Torsional Strabismus

Definitions of Inferior Oblique Overaction, Superior Oblique Overaction, A Pattern, and V Pattern

“Oblique muscle overaction” and A- or V-pattern strabismus are defined based on clinical exam findings. Oblique overaction and pattern strabismus usually are intimately related in well-defined ways. It helps to clarify what these terms mean.

Every ophthalmology resident learns the function of the extraocular muscles based on their direction of “primary field of action” in the six cardinal positions of gaze. The medial rectus moves the eye into adduction when it contracts; the lateral rectus abducts the eye. In turn, the oblique muscles are said to have their greatest or most pure action when the eye is adducted, so the inferior oblique elevates the eye in adduction, whereas the superior oblique depresses the eye in adduction. Thus it seems natural that an excessive elevation of the eye in adduction is called inferior oblique overaction (IO OA), whereas an excessive depression of the eye in adduction is called superior oblique overaction (SO OA). But these labels are deceptive.

The term oblique overaction implies hypertonicity or excess neural stimulation; however, this appearance does not necessarily mean the oblique muscles are truly overacting. Instead, the term oblique overaction is used because the oblique muscles have their greatest vertical action when the eye is adducted, where the hypertropia is seen clinically.

A or V patterns can be defined by the measured difference in horizontal deviation in up gaze compared with down gaze, where the patient is fixing on an accommodative distance target and the vertical gaze angle is about 30° from primary:

• A pattern—at least 10 prism diopter (PD) lesser exodeviation or greater esodeviation in up gaze than down gaze ( Fig. 11.8.1 )

  

• V pattern—at least 15 PD greater exodeviation or lesser esodeviation in up gaze than down gaze ( Fig. 11.8.2 )

  

Precise head positioning when measuring deviation in side gaze or up or down gaze is difficult, especially in children. Thus the incomitant deviation in up and down gaze may vary inadvertently, depending on the variable direction of gaze. Because of this, many clinicians simply label A and V patterns as mild, moderate, or large.

A common system for grading “oblique overaction” is 0 (normal) to 4+ (extreme). In rare cases when the misalignment appears to exceed even 4+ as a result of aberrant orbital anatomy (some types of syndromic torsion), 5+ has been used as well. In turn, restriction of elevation (as can be seen in a patient with Brown syndrome) can vary from 1– to 4–. In addition, and in concert with the dysinnervation of Duane syndrome, the terms upshoot and downshoot reflect hyperelevation or hyperdepression in adduction not typically resulting from oblique muscle function.

The Five Elements Key to Understanding Strabismus

Common childhood strabismus occurs when fusion is lost or is overcome by the imbalance between agonists and antagonists (e.g., intermittent exotropia). Much childhood strabismus can best be understood by considering the five elements that determine the typically progressive misalignments that occur: the three axes of strabismus, the inherent bias between agonist and antagonist pairs, muscle-length adaptation, motor fusion, and the restraint put on the path of extraocular muscles by orbital tissues.

Strabismus occurs when fusion is absent, is lost, or is overcome by the imbalance between agonists and antagonists mechanically, with or without neurological origin. Regardless of the etiology, torsional misalignment may play a major role, often giving a clue to the etiology and directing the best intervention. In a patient with the potential for binocular fusion, addressing the torsional component of strabismus may be critical for maintaining fusion over time.

In addition, strabismus occurs in many forms, including types with distinctive genetic fingerprints, such as congenital cranial dysinnervation disorders like Duane syndrome, Moebius syndrome, congenital fibrosis of the extraocular muscles (CFEOM), and Brown syndrome. Other causes of misalignment include trauma; inflammation from Graves’ or orbital pseudotumor; and neurological conditions, including myasthenia gravis and cranial nerve palsies.

