Ocular Complications Of Strabismus Surgery
Strabismus surgery is typically recommended when a patient’s eye alignment can no longer be treated with conservative measures such as eyeglasses, eye patching, prisms, and orthoptic exercises. Like many other ophthalmic procedures, strabismus surgery is very safe and effective, but complications can occur and need to be diagnosed and treated early to optimize post-operative outcome.
Complications following Strabismus surgery include the following:
- Unsatisfactory eye alignment after surgery is the most common complication. Despite careful pre-operative measurements and utilization of common surgical dosage tables, a certain percentage of patients will be overcorrected or undercorrected after surgery.
- A change in the refraction can occur, especially if two muscles are operated in the same eye. For example, operating on two horizontal muscles can induce a small with-the-rule astigmatism. This change is typically temporary and resolves after a few months.
- Diplopia can occur, particularly in adult patients that are overcorrected. Patients younger than age 10 typically can suppress the deviated eye, but older patients may not have suppression or, if suppression is present pre-operatively, may not be able to shift the suppression scotoma to cover an overcorrection.
- During surgery, scleral perforation can occur from an inadvertent deep pass of the suture needle or during dissection to isolate and disinsert the muscle tendon, particularly in the presence of scarring during a re-operation. In most cases, the perforation does not create a problem other than a chorioretinal scar, but in some cases can trigger endophthalmitis, vitreous hemorrhage, or retinal detachment.
- Post-operative infection can occur, usually withing the first week after surgery. Most infections occur around the initial surgical incision into the conjunctiva. Rarely infections can penetrate deeper into the orbit with proptosis, eyelid swelling, chemosis and erythema in the classical presentation of orbital cellulitis. As mentioned above, sometimes endophthalmitis can develop, either with or without a scleral perforation.
- Allergic reactions can occur in response to either the suture material or post-operative medications. Sometimes, these reactions can be difficult to distinguish from post-operative infections since they tend to occur in the same time frame. Some patients present late with increasing inflammation and discomfort; infection and allergic reaction should be considered in this situation as well.
- A foreign body granuloma can develop, usually a few weeks after surgery. The granuloma typically presents at the suture site as a localized, elevated, hyperemic mass that is less than 1 cm in diameter.
- A conjunctival inclusion cyst can present days to years after surgery. This translucent subconjunctival mass occurs when conjunctival epithelial cells are buried beneath the conjunctival surface during surgery.
- Conjunctival scaring can be a persistent problem after surgery. Instead of returning to the typical translucent white appearance, the conjunctiva can remain chronically hyperemic and pink, particularly after a second or third operation. This complication can be exacerbated by advancement of Tenon’s capsule too close to the limbus, particularly during a resection, or by advancement of the plica semilunaris onto the bulbar conjunctiva. If severe, the conjunctival scarring itself can cause a restrictive strabismus.
- Fat adherence can be cause by a violation of Tenon’s capsule with prolapse of orbital fat. The orbital fat can cause a fibro-fatty scar that is adherent to the muscle and globe, potentially leading to a restrictive strabismus.
- A dellen can occur on either the cornea or sclera when thickened bulbar conjunctiva (either from scarring, hemorrhage, or swelling) prevents adequate and even lubrication of the ocular surface. Fluorescein pools within the indentation of the cornea or sclera without creating true staining.
- Anterior segment ischemia occurs when strabismus surgery creates impaired blood flow to the anterior segment. Most of the blood supply to the anterior segment flows through the ciliary arteries within the four rectus muscles. Simultaneous surgery on three rectus muscles in the same eye, or two rectus muscles in a patient with compromised blood flow from vascular disease, can cause ischemia. Typical findings in anterior segment ischemia include iritis, corneal edema, folds in Descemet’s membrane and, if severe, anterior segment necrosis and phthisis bulbi of the operated eye.
- Eyelid retraction or ptosis can be caused by strabismus surgery on the vertical rectus muscles. The eyelid retractors, particularly in the lower eyelid, are adherent to the intermuscular septum and fascial tissue around the vertical rectus muscles. This connection creates a shift in eyelid position during standard recession or resection surgery of the vertical rectus muscles.
- A lost muscle occurs when the muscle slips free of the sutures or surgical instruments during surgery. This constitutes a surgical emergency and immediate surgical attempts to recover the muscle should be made.
