|INCOMITANT STRABISMUS UPDATE
|Year : 2015 | Volume
| Issue : 3 | Page : 312-319
An approach to some aspects of strabismus from ocular and orbital trauma
Anthony David Neil Murray
Department of Surgery, Division of Ophthalmology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa
|Date of Web Publication||1-Jul-2015|
Anthony David Neil Murray
Division of Ophthalmology, Faculty of Health Sciences, University of Cape, Town Private Bag X3, 7935 Observatory, Cape Town
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Strabismus caused by ocular or orbital trauma can be the result of localized acute soft tissue swelling or may follow orbital fractures, partial or complete loss of extraocular muscle (EOM) and/or cranial nerve function, or damage to surrounding tissues causing mechanical restriction. The strabismus is frequently incomitant and can be difficult, if not impossible to completely correct. The resulting diplopia can affect the individual's ability to function at work, in sports and in common tasks of daily living like driving. The preoperative evaluation should include an assessment of the degree of limitation, muscle function and the condition of the surrounding tissue. In most cases, high resolution computed tomography and/or surface coil dynamic magnetic resonance imaging are required to determine the extent and nature of suspected bony or EOM injury, as well as muscle contractility. If the scan reveals an intact but paretic muscle or only minor muscle injury, surgical intervention is based on the degree of muscle recovery 6 months after the initial insult. If a rectus muscle has been lacerated, and the proximal stump is functional, retrieval should be attempted, either by a direct conjunctival approach if located anteriorly, or by an anterior medial orbitotomy if located deep in the orbit. If a damaged muscle cannot be found, recovered or repaired at any time, then muscle transposition should be considered. If multiple muscles are damaged or scar tissue is excessive, a tether procedure may be indicated. This paper will present an approach to some aspects of strabismus in this setting.
Keywords: Extraocular Muscle Retrieval, Field of Binocular Single Vision, Incomitance, Mechanical Restriction, Ocular Trauma, Orbital Trauma, Strabismus, Transposition
|How to cite this article:|
Murray AD. An approach to some aspects of strabismus from ocular and orbital trauma. Middle East Afr J Ophthalmol 2015;22:312-9
|How to cite this URL:|
Murray AD. An approach to some aspects of strabismus from ocular and orbital trauma. Middle East Afr J Ophthalmol [serial online] 2015 [cited 2019 Sep 19];22:312-9. Available from: http://www.meajo.org/text.asp?2015/22/3/312/159732
| Introduction|| |
Multiple varied or combined etiologies may be responsible for strabismus from ocular, orbital or extraocular muscle (EOM) trauma, and each requires its own individualized and sometimes complex treatment plan. Localized acute changes such as edema hemorrhage and soft tissue swelling may cause diplopia and gaze disturbance. Orbital fractures with muscle entrapment, cranial nerve palsy, disinsertion of an EOM from the globe or direct damage to the EOM itself and/or surrounding tissues cause more long-term sequelae and often require intervention by the orbital surgeon or a strabismologist. Loss of vision after ocular injury may lead to secondary strabismus. The patient's signs and symptoms can vary widely from the quite dramatic to the very subtle, and present the examiner with several unique challenges. Proper diagnosis of the type and severity of injury, and the differentiation between mechanical causes and neurologic involvement is critical in determining the timing and optimal surgical approach for each patient.
Evidence of possible globe perforation or ocular or orbital retained foreign body generally requires prompt surgical exploration and appropriate repair.
While trauma to the orbit may be isolated, it may be associated with other injuries to the head, neck or body, and is frequently associated with events that impart significant high-velocity forces to the body such as motor vehicle accidents or gunshot wounds. Although severe vision-threatening orbital trauma may result from these events, it is widely understood that life-threatening injuries take precedence over all others. Although the repair of a penetrating ocular injury is a top priority, in most cases, ocular or orbital surgery will not occur until the patient's life-threatening injuries are stabilized. This paper will present an approach to some aspects of strabismus from ocular/orbital trauma.
| Evaluation|| |
Diagnosis of strabismus from trauma, including those forms due to a ruptured or lacerated muscle, is based on the history and clinical features in association with the computed tomography (CT) and/or magnetic resonance imaging (MRI) scan results as well as the findings at surgery. Furthermore, a careful history, careful observation of the patient and a general eye examination are necessary to identify preexisting conditions along with the presence of signs associated with orbital or head trauma. Some aspects of the strabismus examination need emphasis.
