatly predominates when the eye is abducted approximately 39°.

References

1.Sherrington CS (1894) Experimental note on two movements of the eyes. J Physiol (Lond) 17

2.Donders FC (1848) Beitrag zur lehre von den Bewegungen des menschlichen Auges. Holland Beitr Anat Physiol Wiss 1:384

3.Chamberlain WP Jr. (1954) Ocular motility in the horizontal plane: an experimental study of the primary and secondary horizontal rotators in the rhesus monkey. Trans Am Ophthalmol Soc 52:751–810

4.Krewson WE (1950) The action of the extraocular muscles: a method of vector-analysis with computations. Trans Am Ophthalmol Soc 48:443–486

5.Jampel RS (1970) The fundamental principle of the action of the oblique ocular muscles. Am J Ophthalmol 69:623–638

6.Jampel RS (1966) The action of the superior oblique muscle. An experimental study in the monkey. Arch Ophthalmol 75:535–544

Traditionally, the goal of strabismus treatment has been to realign the visual axes in order to eliminate diplopia, or to produce, maintain, or restore binocular vision. Additionally, surgery to improve an abnormal head posture, eliminate abnormal eye movements, or simply to restore the normal anatomical position of the eyes are well-accepted indications for surgery [1]. Facilitation of the development of binocular vision is classically demonstrated in the treatment of idiopathic infantile esotropia (congenital esotropia). Historically, Worth’s sensory concept of congenital esotropia proposed a disorder that resulted from a deficit in the “fusion center” within the central nervous system [2]. According to this theory, the goal of producing binocular vision was hopeless. He theorized that it was not possible to restore this congenitally absent neural function. Data supporting Worth’s theory were obtained at a time when strabismus surgery was rarely performed prior to the age of 2 years. Until the 1960s the results of surgical treatment of patients with congenital esotropia almost universally supported this pessimistic view.

3.1 Restoration of Binocular Vision

Chavasse challenged Worth’s theory. He suggested that normal binocular vision could be achieved through facilitation of conditioned reflexes that depend on early ocular alignment for proper development [3]. He considered congenital esotropia to be a strictly mechanical problem. He believed that most cases of congenital esotropia were potentially curable if ocular misalignment could be eliminated early in infancy. Because Chavasse lived before surgery was commonly performed to treat strabismus, support for this theory was limited until Ing and co-workers [4] began to report favorable binocular vision in some infants who underwent strabismus surgery between the ages of 6 months and 2 years. These encouraging results became the basis for the justification of early surgery in children with congenital esotropia.

It is now widely accepted that early surgical intervention can result in restoration of some level of binocular

in a large number of operated children. However, the binocular function obtained in patients with idiopathic tile esotropia is almost uniformly subnormal. Subnormal ocular vision as described by von Noorden [5] or the

fixation syndrome, as described by Parks [6], is considered the optimal result in children with idiopathic infantile esotropia by most pediatric ophthalmologists (>Table 3.1). Monofixation syndrome is associated with peripheral fusion, low-grade stereopsis, and vergence amplitudes capable of maintaining alignment within approximately 10 prism diopters, despite deficient stereopsis and a central suppression scotoma in one eye during binocular viewing. Patients who develop this level of binocular vision are more likely to maintain normal ocular alignment throughout their life [7]. Therefore, early surgery in an effort to achieve monofixation syndrome or subnormal binocular vision is desirable. Most strabismus surgeons recommend strabismus surgery to correct idiopathic infantile esotropia early in infancy, preferably after amblyopia, if present, has been adequately treated.

Older children with normal binocular vision, but who remain immature from the standpoint of cortical visual development, are still at risk for the development of abnormal adaptations in their binocular system if strabismus develops. Suppression and abnormal retinal correspondence are frequently seen in disorders such as intermittent exotropia. These adaptations may allow an intermittent deviation to become manifest more frequently or even to become constant. Once these adaptations develop, they may place a child at higher risk for the recurrence of strabismus later in life, even after successful surgical realignment. In patients with intermittent exotropia, for example, strabismus surgery is often recommended when the deviation increases in frequency and there is evidence that suppression has become more ingrained. The development of suppression may be inferred when the child no longer closes one eye when the deviation is present or when the deviation is manifest for a substantial amount of time or is constant. Surgery before these abnormal adaptations are well developed may be helpful in providing long-term stable ocular alignment. A young child with a deviation that is frequently manifest, but who has not yet developed suppression, may particularly benefit from early surgical intervention.

