Management of Strabismus Associated with Thyroid Eye Disease

9.2.1 Etiology

Enlargement of the extraocular muscles with sparing of their tendons is a common feature of TED that often results in an incomitant strabismus with binocular diplopia. ^{1,​ 2} In the initial acute phase of TED, there is lymphocytic infiltration and interstitial edema of the extraocular muscles with deposition of glycosaminoglycans and hyaluronic acid as well as adipogenesis. ^{3,​ 4} Extraocular muscle inflammation stimulates fibroblast activity resulting in mucopolysaccharide and collagen formation. Later the inflammation subsides, often leaving chronic fibrotic change, which results in limitation of extraocular motility and diplopia in patients with TED. ⁴

9.2.2 Typical Patterns of Strabismus in Thyroid Eye Disease

Mild periorbital edema in association with limitation of elevation of one eye may be the first symptom of strabismus due to TED. Impaired ductions are observed in nearly 80% of TED patients and binocular diplopia is the initial presentation in 15 to 20% of TED patients. ^{5,​ 6,​ 7} Clinically, the inferior rectus muscle is most commonly affected (60–80%), followed by the medial rectus muscle (42–44%) and then the superior rectus muscle, although any extraocular muscle can be affected in TED. ^{4,​ 6,​ 8,​ 9} It is not known why some muscles are more commonly affected than others. The inferior rectus muscle is the bulkiest and most tonically active extraocular muscle and it may be that the degree of muscle activity and hence the blood supply is a factor. ⁴ Magnetic resonance imaging (MRI) of the orbits is useful in assessment of disease activity and in showing muscle involvement and optic nerve compression. ⁴ Finally, the diagnosis of strabismus due to TED can be confirmed by forced duction testing, which reveals restriction of passive movements of the eyeball.

Enlargement and tightening of an extraocular muscle with relative sparing of its antagonist muscle causes the eye to deviate toward the affected muscle. ¹⁰ Symptoms of diplopia in the primary position may not be present if the angle of deviation is small and falls within the patient’s fusional range, which decreases with time. ¹⁰ Sparing of the contralateral yoke muscle may cause a significant imbalance that often results in an esotropia when the medial rectus muscle is involved or a hypotropia from the inferior rectus muscle. Strabismus due to TED is usually incomitant, meaning that the angle of deviation changes depending on the direction of gaze. ^{4,​ 10}

Patients with TED may have isolated horizontal or vertical strabismus, but many patients have a combination of the two depending on the extent of muscle involvement. The most commonly seen vertical deviation is a hypotropia, unilateral or bilateral, that results from a tight inferior rectus muscle. These patients often have concomitant excyclotorsion since the inferior rectus muscle’s secondary action is to excyclotort the globe. Esotropia is by far the most common horizontal deviation in TED due to tightening of the medial rectus muscle and relative sparing of the lateral rectus muscle, its antagonist. Since the lateral rectus muscles are rarely involved, exotropia is almost never seen in TED.

9.2.3 Nonsurgical Management of Strabismus Associated with Thyroid Eye Disease

 Prism

Prisms bend light and can be used to move images onto the fovea of the deviated eye so that disparate images can be fused. Prisms can be grounded into spectacle lenses ( ▶ Fig. 9.1) or a Fresnel prism can be applied on top of a spherocylindrical lens ( ▶ Fig. 9.2). A Fresnel prism is a type of compact lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses. The Fresnel principle states that prismatic power can be achieved by employing a concentric set of prismatic rings and in this way a Fresnel prism can be made much thinner than a comparable conventional prism. In most cases, the cost of Fresnel prisms is much lower than ground-in prisms and they are particularly effective in comitant, small-angle strabismus or for the temporary elimination of small-angle diplopia in primary position while TED patients are waiting for strabismus surgery.

