Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology Current Thought and A Practical Guide_Wilson, Saunders, Trivedi_2008

vergence to accommodation ratio, along with unilateral lateral rectus muscle recession.

When exotropia and DHD coexist or when DHD is frequently manifest in either eye as a result of alternate fixation, bilateral lateral rectus muscle recession is recommended.

In conclusion, dissociated deviations include movements in the vertical, horizontal, and torsional axes. The DVD should not be used to refer to the entire complex. To avoid semantic confusion, DSC should be subdivided into its vertical (DVD), horizontal (DHD), and torsional (DTD) components when patient examination data are recorded. Latent nystagmus should be included as part of the complex.

The DVD and DHD require separate surgical strategies but often are treated surgically together. True hypertropia, hypotropia, esotropia, or exotropia can coexist with DSC. Treatment must be individualized for these patients, addressing both the dissociated and associated portions of the strabismus.

Acknowledgement. This work was supported in part by the Grady Lyman Fund of the MUSC Health Sciences Foundation and an unrestricted grant to MUSCSEI from Research to Prevent Blindness, Inc., New York, N.Y.

References

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Contents

13.2Causes of A or V pattern  . . . . . . . .   165

13.2.4Muscle Laxity orAnomaly  . . . . . . .   168

13.2.5 Primary A or V pattern  . . . . . . . .   169

13.3Clinical Examination Findings  . . . . .   169

13.3.1 Presentation  . . . . . . . . . . . .   169

13.3.2Examination  . . . . . . . . . . . .   169

13.3.3When Is an A or V Pattern Clinically

Important?  . . . . . . . . . . . . .   170

13.4Surgical Options  . . . . . . . . . . .   170

13.4.1Oblique Muscle Weakening  . . . . . .   170

13.4.2Effect of Oblique Muscle Weakening

13.4.4Inferior Oblique Muscle Weakening

Procedures  . . . . . . . . . . . . .   171

13.4.5Superior Oblique Muscle Weakening

Procedures  . . . . . . . . . . . . .   172

Core Messages

•A and V Patterns can be found in association with: (a) “overacting” oblique muscles; (b) orbital abnormality; (c) adduction deficiency, such as in largeangle exotropia or with a slipped muscle;

(d) muscle anomaly, such as lax or missing muscle(s); and (e) no apparent oblique dysfunction or orbital abnormality.

•Optimal treatment of the A/V pattern should account for the etiology causing it.

•In general, if there is oblique muscle dysfunction, surgical correction should be directed at the offending oblique muscles.

•If there is no oblique muscle dysfunction, surgery usually involves vertical shifts of horizontal recti.

13.1 Introduction

13.4.9Complications of Surgery for A and V

Patterns  . . . . . . . . . . . . . .   173

References  . . . . . . . . . . . . . . . . .   176

forms including A, V, X, Y, or lambda pattern; of these, A and V patterns are the most common. An A pattern describes a motility picture where the horizontal strabismus is smallest in upgaze and progressively increases as the eyes move into downgaze. A V pattern is the inverse – the horizontal deviation is smallest in downgaze and increases as the eyes elevate toward upgaze (Figs. 13.1, 13.2) [1–3, 12–15].

The X-pattern describes eyes where the horizontal deviation is smallest in primary and increases in both upgaze and downgaze. The Y-pattern is present when the horizontal deviation is smaller in primary and downgaze but increases in up gaze. Lambda pattern is the inverse of a Y pattern where the deviation remains similar in primary and upgaze and increases in downgaze.

Optimal correction of alphabet pattern strabismus is influenced by the etiology of the pattern. Several etiologies exist, sometimes in combination, to produce the observed vertical incomitance.

13.2 Causes of A or V pattern

13.2.1Oblique Muscle Dysfunction (Overaction)

The terms inferior oblique overaction (IOOA) or superior oblique overaction (SOOA) are ingrained in strabismus language, but they are more descriptive of the clinical appearance than the etiology. The terms overelevation in adduction or over depression in adduction are more accurately descriptive but also more unwieldy to use. We will use inferior oblique overaction or superior oblique overaction to describe this

clinical appearance when it is not due to other orbital or muscle factors (Fig. 13.3) [8].