Absence of bifoveal fixation

Peripheral fusion

Central facultative suppression scotoma

3.2 Diplopia

In older children, and adults, who are visually mature, sensory adaptations to a new-onset ocular deviation typically do not occur. If the patient has a history of normal ocular alignment and develops a deviation in later life, the patient will experience diplopia when the eyes are not aligned. Patients with this history who do not report diplopia are generally ignoring the extra image and are not aware of its presence. The degree of visual disturbance experienced is dependent upon several factors. The frequency of the deviation is often the most important factor in determining the patient’s tolerance of symptoms. However, other factors may be of equal importance. Interestingly, deviations that are large are often less bothersome to the diplopic patient than smaller deviations. The explanation for this is the fact that it is often easier for the patient to detect the “real” object of regard when the second image is separated in space by a large distance, and it is simultaneously easier for the patient to ignore a distant second image. When the two images are close together or overlapping, as occurs with small angle strabismus, the abnormal visual experience is usually much more bothersome.

The position(s) of gaze where the double vision occurs is also an important consideration when evaluating diplopic patients and when discussing the potential benefit of treatment. Deviations that occur in primary position, down gaze, or in the reading position tend to be the most troublesome. Many patients can compensate for deviations that produce diplopia in side gaze and up gaze by turning their head, rather than moving their eyes, when viewing targets in these gaze positions. Such adaptations are less useful for near vision and for down gaze, because of both the frequency with which the eyes are used in these positions, and the need for optical correction (bifocal) for near vision in older patients. Diplopia that occurs only in up gaze tends to be the least bothersome and often well tolerated without treatment. This may not be the case if the diplopia develops within a few degrees of elevation of the eyes from the primary position. There are many exceptions to these general rules of thumb. Patients in some occupations, such as carpentry work that requires frequent use of up gaze, may be less tolerant of diplopia in up gaze, for example.

Patients who are experiencing debilitating diplopia can almost always benefit from treatment. If the deviation is small and relatively comitant, prism added to spectacle lenses may offer significant relief from symptoms. If the deviation is larger, prism will often produce unwanted image distortion and the lenses themselves are often too heavy and uncomfortable for reasonable wear. Though there are exceptions, patients rarely tolerate prism correction of a deviation greater than 8–10 prism diopters, in our experience.

Surgery will eliminate bothersome diplopia in most patients, though the addition of a small amount of prism patient’s spectacle lenses is required for some patients

small residual deviation producing symptomatic diplopia surgery. Patients who otherwise do not require spectacle rection or who prefer wearing contact lenses often request gery even for small deviations to avoid the need for prism

Chapter 3

may seem an unreasonable request on the surface. However, the need for prism glasses to achieve fusion and single vision is not comparable to the need for glasses to correct a refractive error. Even patients with significant uncorrected refractive errors are usually able to function relatively well in many activities of daily living without significant difficulty. In contrast, patients with diplopia are rarely able to comfortably participate in most activities without the need to constantly close one eye to eliminate diplopia. Thus, patients who find the use of prism glasses undesirable may be excellent surgical candidates.

3.3 Incomitant Strabismus

A significantly incomitant deviation with symptomatic diplopia is only occasionally successfully treated with prism and thus surgery is usually the primary treatment option. A surgical plan that takes into account the etiology of the incomitant strabismus can usually provide a significantly expanded field of single vision compared with prism in such patients. Superior oblique paresis is a typical example. Prism correction may provide single vision in primary position but most patients continue to experience bothersome diplopia in other important fields of gaze despite the use of prisms. A well considered surgical procedure can usually collapse the incomitant deviation produced by a superior oblique paresis, resulting in a larger field of single binocular vision.

3.4 Asthenopia

Even if the patient’s deviation remains latent most of the time, it may give rise to bothersome symptoms of asthenopia, in the absence of diplopia. Such deviations may be overlooked on cursory and sometimes even detailed initial examination. Symptoms may include eyestrain, reading difficulties, headaches, vague symptoms of fatigue or other symptoms with prolonged eye use. Asthenopia is usually not present with small horizontal phorias but may be experienced with medium to large horizontal phorias especially those that approach the relatively large horizontal vergence amplitudes. However, we occasionally examine patients with small angle exophorias who experience significant symptoms during reading. Small vertical phorias, on the other hand, often produce asthenopia due to the smaller vertical fusional vergence amplitudes possessed by most normal people (>Table 3.2). Surgery to eliminate a