Fresnel prisms, however, have some major disadvantages. They often cause degradation of visual acuity in the distance compared with traditional ground-in prisms, especially when using a prism greater than 12 prism diopters. ¹¹ They cause increased optical aberrations, loss of contrast, and light scatter that can be intolerable to many patients, and these symptoms typically worsen when using Fresnel prisms of increasing strength. Furthermore, many patients do not like the cosmetic appearance of the lens because the grooves of the Fresnel lens are visible. Overall, Fresnel prisms can be used in select patients and in one study, only 8% of subjects continued using them once satisfactory treatment of diplopia was achieved. ¹²

A more permanent prism option is the ground-in prism, in which the prism is incorporated into the spectacle lens. However, these lenses tend to be thick and relatively heavy, often causing similar optical aberrations as Fresnel prisms. Many optical shops will not grind more than a total of 6 to 10 prism diopters, making this option only good for small-angle, comitant deviations. Since the strabismus associated with TED is usually incomitant, prisms often do not work well because they create a zone of binocular vision that is too small to provide improvement in functionality. ^{9,​ 10} Further, patients with TED often have variation and worsening of their strabismus, making a permanent prism a poor option, as it cannot be frequently and inexpensively changed.

 Botulinum Toxin Injection

Botulinum toxin is a lethal toxin produced by the bacterium Clostridium botulinum. It acts at the presynaptic terminal of the neuromuscular junction to decrease the release of acetylcholine, thereby blocking neuromuscular transmission to cause flaccid muscular paralysis for 3 to 4 months. The toxin begins to work within 1 to 3 days and has its maximum effect in 7 to 10 days.

Injection of botulinum toxin to treat strabismus, reported in 1981, is considered to be its first ever use for therapeutic purposes. A small amount of toxin is injected into the tight muscle in order to weaken its primary action. For example, in a TED patient with esotropia due to an enlarged and tight medial rectus muscle, toxin is injected into the medial rectus.

In the initial phase of TED, patients can experience discomfort with eye movements that results from restriction of ductions. They will often assume a compensatory head turn to put the eyes in a position of least deviation in order to prevent diplopia. ^{10,​ 13} At this stage, botulinum toxin injection into the affected extraocular muscle can produce temporary alignment in primary position and eliminate an abnormal head turn. Several studies have demonstrated that botulinum toxin injections can improve the ocular misalignment in patients with TED resulting in improved ocular ductions and resolution of diplopia, thus avoiding the need for strabismus surgery in some cases. ^{13,​ 14,​ 15,​ 16} The best candidates for botulinum toxin injection are patients with esotropia, relatively small-angle horizontal and vertical deviations, and those with smaller degrees of exclotorsion. ¹⁶ In addition, prisms can be effective at eliminating a small residual angle of deviation following botulinum toxin injection.

A disadvantage of using botulinum toxin for strabismus is that the effect of the toxin cannot be accurately titrated and the resulting effect is unpredictable. Thus, injections can improve the angle of deviation but may not completely eliminate the diplopia. Additionally, TED patients with strabismus often wait many months prior to being a surgical candidate in order to ensure that their misalignment is stable. Since botulinum toxin has an effect for 3 to 4 months, the monitoring period begins after the toxin has worn off, thus delaying definitive surgical treatment by many months.