The sensory exam is critical in deciding which motor tests can and should be used to investigate the strabismus. Sensory deficits resulting from the trauma (such as traumatic cataracts, visual field defects, traumatic optic neuropathy or a central loss of fusion) or preexisting conditions (such as amblyopia, suppression or anomalous correspondence from a childhood onset strabismus) may confound the results of standard motor testing. 
When performing cover tests on patients with traumatic strabismus, each eye in turn must be occluded long enough to allow the patient to take up fixation. These patients may have decreased vision or field defects, and may have difficulty locating the fixation target during alternate cover testing. One should measure to reversal to be certain that the entire deviation has been uncovered. A severely limited eye may not be able to move into the primary position to take up fixation, and it is easy to underestimate traumatic strabismus.  In some patients measurement using Krimsky light reflex testing may be necessary. Both the primary and secondary deviations are important. In many situations, the primary deviation is the target surgical angle. However, in patients with incomitant strabismus that fix with the involved eye, and who will not switch fixation to the contralateral eye (usually because of poorer vision) the target surgical angle should be the secondary deviation.  Both the Lancaster red-green and the Hess screen charts are helpful additional ways of assessing the primary and secondary deviations.
Measurements of ocular torsion in the primary position and in up-gaze and down-gaze are helpful in identifying and quantifying oblique muscle weakness, some types of supranuclear strabismus, and muscle restriction from entrapment. Torsion is measured subjectively using double Maddox rods and objectively by viewing the fundus with indirect ophthalmoscopy. The synoptophore is the only available method to evaluate fusion potential in the presence of horizontal, vertical and torsional deviations, and to determine if the patient has central disruption of fusion.
Ocular versions and ductions are observed. Ocular ductions are typically graded on a scale of − 4 to + 4. If the patient is unable to reach the midline, the ocular duction is recorded as − 5.
Saccadic velocity testing may be helpful in both paretic and restrictive strabismus. Widely spread fixation targets are presented on either side of the midline, and the patient is asked to shift the fixation from one target to another. The examiner observes the latency, facility, amplitude and velocity of fast eye movements. Restricted eyes will be able to make fast saccades up to the limit of their restriction. Paretic eyes will move with subnormal velocity ("floating saccades") and increased latency.
Treating strabismus secondary to restriction is challenging because both the muscle and the surrounding tissues are involved. Since the cause in most cases is multifactorial, the treatment options are often complex and unpredictable. The preoperative evaluation should include an assessment of the degree of restriction, muscle function and the condition of the surrounding tissue. 