- A slipped muscle occurs when the sutures capture only the superficial muscular capsule instead of securing the muscle belly. Post-operatively, the muscle belly retracts within the muscle capsule, leading to clinical weakness of the operated muscle.
Each ocular complication of strabismus surgery has specific contributing factors and causes.
- Unsatisfactory eye alignment can occur sporadically after otherwise uncomplicated strabismus surgery in cooperative patients. It can also occur from pre-operative measurement errors of the eye misalignment, intra-operative measurement errors in extraocular muscle position, and excessive scarring or inflammation. Late causes of poor alignment can come from poor sensory adaptation to the new eye position.
- A change in the refraction post-operatively occurs from a change in the force the extraocular muscle places on the cornea through its attachment to the sclera. Over time, this change in force usually reaches a new equilibrium, typically with restoration of the original corneal refractive shape.
- Diplopia can occur in patients capable of vision in each eye from an imperfect eye alignment. Small amounts of residual vertical or torsional misalignments can be difficult to fuse into a single image, particularly if the misalignment is opposite to the pre-operative misalignment (overcorrection). Rarely, patients may have horror fusionis, the inability to fuse despite well-aligned eyes.
- Scleral perforations can occur from an inadvertent deep pass of the suture needle, particularly if the sclera is thinned from a pathological change in eye structure, such as high myopia with a staphyloma. It may also occur when the suture is placed in the normally thinned sclera directly behind the muscle insertion. Perforations can also occur during the dissection, isolation, and disinsertion of the muscle tendon, particularly in the presence of scarring that may make exposure and isolation of the muscle tendon more challenging.
- Post-operative infections can occur if sterile technique is violated or if the patient has a pre-existing condition such as blepharitis or nasolacrimal stenosis that increases the bacteria count at the surgical site.
- Allergic reactions occur sporadically in patients that are sensitive to the materials or medications used in the peri-operative period.
- Foreign body granulomas occur sporadically in susceptible patients. The exact etiology is unclear, since the occurrence rate appears equal in primary and secondary surgeries and the existence of a prior granuloma does not appear predictive of subsequent granulomas.
- Conjunctival inclusion cysts occur when conjunctival epithelial cells are buried beneath the conjunctival surface during surgery. These cells can multiply over time to create a subconjunctival cyst days to years after the original surgery. Conjunctival scaring is more common after a re-operation and also after a surgical resection, when advancement of the muscle belly may advance the thicker posterior Tenon’s capsule closer to the limbus where it is more visible. Fat adherence is caused by violation of Tenon’s capsule with prolapse of orbital fat. It is more common with extensive posterior strabismus surgery.
- Adellen is more common after a limbal incision than a fornix incision, particularly in the presence of excessive scarring, swelling, or hemorrhage. They are more common after resection surgery than recession surgery. Any disruption of the tear layer on the sclera or cornea can create a dellen.
- Anterior segment ischemia is more common in older patients with microvascular risks factors such as diabetes and hypertension. It also occurs more commonly in patients with extensive prior peri-ocular surgery, such as scleral buckling, that can diminish anterior segment blood flow. Early recognition and treatment of ischemia with corticosteroids is necessary to minimize the chance of adverse consequences.
- Eyelid retraction or ptosis can occur with any vertical rectus muscle surgery, but is more prevalent with larger surgical dosages.
- A lost muscle can occur with any type of strabismus surgery, but is more common when operating on the medial rectus and inferior rectus because of the shorter arc of contact with the globe. A lost muscle is also more common with tight or contracted muscles because the increased passive tension increases the possibility of the muscle tendon pulling free of sutures or clamps during surgery. A lost muscle can occur in association with poor overall systemic health. Pulled In Two Syndrome (PITS) can happen when a weakened muscle ruptures during surgery, typically at the junction between the muscle belly and tendon, resulting in loss of the posterior muscle belly.
- Slipped muscles occur when less than full-thickness bites are used to capture the muscle tendon. Superficial sutures only incarcerate the muscle capsule, allowing the muscle belly to retract posteriorly when force is exerted during contraction.
Each complication has its own particular risk factors.