The Three Axes of Strabismus

To understand torsional strabismus, it is important to understand the three axes of ocular motion. Each globe has three rotational degrees of freedom, and thus the rotations of the eye fall into three orthogonal (perpendicular to each other) axes: horizontal, vertical, and torsional. These axes also have been described in analogy with the motions of an airplane, as yaw (horizontal), pitch (vertical), and roll (torsional), as emphasized by Dr. Michael Brodsky. Horizontal misalignment is seen as esotropia (ET) or exotropia (XT), whereas vertical misalignment is seen as hypertropia, with one eye being higher than the other in the primary position. However, torsional misalignment is invisible to the clinician when the eyes are in the primary position: detection of torsional misalignment depends on observing the relatively skewed ocular deviations in various gaze directions (e.g., hypertropia of the adducting eye in the presence of excyclotorsion) or on fundoscopy by noting the incyclo- or excyclorotated fundus appearance. Because the muscle insertions are torted along with the rest of the globe, the direction of motion of the eye in attempted versions induces the appearance of oblique overaction. Thus incyclotorsional misalignment is often attributed to SO OA, whereas excyclotorsion is often attributed to IO OA. These are so named because these vertical deviations in adduction correspond to the “primary fields of action” of the respective oblique muscles. How this occurs will be described in detail later in this chapter.

Example: Child With Accommodative Esotropia Who Develops a V Pattern

As an example of “primary overaction of the inferior obliques,” let us consider the common story of a 2-year-old child with a cycloplegic refractive error of +4.5 D OU, who presents with accommodative esotropia. The initial examination shows no oblique overaction, pattern strabismus, or fundus torsion, and when given a prescription for +4.5 D lenses, the spectacles fully correct his deviation. The child then is lost to follow-up, and over 2 years, he fails to wear the glasses. He presents again at age 4 with partially accommodative esotropia: when given glasses to correct his full cycloplegic refraction, his esotropia (ET) cc = 25 prism diopters (PD). He also has 3+ IO OU, a moderate V pattern, and approximately 15 degrees of excyclotorsion in both eyes ( Fig. 11.8.3 ). Because no apparent cause exists for the IO OA, such as cranial nerve IV palsy or craniosynostosis, his strabismus is called V-pattern esotropia with “primary inferior oblique overaction.” Surgery is performed: a recession of the medial rectus (MR) 5.0 mm OU and a recession of the IO 14 mm OU. At his 2-month postoperative exam, he is straight in all positions of gaze, with no IO OA, no V pattern, and no torsion on fundus exam.

Why did this patient develop a V pattern with apparent IO OA, and how did recession of the IO correct both conditions?

Summary of Upcoming Sections

Many causes occur for “oblique overactions” and pattern strabismus. In this chapter, we show how ocular torsion and torsional displacement (also called heterotopy ) of the muscles and orbital tissues are the keys to understanding many patients with oblique muscle dysfunctions and V- and A- pattern strabismus. We discuss clinical observations, the place of torsion within the three axes of strabismus, how torsion of the rectus pathways results in pattern strabismus, and the appearance of oblique overaction. We will discuss how lack of fusion can lead to torsional strabismus, as well as other causes of oblique dysfunction.

Age of Clinical Examination

Oblique overactions and A and V patterns rarely are observed clinically in infants with esotropia, perhaps because of their limited effort or cooperation. It is difficult to get a young child to follow an object into far side gaze or to precisely measure alignment in chin-up or chin-down posture. Many patients either develop or are found to have these signs of torsional strabismus when they are 3–6 years of age rather than in the first year of life, even when torsion is observed objectively on fundus exam.

Epidemiology

In a study by Wilson and Parks, IO OA was observed to develop in about 72% of patients with congenital esotropia, 34% of those with accommodative esotropia, and 32% of those with intermittent exotropia who were followed up for longer than 5 years. The incidence of IO OA in patients with congenital esotropia was not related to age at strabismus onset, time from onset to surgery for the esotropia, age at first surgery, or decompensation of horizontal alignment; it was, however, related to the number of surgeries necessary to align the eyes horizontally. The mean age of onset or detection was 3.6 years, with many demonstrating pattern deviation much before.

Eustis and Nussdorf studied infants undergoing bilateral medial rectus muscle recession for esotropia and found that of the eyes with excyclotorsion observed at surgery, all demonstrated IO OA at follow-up. Of those with no torsion at the time of surgery, only 9 of 21 demonstrated IO OA at follow-up.

In children with SO OA, there is a higher rate of concurrent neurological disease, up to 40% in one series.

How to Observe or Measure Torsion

Directly With Fundoscopy or Photography

Clinical fundoscopy can be used to estimate the degree of fundus torsion, based on a 1+ to 4+ scale, a protractor, or estimation of the angle, as is done for free-lens retinoscopy refractions. Torsion also can be documented by fundus photography in cooperative patients ( Fig. 11.8.4 ) or during surgery.