Forced duction testing
The most important information needed to plan the surgical intervention is the amount of restriction, easily assessed by performing a forced duction test (FDT). This should preferably be done in the office. In preparing the patient for the procedure, the ophthalmologist should clearly communicate the goals and expectations of the test, and talk the patient through it. , "Vocal" anesthesia is crucial, and when properly performed the test is short and relatively painless. A topical anesthetic is applied, first as an eye drop and then using a cotton-tipped applicator soaked with topical anesthetic, held against the conjunctiva (where the eye will be grasped), for about 2 min. Thereafter the injection of about half a milliliter of local anesthetic to the same site adds significantly to patient comfort and their ability to tolerate the procedure. The patient is instructed to look in the direction of the muscle suspected of having limited rotation. The crucial question to answer is "can the forceps rotate the globe further than the patient can, using maximal innervation in that gaze field?" Holding the lids apart with one hand, the examiner uses toothed forceps for globe fixation. The examiner should first demonstrate to the patient that they can't feel the forceps touching the anesthetized eye. This puts the patient at ease. The examiner should then grasp the conjunctiva as close to the limbus as possible. This area corresponds to that where Tenon's capsule and conjunctiva are fused in one layer, which limits stretching the conjunctiva and gives the examiner a firmer grasp in rotating the globe.  The globe should be held on the same side as the gaze limitation (if abduction is limited the conjunctiva is grasped temporally-this usually helps avoid a corneal abrasion, should the forceps slip). It is important not to push the eye into the orbit, which relaxes the rectus muscles and may give a false negative result. If the globe cannot be passively rotated further than the patient can, a restriction is present. Entrapped muscles will show restriction while nerve or muscle injury will typically show unrestricted movement. However, in the early stages of muscle injury edema and hemorrhage will also restrict movement. It is important to determine the amount of limitation in both directions for each of the involved muscles. For example, in a patient with restricted elevation, both elevation and depression must be evaluated. In such situations, there can be limitations in upgaze due to entrapment and in downgaze due to a reverse leash effect. Identifying all of the positions in which limitation exists, is important for a successful surgical strategy. ,
This type of injury can occur after orbital blow-out fractures, which are well described and beyond the scope of this presentation.
Forced duction testing in the office increases diagnostic accuracy and helps in choosing a surgical strategy. It is also particularly useful in predicting how a patient is likely to respond to adjustable sutures. However, there will be patients who cannot co-operate fully during the test, despite ideal circumstances. Stimulating the vagal response may cause syncopal attacks. In these situations, the test may be done in the operating room at the commencement of surgery. The exaggerated traction test, which tests tightness (or restriction) of the superior, and inferior oblique muscles must be done under anesthesia. 
Force generation testing
Because active co-operation is required the force generation test cannot be done with the patient under anesthesia or heavy sedation. After the conjunctiva is anesthetized, the patient is instructed to voluntarily look maximally in the field of action of the suspected paretic muscle. The surgeon then grasps the eye at the limbus opposite the muscle producing the duction effort and pulls against it to evaluate the force or more appropriately the resistance encountered.  This allows one to detect even subtle residual amounts of force in the affected muscle.
| Treatment Strategies|| |
Retrieval of an extraocular muscle
After the trauma, the decision to perform a retrieval of an EOM should be based on a systematic surgical plan. In the past the value of preoperative diagnostic orbital imaging by CT or MRI was thought to be generally limited, and that a disinserted muscle usually remains in its normal location in the orbit at all points posterior to the equator. ,, However in most cases state of the art high-resolution CT and/or surface coil MRI have today proved essential in determining the extent and nature of muscle and surrounding tissue injury. CT with 1-3 mm slice thickness is particularly advantageous in evaluating the bony orbit. Before proceeding with an MRI, retained ferromagnetic (metallic) foreign bodies must be excluded. Muscles that are believed to be damaged during endoscopic sinus surgery (ESS), or from ocular trauma should undergo dynamic (multipositional) MRI scanning. In some instances, this may also be indicated when EOM damage is suspected following blepharoplasty or strabismus, retinal, glaucoma or pterygium surgery. Dynamic scanning refers to viewing the EOMs during different maintained gaze positions. ,,,,, It demonstrates that during contraction the cross-sectional area of the muscle increases and the plane of maximum cross-sectional area moves posteriorly in the orbit and vice versa.  By demonstrating the entire length and contractility of EOMs, as well as providing detailed information about their anatomical location relative to other ocular structures, dynamic MRI provides unique information unavailable from the clinical examination alone. ,,, It can thus enable precise diagnosis and help devise the most suitable treatment plan. The surgeon can then select an individualized approach based on specific EOM involvement, surrounding tissue damage and restriction, as well as patient visual needs.  If a muscle has been lacerated in the posterior part of the orbit, dynamic MRI will demonstrate whether the proximal stump is functional, and retrieval should be attempted.