- There is a higher risk of an unsatisfactory eye alignment after surgery in patients with poor fusion potential and in patients with more complicated types of strabismus. Patients with dense amblyopia or structural problems in one or both eyes have limited potential for binocular vision and will not employ fusional mechanisms to improve or maintain eye alignment. Similarly, patients with neuro-developmental anomalies have been shown to have higher rates of undercorrection and overcorrection after strabismus surgery. Also, patients with more unusual and severe forms of strabismus, such as 3rd nerve palsies, are more difficult to align satisfactorily with surgery.
- Operating on more than one muscle per eye, particularly utilizing larger amounts of recession or resection, can increase the risk of a change in refraction.
- The risk of diplopia is increased in adult patients who possess limited ability to suppress the second image. The risk is also higher in more complicated types of strabismus, particularly vertical, torsional, or paretic forms of strabismus. In those patients, it may be more difficult to create a useful area of single binocular vision.
- The risk of scleral perforations is increased if the sclera is thinned and also if there is significant scarring or hemorrhage (more common during a re-operation) that may impede exposure of the sclera for suture placement. The risk is also greater with very posterior suture placement, such as for posterior fixation sutures, because it is more difficult to visualize and place the suture at the proper depth in the sclera in the retroequatorial globe.
- The risk of a post-operative infection is increased if the patient has a pre-existing condition such as blepharitis or nasolacrimal stenosis that increases the bacteria count at the surgical site. It may also be increased in very young patients, particularly those with developmental delay, that may have difficulty cooperating with hygiene and antibiotic eye drops after surgery.
- The risk of allergic reactions is increased in patients with a history of hypersensitivity reactions or systemic allergies or asthma.
- The risk of a foreign body granuloma appears to be related to the suture material. With the elimination of gut sutures in most strabismus surgeries, granulomas have become uncommon.
- The risk of a conjunctival inclusion cyst is increased when the conjunctival wound is not closed meticulously. In particular, for fornix incisions, relying on simple apposition of the conjunctival wound without sutures appears to increase the risk of subsequent cyst formation.
- The risk of conjunctival scarring is increased for a re-operation and also after a surgical resection, when advancement of the muscle belly may advance the thicker posterior Tenon’s capsule closer to the limbus where it is more visible. Care must be taken to avoid creation of conjunctival foreshortening or symblepharon.
- The risk of fat adherence is increased with more posterior strabismus surgery, such as operating on the inferior oblique muscle, exploring posteriorly to retrieve a lost muscle, or dissecting more posteriorly to place a posterior fixation suture.
- The risk of a dellen is higher for a limbal incision than a fornix incision, because the subsequent irregularity of the perilimbal conjunctiva can cause a disruption of the tear layer in the anterior sclera and cornea.
- The risk of anterior segment ischemia is higher when operating on multiple muscles in the same eye, in older patients with microvascular disease, and in patients with prior extensive eye surgery that might also disrupt the ciliary vessels, such as scleral buckling procedures.
- The risk of eyelid retraction or ptosis is increased in subjects undergoing vertical rectus recessions or resection with high surgical dosages, particularly surgeries involving the inferior rectus.
- The risk of a lost muscle increases with poor overall systemic health, with extensive scarring from prior eye surgery, and with surgery on a tight, contracted muscle.
- The risk of a slipped muscle is increased when the muscle is tight, such as with dysthyroid orbitopathy or the contracted antagonist of a paretic muscle. It can be difficult to create space at the muscle insertion to allow a full-thickness suture pass to encompass both the muscle capsule and muscle belly. A superficial suture pass is more likely to capture just the muscle capsule, and the tight muscle is more likely to retract within the muscle capsule after surgery.
A few of the complications of strabismus surgery are related to pathology at the site of surgery. Scleral perforations are more likely if the sclera is thinned from any of a number of eye conditions, most commonly high myopia. Typically, the thinned sclera shows a darker, off-white coloration because of the translucent appearance of the underlying choroid.
Post-operative infections can occur whenever sterile technique has been violated. Most of the time, the infections are caused organisms that colonize the patient’s skin, lashes, and conjunctiva, so conditions that increase the bacterial count in those areas, such as blepharitis or a pre-existing nasolacrimal duct obstruction, can increase the risk of infection.
Conjunctival inclusion cysts occur when conjunctival epithelial cells are inadvertently buried beneath the conjunctival surface. These cells continue to replicate and over time grow into a cystic mass.