Bilateral overelevation in adduction will cause a V pattern and can be found in association with either aprimarypositionesotropiaorexotropia.Conversely, over depression in adduction will cause an A pattern esotropia or exotropia.

Why does this apparent “overaction” occur? There is no evidence or even logic to suggest that these muscles are somehow “over innervated” or excessively strong. So why do we see the clinical picture commonly described as inferior or superior oblique overaction?

Production of a V pattern secondary to IOOA (and associated SO underaction) can be caused by innervation anomaly alone as in bilateral acquired superior oblique palsy resulting from neurotrauma. It is noteworthy that the degree of inferior oblique overaction in these clearly acquired cases is usually mild, if present at all. The underaction of the SOs and V pattern with esotropia in downgaze may be more apparent than the IOOA.

Congenital superior oblique palsies usually have more pronounced IOOA and these are frequently found in association with lax SO tendons [10]. This suggests the possibility that the apparent IOOA is really an “under checking” phenomenon caused by the SO tendon abnormality. Clearly, the ultimate SO tendon abnormality – absence – is associated with marked inferior oblique overaction.

It is also interesting to note that the patients with bilateral IOOA associated with the congenital esotropia complex frequently have marked bilateral SO tendon laxity (unpublished data). This, however, is not always the case, which raises the question about both the etiology and importance of this finding. Whatever the cause of this clinical association, it is the most common scenario for bilateral inferior oblique overaction and resultant V pattern.

The oblique dysfunction and V pattern can be apparent in early infancy concurrent with the initial onset of esotropia, but more frequently is first noted at age 3−4 years, or later. It can develop in the years following horizontal surgery for the esotropia or can become manifest before the horizontal misalignment is addressed.

13.2.2 Orbital Abnormality

The prototype anomalies causing usually a V pattern with overelevation in adduction are the cranial facial syndromes such as Crouzon syndrome (Fig. 13.4) [17]. Imaging of orbits in such patients frequently shows excyclorotation of the entire orbit including all muscles (Fig. 13.5). One can hypothesize that adduction of a medial rectus that is displaced superiorly will impart a vertical force vector on attempted adduction. Less dramatic muscle position abnormalities are found in patients with displaced muscle pulleys without a cranial facial syndrome (Fig.13.6) [18]. For instance, in patients with downward displacement of the lateral rectus muscles, it has been hypothesized by Clark et al. [18] that the downward vector of a

Fig. 13.6  An MR scan of a patient with pulley displacement.

Note vertical displacement of horizontal rectus muscles.

(Courtesy of J. Demer)

fixating abducting eye with an inferiorly displaced lateral rectus will cause a compensatory firing of the elevators of the same eye to maintain fixation which, in turn (by Hering’s law), causes a firing of the elevators of the opposite eye, giving an apparent overelevation in adduction.

13.2.3Adduction Deficiency with Large Exodeviation

found with slipped medial rectus muscles. In such cases, when the patient is asked to look up or down with the eye in maximal adduction, a pseudooveraction of the inferior oblique is seen on upgaze and of the superior oblique on downgaze (Fig. 13.7) This apparent overaction is due to the orbital connective tissue anatomy that allows an eye to move further up or down when it is not maximally adducted. In these cases, the pseudooveraction and X pattern decrease

when the exodeviation and adduction deficiency are corrected.

13.2.4 Muscle Laxity or Anomaly

Overelevation in adduction can be found in association with laxity of the superior oblique tendon, as

Fig. 13.8a–c  Typical appearance of marked inferior oblique overaction and superior oblique underaction in a patient with clinical picture of superior oblique palsy. a The MR shows similar patient with absent left superior oblique muscle. b Upgaze right; c downgaze right

is frequently found in cases of congenital superior oblique palsy. Although usually unilateral, this finding can be present bilaterally. The ultimate tendon abnormality, absent tendon/muscle, is always accompanied by overaction of the antagonist muscle. This is usually found in cases of absent superior oblique tendon or muscle (Fig. 13.8). Theoretically, the inverse of this − over depression in adduction − would

13.2.5 Primary A or V pattern

Some patients can exhibit an A or V pattern without evidence of oblique muscle overaction, large exodeviation, or craniofacial anomaly. Presumably, most of these patients have subtle abnormalities of the course or insertion of the rectus muscles or their associated pulleys.