A direct conjunctival approach may be feasible if the muscle is located sufficiently anteriorly or an anterior medial orbitotomy approach along the adjacent orbital wall may be required if the muscle is located deep in the orbit. , If the proximal stump is nonfunctional and the muscles nerve supply has been damaged, retrieval is not justified, and an alternate procedure should be considered. Optimal therapy generally involves an initial attempt at targeted exploration with attempted recovery and/or repair of the damaged, transected, or lost EOM.  This may be performed early, immediately following the injury. However, because initial tissue disruption and hemorrhage may compromise both imaging studies and surgical visualization of normal tissue planes and landmarks, re-imaging and exploration may have to be considered only after edema and hemorrhage have settled. , If the swelling is significant, and if there is no contraindication, the patient should be treated with systemic corticosteroids, which rapidly reduce it.
After trauma and especially penetrating trauma of the orbit, although the EOM may be lacerated, the check ligaments and intermuscular membrane are usually not extensively damaged and hold the detached EOM in close to normal position, even though it may have disappeared from view. ,
Direct anterior conjunctival approach
This type of surgery is complex and requires expert knowledge of the relevant anatomy and experience with the surgical techniques involved, generally gained only by previously mentored surgery of similar cases. Recovery of lost or damaged EOM is best attempted by the most experienced strabismus surgeon available. 
In order to prevent orbital fat adherence and scarring, every effort should be made to isolate and avoid damage to the fascial planes, or entering the orbital fat. One needs to know where to look for the EOM or its fragments and how to identify and confirm its recovery. For example, when trying to locate the medial rectus muscle (MRM), the direction of the exploration should be parallel to the medial wall of the orbit and not along the curve of the globe. , Widespread exploration and traction on orbital tissues can result in further posterior migration of the transected EOM.
Adequate illumination, good exposure and appropriate magnification are essential. Ideally, a zoom operating microscope, with a lowest power of 4 times (which ensures a wide enough surgical field) should be used. Two experienced assistants should be present to assist with the operation. The microscope should be fitted with an attachment for each assistant who is then able to view the same operative field as the surgeon, thereby ensuring gentle, sustained good exposure throughout the procedure. , If the surgeon chooses to use an operating loupe it is much more difficult for the assistants to obtain an adequate view of the operative site.
Malleable retractors are used to expose the appropriate tissue planes. They cause much less tissue damage then that induced by toothed forceps or sharp instruments, which should be avoided. A limbal approach is usually best for surgery after trauma as it allows maximal exposure with only gentle tissue traction.
Until it is on a hook, and to avoid inadvertently transecting residual connections, never dissect (even bluntly) toward a presumed damaged EOM. Instead, bluntly dissect into the adjacent quadrants through undamaged tissue, away from the presumed muscle location, until bare sclera is present posteriorly in the subtenon's space.
Using a Steven's muscle hook inserted through these quadrant incisions, an attempt is then made to isolate any possible new or residual insertion of transected EOM as well as to identify any scar tissue inserting at the original muscle insertion site. 
Always attempt to determine if the EOM is attached to the globe. If attached, it is the most posterior dense tissue band adherent to the sclera that provides a firm, passive posterior force toward the orbital apex, not the weaker stretchy tissue with anteriorly directed traction force that is associated with a pseudotendon or other adherent fibrous scar band.  Only after an appropriate firm potential muscle structure is hooked, should that presumed EOM be isolated by blunt and sharp dissection to expose its insertion site, and then cleaned posteriorly to document its integrity, size, location, and path.