Dellen result from local dehydration of the scleral or corneal tissue. Although the thinning can appear very dramatic, the rate of perforation appears very low.
Anterior segment ischemia can occur in patients with pre-existing vascular disease of the ciliary vessels. Disinsertion of the rectus extraocular muscles exacerbates the underlying poor perfusion by removing one or more ciliary arteries from the anterior segment circulation.
The risk of a lost or slipped muscle increases with poor overall systemic health, with extensive scarring from prior eye surgery, and with a tight muscle, such as with dysthyroid orbitopathy or the contracted antagonist of a paretic muscle.
See comments in “General Pathology”.
Since all strabismus surgeries carry an element of risk, the goal is to reduce the incidence of adverse post-postoperative outcomes with the realization that is not possible to eliminate surgical complications.
- The risk of an unsatisfactory eye alignment after surgery is minimized by a careful and complete pre-operative evaluation and by meticulous surgical technique with emphasis on good exposure, lighting, and precise intraoperative measurements to verify that proper surgical dosages were obtained. The pre-operative evaluation can include multiple consistent measurements of the eye deviation and prism adaptation to assess both surgical angle and post-operative fusion potential. The intra-operative procedure can include steps to minimize the measurement error induced by the curvature of the globe, particularly for large recessions, and to ensure proper placement of the muscle sutures to capture of the muscle belly, not just the muscle capsule, and minimize sag of the new muscle insertion.
- Limiting surgical dosages could reduce the risk of a change in the post-operative refraction, but as a practical matter simply monitoring post-operative vision and adjusting the refractive correction to maximize vision if necessary can treat this self-limited complication.
- The risk of post-operative intractable diplopia can be minimized by careful assessment of the binocular potential prior to surgery and by choosing conservative surgical dosages and employing adjustable suture techniques to minimize the chance of an overcorrection. Diplopia is a necessary but short-lived complication of eye muscle surgery in many patients.
- The risk of scleral perforation can be minimized by using a magnified view of the surgical field and placing flexible spatulated needles through the sclera with good exposure and clear visibility of the needle tip at all times. Also, the depth of the scleral suture is easier to judge when placing the scleral sutures through the original insertion and using a “hang-back” technique for recessions.
- The risk of a post-operative infection can be minimized by aggressively treating any superficial infection or bacterial overgrowth pre-operatively, by using meticulous sterile technique during surgery, and by using post-operative antibiotics.
- The risk of an allergic reaction can be minimized by carefully reviewing the patient’s medical history and avoiding medications that might cross-react with known medication allergies.
- The risk of a foreign body granuloma can be reduced by avoiding gut sutures and by proper draping to keep lashes out of the surgical field.
- The risk of a conjunctival inclusion cyst can be minimized with careful and complete wound closure. Preplaced marking sutures can allow easy identification of the edges of the conjunctival wound at the end of surgery, allowing easy distinction from the underlying Tenon’s capsule.
- The risk of conjunctival scarring can also be minimized by careful wound closure. For resections, debridement of Tenon’s capsule from the undersurface of the conjunctiva and recession of the conjunctival wound from the corneal limbus may help prevent the thickened anterior conjunctival scar that can occur after a large resection.
- The risk of fat adherence can be minimized by recognition of the violation of the retro-orbital fat intra-operatively. The posterior opening in Tenon’s capsule can be repaired in layers with absorbable sutures to prevent contact between the globe and muscles and orbital fat.
- The risk of a dellen can be minimized by utilizing a fornix incision instead of a limbal incision or by recessing the conjunctiva away from the limbus after a limbal incision.
- The risk of anterior segment ischemia can be minimized by limiting the number of muscles operated on in each eye, by utilizing botulinum toxin, and by using special surgical techniques to spare the ciliary vessels during muscle surgery. A fornix conjunctival incision may have a slightly reduced effect on anterior segment perfusion compared with a limbal incision.
- The risk of eyelid retraction or ptosis can be minimized by limiting the surgical dosages applied to the vertical rectus muscles and by carefully dissecting the lid retractors away from the extraocular muscles during surgery at least 12-15 mm posterior to the insertion.
