may be associated with the clinical findings of limitation of active duction in the EOM’s field of action due to pulley hindrance (discussed below), as well as mechanical restriction to the opposite direction of passive rotation using for-

6ceps. In the usual clinical setting of generalized orbital and eyelid edema, these clinical findings can be indistinguishable from those of EOM entrapment in an orbital fracture. It is therefore desirable to promptly obtain an adequate imaging study, such as a CT or MRI scan, that can identify any possible tissue entrapped in an orbital fracture. Expeditious, if not emergent, release of entrapped EOMs or pulley tissue should be performed within several days before scarring makes repositioning impossible [11].

Summary for the Clinician

■Orbital pulley disorders can cause strabismus.

■Strabismus due to pulley disorders can clinically mimic restrictive or paralytic strabismus.

6.3Congenital Pulley Heterotopy

The direction of ocular rotation imparted by any EOM is defined by the relative locations of its scleral insertion and pulley; EOM path direction posterior to the pulley is not directionally important [12–14]. Every EOM can produce horizontal, vertical, and torsional actions, in relative proportions depending on pulley and insertion locations. Thus, alterations in positions of the horizontal rectus pulleys can impart substantial vertical and torsional actions to the medial and lateral rectus (LR) EOMs, while alterations in positions of the vertical rectus pulleys can impart substantial horizontal actions to the vertical rectus EOMs (Table 6.3).

The MR and LR pulleys are directly suspended by fibroelastic connective tissues from anteriorly located entheses, or anchors, on the orbital bones [15]. The medial enthesis is at the posterior lacrimal crest, while the lateral enthesis is at Whitnall’s tubercle. The inferior (IR) and superior rectus (SR) pulleys are somewhat indirectly supported by, in both cases, the medial and lateral enthe-

ses. Malpositioning of the entheses, or malpositioning of the orbital bones to which the entheses join, can therefore cause significant alterations in rectus EOM pulling directions. More significant still, the pulling directions of the four horizontal rectus EOMs can be purely horizontal only if their respective pulleys all lie on a horizontal line exactly transverse to the mid-sagittal plane of the skull. Any other orientation of the horizontal rectus pulleys in the two orbits will impart vertically imbalanced actions to the binocularly yoked agonist pairs: the MR in one orbit and the LR in the opposite orbit. This e ect is not related to the activity of the oblique EOMs, and probably cannot be counteracted by them.

Symmetric heterotopy of the rectus pulley arrays in the orbits produces two clinical findings: imbalanced versions in oblique gaze directions (formerly but incorrectly attributed to oblique EOM dysfunction) and vertically incomitant horizontal strabismus [16, 17]. MRI has demonstrated the coronal plane locations of rectus EOM pulleys to be stereotypic in normal [17, 18] and most strabismic subjects [18]. The 95% confidence intervals of coronal plane pulley coordinates are less than ±1 mm [18]. A computer model of binocular alignment incorporates passive elastic pulleys [19] and is now available as the application Orbit. The expected e ect of coronal plane heterotopy (malpositioning) of pulleys can be computed using Orbit [20]. Many cases of incomitant cyclovertical strabismus are associated with heterotopy of one or more rectus EOM pulleys exceeding two standard deviations from normal. Patterns of incomitance in individual patients consistently match those predicted by Orbit simulation based on measured pulley locations, suggesting that pulley heterotopy caused the strabismus [21, 22].

When the LR pulley is located superiorly to the MR pulley in both orbits (Fig. 6.3a), the MR exerts an infraducting action in adduction relative to that of the LR, causing excessive infraduction in extreme adduction, since only the abducting eye can fixate a target in this position. This heterotopic pulley configuration is typically associated with a nasal placement of the SR pulley relative to the IR pulley, such that the array of the four rectus pulleys has been incyclo rotated about the orbital

center. In supraversion, the SR exerts an adducting action, while in infraversion, the IR exerts an abducting action. Binocular alignment is consequently more divergent in infraversion than in supraversion, constituting an A pattern strabismus.

When the LR pulley is located inferiorly to the MR pulley in both orbits (Fig. 6.3b), the MR exerts a supraducting action in adduction relative to that of the LR, causing excessive supraduction in extreme adduction, since only the abducting eye can fixate a target in this position. This heterotopic pulley configuration is typically associated with a temporal placement of the SR pulley relative to the IR pulley, such that the array of the four rectus pulleys has been excyclo rotated about the orbital center [16]. In supraversion, the SR exerts an abducting action, while in infraversion, the IR exerts an adducting action. Binocular alignment is consequently more convergent in infraversion than in supraversion, constituting a V pattern strabismus.

Bony deformity of the orbits, such as that associated with craniosynostosis, is a common cause of congenital pulley heterotopy. Such a deformity and pulley heterotopy

need not be bilaterally symmetrical; when asymmetrical, the resulting strabismus may be horizontally as well as vertically incomitant, resembling dysfunction of a single oblique EOM.

Osseous deformity with pulley heterotopy may be suspected when external facial features are asymmetrical, or when there is a significant inclination to one or both the palpebral apertures [12, 23]. The medial and lateral canthal tendons normally insert on the orbital bones near the medial and lateral entheses of the pulley system, respectively. A superior (“mongoloid”) inclination of the lateral palpebral canthus is associated with A pattern incomitance, while an inferior inclination of the lateral palpebral canthus is associated with V pattern incomitance.

6.4Acquired Pulley Heterotopy

The inferior oblique’s (IO’s) orbital layer inserts partly on the conjoined IO–IR pulleys, partly on the IO sheath temporally and partly on the LR pulley’s inferior aspect