If the EOM has retracted behind Tenon's capsule within the muscle sheath, and no EOM insertion or other adherence can be found attached to the sclera, one should find the EOM capsule tunnel (muscle pulley) passing through Tenon's tissue by tracing the intermuscular membrane from the adjacent two rectus EOM toward the presumed lost EOM. , If the sleeve is found it should be held open by the two assistants, and the surgeon should search within it. , If muscle tissue is located it can be further isolated from the surrounding Tenon's capsule and intermuscular septum by blunt and sharp dissection. Fine locking forceps are useful to maintain the grasp on the recovered muscle tissue, which tends to retract further into the orbit when cleaned. Once isolated and cleaned the muscle tissue can be secured with a double-armed 6-0 polyglactin suture. A generous full thickness central bite is placed several millimeter posterior to the lacerated edge of the muscle and a knot tied at that location. Thereafter the suture is threaded toward the lacerated end of the muscle and locked at each edge, prior to reattachment to the sclera. If the suture bites are only placed near the irregular lacerated end of the stump, the suture may easily pull out. Since the new force of the recovered muscle may be increased (from traumatic resection, contracture or surrounding scar tissue) or weakened (from palsy or partial loss of muscle tissue) an adjustable suture may be useful when possible. , Alternatively a temporary bow knot can be used intra-operatively, and adjusted to orthotropia in the primary position, using the FDT and spring back test as guides.  If the surgery is delayed and the antagonist has become contractured, it may need to be recessed, also on an adjustable suture. After prolonged surgery, a megadose (8 mg intravenously) of dexamethasone may reduce postoperative edema and nausea significantly. Provided there are no contraindications, its use should be considered. 
If at attempted early repair the transected muscle cannot be found quickly, or if visualization is poor, it is frequently better to gently close and wait for a later attempt at surgical recovery.
Muscles that are transected posteriorly are the most difficult to retrieve and are often impossible to retrieve from a standard anterior approach. ,,,, Recovery may be possible by one of the several alternate approaches.
Medial wall approach (subperiosteal)
Awad et al.  reported successful use of a subperiosteal medial orbitotomy approach through a transcaruncular incision in the management of one patient. Lenart et al.  retrieved 2/2 lost MRMs by performing a subperiosteal medial orbitotomy through a transcutaneous modified Lynch incision.
Orbital wall approach (internal periosteal surface)
Underdahl et al. , subsequently used an orbital wall approach (that required the added expertise of an orbital surgeon) through a fornix-based transconjunctival incision, dissecting along the periosteal surface (and not in the subperiosteal space) toward the orbital apex. Because the muscle belly is situated immediately adjacent to the periosteum in the posterior orbit, it is located easily. Once the cut anterior edge of the muscle was identified, it was freed of adjacent fat and scar tissue and a nonabsorbable double-armed suture was placed through the muscle. A fine mosquito snap with the tips closed was then passed from the conjunctival incision through the medial orbital fat, into the peripheral surgical space, and the sutures grasped with the snap. The snap was withdrawn, bringing the sutures (attached to the muscle stump) into the peribulbar space The muscle stump was then advanced as far as possible anteriorly, without creating significant restriction to ocular rotation in the opposite field of gaze, and sutured to the sclera. 7/7 (100%) rectus muscles (five medial, one lateral, one inferior) were successfully recovered. 5/7 (71%) had previously undergone a failed attempted retrieval from a standard transconjunctival approach. ,
While the assistance of an orbital surgeon is required, the orbital wall approach along the periosteal surface offers better exposure of the muscle compared to subperiosteal retrieval, avoids the visible scar along the medial orbital rim created by a modified Lynch incision, can be used for any transected rectus muscle (not just the medial) and has become the author's preferred technique. ,
Pineles et al.  recently reported on the function of transected or avulsed rectus muscles, that they had recovered using the anterior orbitotomy approach in 11 patients. There was a significant improvement in duction (≥2 U) in the field of action of the effected muscle (P = 0.002) and 6/11 (55%) patients achieved single binocular vision in the primary position. Of interest, several of the patients underwent muscle retrieval months after the initial injury and had a successful recovery of single binocular vision.