- The risk of a lost muscle can be minimized by using gentle surgical techniques when isolating and securing the muscle belly and rotating the globe towards a resected muscle rather than pulling the tight muscle anteriorly. The risk can also be reduced by direct placement of scleral sutures at the site of the new insertion, instead of utilizing a hang-back technique from the original insertion.
- The risk of a slipped muscle can be minimized by using full-thickness bites when passing the muscle suture. Special groove hooks or muscle clamps can be used with tight muscles to provide more space to securely place the muscle sutures to encompass the muscle belly in addition to the muscle capsule. Some physicians prefer to use two locking bites instead of the traditional one locking bite to ensure adequate capture of the muscle fibers.
In most cases, the diagnosis of a surgical complication after strabismus surgery is straightforward. The most important part of the diagnosis is to evaluate the patient early in the post-operative period and to be available for additional consultation and evaluation if the patient reports any complaints or change in status.
A focused history should narrow the diagnostic possibilities, with an emphasis on the time course after surgery, any change in vision, diplopia, discharge, pain, swelling, and other specific complaints.
The standard ophthalmology exam should be performed with a special emphasis on the motility exam and the slit lamp exam. During the motility exam, specialized testing such as saccadic velocities, force generations, and forced ductions can distinguish between lost or slipped muscles and excessive scarring or fat adherence. The slit lamp can distinguish signs of anterior uveitis, an early indicator of anterior segment ischemia, from conjunctival inflammation secondary to an infection or allergic reaction. In addition, dellen or exposed and loosened sutures can be identified and treated. Pupillary reactions should be assessed because an abnormally slow or asymmetric pupillary reaction to light can be an early sign of anterior segment ischemia. A dilated exam should be performed when there is a potential scleral perforation, to identify potential retinal or vitreal problems that may require further treatment.
In many cases, the clinical signs make the diagnosis. Worsening eye alignment, abnormal eyelid position, diplopia, and a new subconjunctival mass all are specific for certain types of post-operative complications. Other signs are more nonspecific and require careful evaluation to determine the diagnosis. Increasing redness, swelling, and a change in vision may be secondary to a post-operative infection, but may also be a sign of an allergic reaction or anterior segment ischemia, particularly in the early post-operative period.
Typical symptoms involve either increasing pain, redness, and swelling, from increasing inflammation from an infection, allergy, or ischemia or worsening vision, diplopia, and/or eye alignment. Occasionally, the patient may not notice a problem for many months or even years, as in the case of a conjunctival inclusion cyst or late change in eye alignment. Most complications occur in the immediate post-operative period.
Most of the time the diagnosis can be made using only the clinical ophthalmic exam. The diagnosis can be aided in some cases by specialized testing, as mentioned below. For the most common area of confusion, allergic reaction versus early post-operative infection, an exact diagnosis at presentation may be difficult and an empiric change in antibiotic medication may be warranted with careful follow up to monitor the clinical course.
Specialized diagnostic procedures can occasionally aid in the diagnosis and treatment of post-operative complications of strabismus surgery. For infections, gram stains and cultures (either of the conjunctiva or, in the case of scleral perforation and possible endophthalmitis, of the vitreous) can aid with diagnosis and treatment. For unsatisfactory eye alignment, special components of the orthoptic exam such as saccadic velocity, force generation, and forced duction testing, can help distinguish between lost and slipped muscles and a restrictive strabismus from excessive scarring or fat adherence. For decreased vision, a refraction can help distinguish a refractive change from a more serious post-operative complication.
Only a few laboratory tests have a role in the evaluation for post-operative complications of strabismus surgery. As mentioned above, gram stains and cultures can help with the diagnosis and treatment of infections. Imaging can help identify pre-septal and orbital cellulitis and also help localize lost muscles. Other forms of specialized testing, such as an iris angiogram for anterior segment ischemia, are typically performed in a research setting rather than in standard clinical practice.
Two types of post-operative symptoms carry a differential diagnosis.
Severe post-operative vision loss can be caused by endophthalmitis, vitreous hemorrhage, retinal detachment, anterior segment ischemia, or complications from retrobulbar anesthesia.
Persistent post-operative foreign body sensation can be cause by a foreign body granuloma, conjunctival inclusion cyst, dellen, or loose and exposed suture.
The management of post-operative complications of strabismus surgery is directed towards treating infections, relieving pain and discomfort, and restoring functional vision as soon as possible.