9.2.4 Indications for Surgical Intervention

Strabismus surgery is indicated in a patient with TED who has binocular diplopia in primary position or an abnormal head position that is adopted to minimize or eliminate diplopia. Once it is determined that there is an indication to perform surgery, the surgeon must decide on the correct timing for surgery, as it should not be carried out until the inflammatory phase has become inactive, thyroid function has normalized, and orbital decompression has been performed, if indicated. ⁴ Since the inflammatory phase typically lasts 12 to 18 months, it is important to document the approximate start date of the eye symptoms. Prior to performing surgery, the surgeon must make sure that strabismus measurements are stable over a period of 4 to 6 months. ^{4,​ 9,​ 10} Patients who have a prolonged clinical disease course and who are experiencing debilitating diplopia are often eager to have surgery as soon as possible and it is important to properly manage patient expectations. The surgeon should explain that despite careful surgical planning, up to 30% of patients will develop clinically significant changes in their deviation after 6 months of stable measurements. ^{4,​ 17} As mentioned earlier, if orbital decompression is planned, it should occur prior to any strabismus surgery because eye misalignment will worsen in about 20% and improve in 25 to 36% of patients due to shifting of orbital contents. ¹⁸ Eyelid surgery should be delayed until after strabismus surgery because operating on the inferior rectus or superior rectus muscles may result in eyelid retraction given the proximity of the eyelid retractor muscles to the vertical rectus muscles.

9.2.5 Surgical Management of Strabismus Associated with Thyroid Eye Disease

 Surgical Goal

Surgical goals and realistic expectations should be defined and discussed with the patient prior to surgery. Surgery of restrictive strabismus, particularly in TED, is challenging, because a full field of binocular single vision in patients with TED is difficult to obtain due to the incomitant characteristics of the strabismus. ^{10,​ 19} Therefore, the goal of strabismus surgery in TED patients should be to create a zone of binocular single vision that is as large as possible. ^{18,​ 19,​ 20,​ 21} The practical goal should be to achieve a zone of binocular single vision in primary position at distance and near, in addition to the reading position with or without the aid of prisms. ^{4,​ 9,​ 10} It is important to make the patient understand the possibility of residual deviation and new diplopia in certain gaze directions requiring a new head turn or maneuver, and the possibility of having multiple surgeries. ¹⁰ There are many possible surgical methods to accomplish this goal including recessions of the affected extraocular muscles with or without adjustable sutures, Faden procedures, or interposition of spacers to lengthen the involved muscle. ^{19,​ 21,​ 22,​ 23,​ 24}

 Recessions of the Affected Extraocular Muscles with or without Adjustable Sutures

The affected extraocular muscles in patients with TED are usually tight and fibrotic. Standard surgical treatment for strabismus associated with TED is recession of the tight muscles. Resection is rarely carried out, and gaining access to the muscles can be difficult. ⁴ Surgery is performed under general anesthesia and an experienced assistant is recommended. A Fison retractor is helpful to expose tight muscles. When the muscle is extremely tight, it may be necessary to disinsert it using a scalpel while protecting the globe with the strabismus hook. A forced duction test should be performed in order to grade the tightness of all the muscles. The position of the muscle insertion should be measured prior to and after disinsertion. ^{4,​ 10} The insertion of a very tight muscle may move up to 2 mm anteriorly after disinsertion of the muscle, and the muscle could be under-recessed if this is not recognized during surgery. ^{4,​ 10}

Surgical results in strabismus associated with TED can be highly unpredictable, due to the intrinsic changes to the extraocular muscles, which may lead to variable responses to standard dose surgery, with reoperation rates reported between 17 and 45%. ^{25,​ 26,​ 27,​ 28} The adjustable suture technique is effective in improving outcomes, as it allows more precise alignment of the eyes in the immediate postoperative period. ²⁹ However, there is some controversy over using adjustable sutures when recessing the inferior rectus muscle. Several authors have reported late overcorrection of preoperative hypotropia using adjustable sutures in strabismus associated with TED. ^{25,​ 29,​ 30} Kerr found that nonabsorbable sutures provided an advantage in reducing late overcorrections. ³¹ In addition, several authors suggest targeting the postoperative deviation angle to undercorrect preoperative hypotropia in order to prevent a late overcorrection. ^{4,​ 29} The aim should be to achieve binocular single vision in about 10 degrees of downgaze. This facilitates reading and walking down steps while requiring only a minimal chin-up head position for distance vision. ^{4,​ 29} In addition, semi-adjustable suture techniques have been proposed to prevent late overcorrection due to muscle slippage, including in patients with TED. ³² An alternative method is to recess the inferior rectus muscle with a fixed suture while recessing the contralateral superior rectus with an adjustable suture. ⁴