When compared to lack of diplopia only in the primary position, or only an improvement in ductions, the field of binocular single vision (BSV) (diplopia free field) is regarded as a better indication of the patient's ability to meet their own particular visual requirements. , Unfortunately, the measurement of fields of BSV is not routinely tested in the author's clinics, and they do not have any data to analyze any changes in this parameter associated with rectus muscle retrieval.  Nevertheless they report their results as being similar to historical series in which muscles were not retrieved, and transpositions were performed. , In the largest study of traumatic muscle disinsertion 17/25 (68%) patients were satisfactorily aligned and had a functional field of BSV.  The muscles were retrieved via a standard anterior approach in 12/17 (71%) patients. 5/17 (29%) patients required transposition procedures, which resulted in a functional field of BSV in all five. 8/25 (32%) had an unsatisfactory result, related to adjacent rectus muscle involvement and/or extensive scar tissue. 1/8 (13%) patients had a transposition procedure. The medial orbitotomy approach, which was first reported in one patient in 1997,  was not used on any patient in that series, which included patients up to December that year. 
Pineles et al.  suggest that given the availability of an orbital surgeon to assist with the anterior orbitotomy approach to retrieve rectus muscles, it is reasonable to attempt muscle recovery for the possibility of a larger field of BSV due to active muscle contractility. Retrieval would also reduce the risk of anterior segment ischemia after transposition surgery, which would affect the anterior ciliary vessels of two additional rectus muscles (in addition to the transected muscle). Their recommendation is to attempt muscle retrieval if there is contractility apparent on dynamic MRI scan, and if the proximal rectus muscle stump is longer than 20 mm. ,
Transnasal endoscopic retrieval
Transnasal endoscopic retrieval of the proximal part of a disinserted MRM is feasible. ,, However, it requires a combined ophthalmologic and otolaryngologic surgical approach and is more difficult than the orbital wall approach along the periosteal surface, which has replaced it. 
Transposition of extraocular muscle
While technically easier and more predictable than EOM recovery surgery in most cases, and able to grossly center involved eyes, EOM transposition procedures are regarded by some as being inferior to direct muscle recovery/repair because (at best), they can establish fusion within a relatively small area, resulting in a limited field of BSV. , Only by successful recovery and/or repair of traumatized, yet functional EOM and successful prevention or release of restrictive scarring can optimal restoration of normal ocular motility with elimination of diplopia and a maximal field of BSV occur.
It is rarely advisable to perform a transposition procedure at the time of acute injury and presumed EOM loss.
However, if a transected EOM cannot be found, recovered or repaired at any time, then the creation of alternative ducting forces can be accomplished through transposition of either rectus or oblique muscle insertions to new locations. ,,,,,,, Compared to other transposition procedures full-tendon width transposition with augmentation sutures provides maximal transposition force and can restore a useful field of single binocular vision of about 60% of normal (average 71° compared to normal average of 110°). , If the vertical rectus muscles need to be transposed toward the medial rectus insertion, each must also be resected 5 mm to reduce the created slack and enhance the effect.  However most patients with strabismus following transection of a rectus muscle will have lost that muscles anterior ciliary blood supply. Full-tendon width transposition of two additional rectus muscles creates a real risk of anterior segment ischemia, and in this setting a partial rectus muscle augmented transposition or a vessel sparing technique should be considered. ,,,,,,
Nishida et al. , recently reported their technique of transposition of the vertical rectus muscle bellies, without splitting the muscles or disinserting the distal ends of the muscle from their insertions, which spares the anterior ciliary arteries. In a small series of 9 patients with limited abduction after a sixth nerve palsy, alignment improved (up to 60 Δ when combined with a recession of the MRM) and the field of BSV increased significantly (from 30°-60°). This promising procedure, which is simple to perform, and only requires a suture from muscle to sclera needs evaluation in a larger series of patients with longer follow-up. Its role in vertical transposition warrants investigation.
For an abduction limitation transposition of only the superior rectus (SR) muscle to the spiral of Tillaux just above the lateral rectus (LR) muscle, combined with an adjustable MRM recession, avoids inferior rectus (IR) muscle surgery and thereby spares the two IR muscle anterior ciliary arteries. ,, Long-term follow-up of a larger series of patients, as well as pre and postoperative assessment of their field of BSV, would give a better idea of the indications for the procedure.
