[15, 24]. Consequently, the IO exerts a tonic nasalward force on the IR pulley, and a tonic inferior force on the LR pulley [24]. In youth, these active muscular forces are balanced by the elastic sti ness of the pulley connective tis-

6sue suspensions,particularly by the elasticity of a ligament connecting the LR with the SR pulleys that is termed the LR–SR band [15, 25]. The suspensory tissues of the orbital pulleys become gradually attenuated during normal aging [15, 25], causing predictable inferior shifts in horizontal rectus pulley positions [26], and making the pulleys of order people more susceptible to the e ects of trauma and surgery.

Summary for the Clinician

■Pulley connective tissue degeneration in older people can cause horizontal or vertical strabismus.

■Involutional eyelid changes and blepharoptosis suggest that pulley tissues may also be degenerating.

6.5 “Divergence Paralysis” Esotropia

While the locations of the vertical rectus pulleys remain constant during the lifespan of a normal person, the horizontal rectus pulleys gradually “sag” inferiorly by 2–3 mm by the seventh decade of life [26]. This converts some of the horizontal force of the horizontal rectus EOMs to infraducting force, without any abducens neuropathy or deficiency of the magnitude of LR force generation. Abducting saccades maintain normal peak velocities [27].

When horizontal pulley sag occurs symmetrically, there is no e ect on horizontal binocular alignment, since the MR and LR muscles experience balanced force reductions [25]. The additional infraducting force contributed by the horizontal rectus EOMs is most likely to be the cause of the predictably reduced supraducting ability of older people [28].

More severe LR–SR band degeneration may permit the LR to shift farther inferiorly than does the MR pulley (Fig. 6.4). In this case, more of LR abducting force is converted to infraducting force than is the corresponding situation for MR adducting force. The imbalance leads to a convergent shift in alignment most evident during distance viewing when the visual axes of the eyes should be parallel, while there may be little or no esodeviation during near viewing where physiologic convergence is required. This situation has been described as “divergence paralysis esotropia,” a clinical entity in which there is esotropia predominantly or exclusively present during distance but not near viewing, and in which there is no evidence of LR paresis, e.g., abducting saccadic velocities and abduction range are normal [27]. When bilaterally symmetrical, the vertical e ect in the two eyes is matched, avoiding vertical strabismus. “Divergence paralysis esotropia” due to LR pulley sag typically occurs in older people with retracted upper eyelid creases and blepharoptosis due to dehiscence of the levator tendon from the tarsal plate [25]. Both the blepharoptosis and strabismus presumably result from orbital connective tissue degeneration in the absence of EOM neuropathy or myopathy. Patients typically retain excellent fusional convergence and binocular fusional potential. While divergence paralysis esotropia can be

Table 6.4. Alignment e ect of LR–SR band degeneration

very successfully treated by multiple conventional strabismus surgical approaches that counteract esodeviation (e.g., MR recession or LR resection), it is the author’s experience that the required surgical dosage must be about double that required for other forms of esotropia. Surgical repair of LR pulley sag is not typically required in divergence paralysis esotropia (Table 6.4).

6.5.1Vertical Strabismus Due to Sagging Eye Syndrome

Asymmetric stretching or catastrophic rupture of the LR–SR band may suddenly impart a marked infraducting action to the involved LR muscle, even creating restriction to passive supraduction [25] (Fig. 6.5). The clinical presentation may be acute onset of hypotropia with deficiency of supraduction that might be mistaken for SR paralysis or IR restriction in the absence of adequate orbital imaging. Orbital imaging secures the

Fig. 6.5 Coronal MRI of left orbit of older patient demonstrating marked inferior displacement of LR pulley in sagging eye syndrome associated with acute onset hypotropia. LR lateral rectus muscle; MR medial rectus muscle

diagnosis of sagging eye syndrome (Fig. 6.5). While it may sometimes be possible to surgically repair the ruptured or stretched LR–SR band to normalize LR pulley position, severe degeneration may render this ligament irreparable. In that event, posterior surgical ligature between the lateral margin of the SR muscle and the superior margin of the LR muscle may be required to normalize LR path [25].

6.5.2Postsurgical and Traumatic Pulley Heterotopy

Rectus pulley suspensions may be damaged by surgical dissections.Again, the LR pulley is most susceptible to this e ect of aggressive anterior dissection at strabismus, retinal, or orbital surgery. For instance, damage to the LR–SR band during endoscopic orbital decompression surgery may present as restrictive hypotropia in adduction.

6.5.3Axial High Myopia

Inferior displacement of the LR muscle is also a well-rec- ognized cause of strabismus in high myopes [29]. Known as “heavy eye” syndrome or myopic strabismus fixus, this syndrome is characterized by esotropia and hypotropia due to conversion of LR muscle action from abduction to infraduction [29, 30]. Patients with “heavy eye” syndrome have impaired abduction and supraduction due to degeneration of the LR–SR band, allowing inferior LR pulley displacement causing inferior shift in LR muscle path that may become so extreme as to approach that of the LR. Abducting LR force is converted into infraducting force, resulting in large-angle esotropia and hypotropia. Since axial length in this condition is typically 30 mm or more, strabismus associated with axial high myopia was formerly (but misleadingly) termed the “heavy eye syndrome” under the assumption that an enlarged globe would sink inferiorly in the orbit [31]. Clinical orbital imaging is of great value in diagnosis of this condition, since it confirms the diagnosis of LR displacement, and excludes alternative or coexisting conditions that may require di erent surgical treatment, or preclude treatment altogether. For example, with or without inferior displacement of the LR pulley, a severely staphyomatous globe may fill the bony orbit so completely that duction is limited [32], or the LR muscle may have su ered neuropathic paralysis and have become atrophic. If the cause of the esotropia is simply inferior displacement of the LR pulley due to LR–SR band degeneration, an e ective treatment may be identical to that used in the sagging eye