13.3 Clinical Examination Findings

13.3.1 Presentation

Young children may present with abnormal head posture, e.g., chin up for an A-pattern esotropia, if the assumed posture promotes fusion. Older patients may also assume a compensatory head position but will frequently complain of asthenopia or even diplopia.

Misalignment evident only in upgaze is usually not functionally significant. Strabismus in the functionally important primary and reading (downgaze) positions will promote symptoms.

13.3.2 Examination

Diagnosis of A- or V-pattern strabismus requires prism cover measurements in primary gaze, upgaze, and downgaze. Fixation should be for distance when the measurements are made. Upand downgaze should be approximately 30º from primary gaze. Von Noorden and Olson have further specified that optimum measurements should be made at 35º in downgaze and 25º in upgaze [16].

Inexperienced examiners will sometimes misdiagnose increased esotropia deviation for near viewing obtained in downgaze as a high AC/A ratio, when in fact they have encountered a V pattern. This mistake

can be avoided by ensuring that near deviations are measured with the eyes in level (primary) gaze, not in the more natural downgaze reading position.

13.3.3When Is an A or V Pattern Clinically Important?

Any deviation that causes symptoms or a compensatory head posture is clinically relevant, but in general and by convention, V patterns greater than 15 prism diopters (PD) from up to downgaze and A patterns greater than 10 PD are considered significant. Again it must be stressed that the threshold for functional significance is far more likely to be crossed if the largest deviation is present in downgaze rather than upgaze.

13.4 Surgical Options

In general, surgery directed at collapse of the A or V pattern involves either weakening of oblique muscles or vertical transpositions of horizontal rectus muscles. The choice must be made with knowledge of the etiology of the pattern for each individual patient. A basic tenet of surgery for pattern strabismus is that surgery should be directed at the oblique muscles when significant oblique “overaction” is present and surgery aimed at displacement of the rectus muscles perpendicular to their direction of action should be used when oblique overaction is not the cause of the pattern. This treatment algorithm is basic, crucial, sometimes overlooked, and not new − it was suggested by Knapp nearly 50 years ago [5−7, 9, 16].

13.4.1 Oblique Muscle Weakening

13.4.1.1V-pattern Esotropia

In V-pattern esotropia with over elevation in adduction with no muscle or orbital anomaly, bilateral

weakening of the inferior obliques is highly effective for collapsing the pattern. Weakening can be accomplished with myectomy or recession at the surgeon’s preference. The amount of deviation decreased seems relative to the amount of inferior oblique overaction, but 20−25 PD can be expected.

A special circumstance arises when IOOA and V pattern is found in association with dissociated vertical deviation (DVD). This is usually in the setting of congenital esotropia (ET) complex. Anterior transposition of the IO is as effective for decreasing IOOA and collapsing the V pattern as myectomy or recession procedures, but has the added effect of decreasing the DVD. In patients with congenital ET, IOOA, and V pattern, our practice is to perform the anterior transposition even without manifest DVD as a “prophylactic” procedure for the presently inapparent DVD. Since the oblique surgery will have a negligible effect on primary position deviation, appropriate rectus muscle surgery to eliminate the primary position deviation should be carried out at the same time.

13.4.1.2V-pattern XT

Intermittent or constant XT with overelevation on adduction and a larger deviation in upgaze is common. Weakening of the IOs along with horizontal rectus surgery for the primary position deviation is very effective. These patients frequently maintain excellent binocular vision and stereoacuity despite their deviation. A dissociated strabismus complex component is rare. Fundus excyclotorsion is usually apparent with ophthalmoscopy. Weakening the inferior obliques in such cases does not cause problems with subjective torsion post-operatively.

13.4.1.3A-Pattern ET

A-pattern ET is relatively uncommon compared with V-pattern ET or XT. When accompanied by significant degrees of SOOA and a larger A pattern (>25 PD) bilateral SO weakening is effective at collapsing the A pattern. Several weakening strategies can work including free tenotomy or guarded tenotomy with insertion of a spacer. Concomitant hori-