Anterior transposition of the inferior oblique muscle for loss of IR muscle function is well established. ,, Transposition of the superior oblique tendon to the nasal edge of the SR has a role when medial, and vertical rectus muscle function is lost. 
Procedures to address incomitance
Although satisfactory central or near central alignment with fusion can be established by muscle recovery surgery or transposition surgery, in some patients significant incomitance with diplopia in important gaze positions persists.
The field of BSV is a particularly useful means of assessing the effect of diplopia on the patients work and recreation related visual requirements.  In order to restore a larger more useful field of BSV the incomitance can frequently be addressed by additional medical or surgical procedures such as partial prism therapy, posterior fixation sutures (fadenoperation), , Alan Scott's adjustable posterior fixation recess-resect procedure, ,,, or pulley surgery. ,
If the recovery is not possible, and if multiple EOM involvement precludes transposition, various tether procedures may be considered. ,,,,,
Just prior to any surgical intervention, the FDT should be repeated intraoperatively, and the results compared with those obtained in the office and with the clinically observed limitation of ocular rotations. During surgery, after the cause of restriction has been identified, and, for example, fibrous tissue excised, or a contracted muscle detached from its insertion, the FDT should be repeated. This confirms the identified cause and the surgical efficacy of releasing the restriction.  Any additional mechanical limitation resulting from symblepharon or other scar tissue must be surgically released. This may require conjunctival Z-plasty, a bare scleral approach with a ring conformer, or a conjunctival, mucous membrane or posterior lamellar graft as well as fornix deepening sutures. 
Specific approach to strabismus after sinus surgery
During the last 25 years, the introduction of ESS has dramatically improved the treatment of sinus disorders. However, a variety of orbital complications have occurred including optic nerve damage and strabismus. ,,,, ESS risks damage to the EOM, specifically when the procedure extends into the middle or posterior ethmoid sinus.  The bony wall between the ethmoidal sinus and the orbit is normally very thin and is often eroded by chronic sinusitis. Because of its proximity the MRM is at risk of endoscopic trauma. The superior oblique tendon, which is only a few millimeters above the MRM, may also be damaged. Injuries range from mild contusion to almost total destruction of the EOM and may cause permanent strabismus with troublesome diplopia. ,,,,
While forced duction and force generation testing demonstrate the extent of muscle paresis and the presence or absence of mechanical restriction, in these cases dynamic MRI is particularly helpful to confirm the presence and degree of actual muscle destruction, to identify both ends of the transected or traumatized muscle and to determine if the remaining proximal muscle segment is still functional. 
If dynamic MRI reveals an intact, but paretic MRM, or if the MRM injury is minor, botulinum toxin injection of the ipsilateral LR may prevent secondary contracture. Definitive surgical intervention is then based on the degree of MRM recovery, 6 months or longer after the initial insult. Immediate repair of an MRM laceration or transection is indicated.  If a portion of the MRM was destroyed, but a large functioning proximal segment of the MRM remains, it may be recovered via a medial orbitotomy incision and attached to the globe.  If required a bridge of temporalis fascia between the MRM and the globe may be used.  Recession of the LR on an adjustable suture may also be necessary. If dynamic MRI reveals a large segment of MRM destruction and the proximal stump is noncontractile or shorter than 20 mm, appropriate vessel sparing vertical rectus muscle transpositions to the MRM may be done earlier because spontaneous improvement in muscle function will not occur. , Botulinum toxin to the ipsilateral LR may be necessary. If the superior oblique has been tenectomized, attempts to retrieve the ends of the tendon may be made if they are demonstrated by dynamic MRI, joining them with a silicone bridge.  If this is not possible, one would treat the patient as one would for a superior oblique palsy with a weakening procedure of the antagonist inferior oblique and/or appropriate vertical rectus muscle surgery. If first treated several months after the sinus surgery, secondary scarring may have taken place. These cicatricial adhesions may have to be excised at the time of strabismus surgery. 
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