The treatments can be divided into three broad categories.
Optical treatments include a change in refraction, temporary or permanent prisms, or optical blur or occlusion.
Medical treatments include antibiotics – topical, systemic, or intravitreal –and anti-inflammatory medications, especially topical corticosteroids.
Surgical treatments include excisional biopsies, release of scar tissue, treatment of any retinovitreal disorders, and repeat strabismus surgeries.
Optical therapy is directed at correcting any change in refractive error following surgery. Prisms may also be used in select cases to alleviate diplopia.
Medical therapy is the first line of treatment for several post-operative complications. For infections, topical antibiotics are used to treat conjunctivitis, systemic antibiotics are used to treat preseptal and orbital cellulitis, and intraviteal antibiotics are used to treat endophthalmitis. For granulomas and conjunctival inclusion cysts, topical corticosteroids are used for several weeks, with possible surgical excision if no clinical response is observed. For allergies, the antibiotic eye drops should be changed, with the possible addition of a topical corticosteroid or antihistamine if the symptoms persist. For dellen formation, aggressive topical lubrication with artificial tears, sometimes in conjunction with eye patching, can help until the chemosis and swelling subside and the ocular surface becomes smooth again. For anterior segment ischemia, topical and systemic corticosteroids can relieve inflammation until collateral vascularization can occur.
Medical follow up
The medical follow up is determined by the severity of the clinical problem. For serious infections, the patient may need to be followed daily to monitor and adjust therapy. For less serious problems, the follow up period can be longer, with specific instructions to call for any apparent deterioration in clinical status.
Surgery is the first line treatment for other post-operative complications.
For a lost muscle, an attempt should be made to retrieve the muscle promptly, during the same surgery if possible. If the muscle cannot be retrieved, a transposition surgery can be considered, although there is a risk of anterior segment ischemia by performing surgery on three eye muscles (the lost muscle plus two transposed muscles) at the same time.
An initially unsatisfactory post-operative alignment may or may not require surgical correction. Sometimes prisms can be incorporated temporarily (fresnel prisms) or permanently into eyeglasses and further surgery deferred. In other situations, the prisms can be weaned slowly over time as fusional mechanisms build, allowing single binocular vision without prisms. Many cases of unsatisfactory eye alignment will need another strabismus surgery, particularly in the case of a slipped muscle or restrictive scarring where the eye alignment will not improve over time.
Likewise, a scleral perforation may or may not require additional surgery. Most cases resolve without treatment, but the presence of infection, significant hemorrhage, or retinal detachment may require a vitrectomy or other surgeries as indicated. The use of cryotherapy or laser retinopexy at the time of perforation is controversial in the absence of retinal perforation.
Foreign body granulomas and conjunctival inclusion cysts may require excision if they are symptomatic and fail to respond to topical corticosteroids. Likewise, persistent conjunctival scarring or a dellen may require surgery to remove scar tissue and smooth the conjunctival surface.
Eyelid retraction or ptosis may require oculoplastic surgery to restore the normal lid configuration, particularly if the defect prevents proper eyelid closure or affects peripheral vision.
Surgery for a lost muscle can benefit from pre-operative imaging to help locate the muscle. Depending on the location, a posterior orbital approach with the aid of an orbital surgeon can sometimes successfully isolate and retrieve the lost muscle. If the attempt to locate the muscle is unsuccessful, transposition surgery can be effective at restoring primary gaze alignment.
Surgical follow up
Post-operative follow up should follow the standard of care for the surgery performed.
Each subsequent surgery carries the risk of additional complications similar to those discussed above. Fortunately, the risks remain low, with the exception that the risk of excessive conjunctival scarring increases with each surgery performed.
All of the complications of strabismus surgery, with the notable exception of endophthalmitis, have an excellent prognosis for recovery with proper treatment. Many of the complications lessen or disappear with time and conservative treatment, while others respond well to additional surgery. When severe, anterior segment ischemia can progress to necrosis and phthisis bulbi, but most cases resolve with corticosteroids over time. Endophthalmitis, on the other hand, carries a significant risk of permanent vision loss even with prompt treatment.
1. AAO, Basic and Clinical Science Course, Section 6: Pediatric Ophthalmology and Strabismus, 2009-2010.
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