Recession of an inferior rectus muscle in patients with TED results in between 3 and 4 prism diopters of effect per millimeter of recession. ^{4,​ 29} Large recession of an inferior rectus muscle results in limitation of depression. ¹⁰ With very large recession of the inferior rectus, the superior oblique increases its action of downgaze. Considering that the inferior rectus is an adductor and the superior oblique is an abductor, the eye abducts in downgaze, resulting in an A-pattern with incyclotorsion and diplopia occurring after a very large recession of the inferior rectus. ^{4,​ 10} To prevent this phenomenon, the inferior rectus should be moved in a nasal direction by half an insertion width during surgery in order to reduce the A-pattern. ^{4,​ 10} When there is no excyclotorsion in the presence of a tight inferior rectus muscle, a tight superior oblique muscle should be considered and if present, the superior oblique muscle will be tight during the traction test after disinsertion of the interior rectus muscle and should be recessed as well ^{4,​ 10,​ 33,​ 34}

Large muscle recessions may need to be accompanied by additional conjunctival recession to avoid postoperative restriction through conjunctival tethering. To prevent this, the forced duction test should be performed not only prior to surgery, but also after conjunctival suturing. ¹⁰ In addition, close attention should be paid when dissecting Tenon’s capsule, particularly around the inferior rectus muscle, because the possibility of a slipped muscle after large recession is somewhat high, partly due to its short arc of contact with the globe and the presence of Tenon’s capsule, which may predispose to nonadhesion to the sclera. ^{4,​ 10}

In summary, strabismus surgery, particularly with the adjustable suture technique, can be highly effective in patients with strabismus due to TED. Correcting restrictive strabismus caused by TED is challenging, because routine surgical dosing tables do not seem to be as effective in cases of strabismus due to TED. ²¹ The adjustable suture technique is effective in improving outcomes, as it allows more precise alignment of the eyes in the immediate postoperative period ^{21,​ 29} However, the adjustable technique cannot prevent late overcorrection after inferior rectus recession in patients with strabismus due to TED. Therefore, some authors suggest that adjustment of these patients to undercorrection of at least 2 prism diopters to compensate for postoperative overcorrection after inferior rectus recession. ²⁹ Most patients are very grateful to have their most disabling problem related to TED resolved or improved as much as possible.

9.3 Management of Eyelid Malposition Associated with Thyroid Eye Disease

9.3.1 Eyelid Retraction

Eyelid retraction is the most common sign of TED and is found in 70% of upper eyelids and 20% of lower eyelids of patients recently diagnosed with TED. ³⁵ Studies suggest that the levator palpebrae superioris muscle is the most commonly involved extraocular muscle in thyroid orbitopathy. ^{36,​ 37} The exact mechanism of upper eyelid retraction is poorly understood, but is thought to be the result of levator inflammation acutely, which is followed later by fibrosis. Müller’s muscle has also been implicated in upper eyelid retraction as a result of sympathetic stimulation during the acute phase of TED and by enlargement and fibrosis in the chronic phase. ³⁸ Lower eyelid retraction has been postulated to be a result of either proptosis or lamellar shortening from retractor muscle inflammation and fibrosis. Studies measuring inferior fornix depth and analyzing postsurgical decompression data have suggested that proptosis is likely the primary cause of lower eyelid retraction. ^{39,​ 40}

Eyelid retraction may result in symptoms related to dryness such as pain, foreign body sensation, and epiphora, and contact lenses may become intolerable. Other autoimmune mechanisms directly affecting the ocular surface have also been implicated as a cause of these symptoms. ⁴¹ Eyelid retraction is often cosmetically troublesome to patients, resulting in poor self-esteem, depression, and avoidance of social situations ( ▶ Fig. 9.3). ⁴²

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