Ординатура / Офтальмология / Английские материалы / Strabismus Surgery and Its Complications_Coats, Olitsky_2007

Effective and safe anesthesia is obviously important to patient safety and surgical outcomes. Difficulties that arise related to anesthesia can produce anxiety on the part of the surgeon as well as other operating room staff. A straightforward surgical case can rapidly evolve into a very difficult and even dangerous situation in the rare event that an anesthetic complication arises.

nized beforehand, preparation for fiber-optic intubation, use of laryngeal mask, or a decision to perform surgery under local anesthesia may be made as appropriate. Patients with Down syndrome may have atlantoaxial instability making intubation problematic and this condition can be unrecognized in some patients. The value of preoperative screening remains debatable among pediatricians, orthopedists, and anesthesiologists [2].

The majority of strabismus surgery operations are performed under general anesthesia. Fortunately, serious complications involving general anesthesia are uncommon. The incidence of serious complications can be reduced through appropriate risk management. The specific details of medical disorders that increase the risk of an adverse event in patients undergoing general anesthesia are beyond the scope of this chapter. However, it is important to have an understanding of the patient’s general medical health prior to surgery. Patients with significant health disorders should discuss their upcoming surgery with their primary medical physician. In addition, consultation with an anesthesiologist prior to surgery can be helpful for patients with complex underlying medical disorders.

Strabismus surgery is virtually always performed as an elective procedure and few cases cannot be postponed until the medical condition of the patient has been optimized. A note from the primary care physician that a complex patient may undergo general anesthesia is useful, but alone is not sufficient. It is the responsibility of the anesthesiologist to make a determination about the safety of the proposed anesthetic procedure given the medical background supplied by the patient, laboratory studies, and reports from other physicians treating the patient, where appropriate,

Mechanical concerns regarding endotracheal intubation include patients with difficult airways and patients with

cal neck disorders. Patients with a short, thick neck or spondylosis may be difficult to intubate. While generally sidered primarily an anesthesia issue, Arnold and

[1] reported a surgical complication related to a cervical abnormality. Unknown to the surgeon, the patient’s not in contact with the operating table, and movement head during surgery resulted in perforation of the globe suture needle. If the potential for a difficult intubation is

Malignant hyperthermia is a potentially lethal disorder that occurs in genetically susceptible people. The general signs of malignant hyperthermia include tachycardia, increased body metabolism, muscle rigidity, and fever that may exceed 43°C (110°F) (>Table 28.1). Sequelae of malignant hyperthermia can include cardiac arrest, brain damage, organ system failure, and death. The genetic predisposition for malignant hyperthermia occurs in 1:10,000 people and the clinical incidence is approximately 1:62,000–1:84,000 cases, which means that not every patient with a genetic predisposition for malignant hyperthermia will develop the condition [3, 4]. Multiple genetic defects have been associated with the condition. The susceptibility to develop malignant hyperthermia is inherited in an autosomal dominant fashion. This means that children and siblings of a patient who has been identified as a malig- nant-hyperthermia-susceptible individual have a 50% chance of inheriting a genetic defect predisposing them to malignant hyperthermia as well.

Malignant hyperthermia occurs as the result of a biochemical chain reaction within the skeletal muscles of susceptible

Temperature elevation

Tachycardia

Trismus

Tachypnea

Metabolic acidosis

Rise of end-expiratory CO2

patients as a response to commonly used general anesthetic agents. It is due to a dysregulation of intracellular calcium within skeletal muscles. The dysregulation occurs when a susceptible individual is exposed to certain anesthetic agents. Anesthetic agents that are known to trigger malignant hyperthermia include volatile inhalational agents as well as succinylcholine. Many agents are safe and may be used in patients who are thought to be at risk for malignant hyperthermia. During a malignant hyperthermia crisis, cellular calcium levels increase. This in turn causes an increase in metabolic rate, heat production, and muscle rigidity. Early signs of malignant hyperthermia may include masseter muscle spasm, metabolic acidosis, sinus tachycardia, a rise of end-expiratory CO2 and flushing of the skin. Later, complex arrhythmias, hypoxemia, hypotension, electrolyte abnormalities, rhabdomyolysis, and hyperthermia develop. Treatment of a possible early malignant hyperthermia crisis includes immediate discontinuation of triggering anesthetic agent(s), administration of 100% oxygen, adjustment of ventilation according to blood gas analysis, and administration of insulin to treat hyperkalemia. Dantrolene (1 mg/kg) is given via rapid intravenous infusion up to a total of 10 mg until symptoms of malignant hyperthermia resolve. Anti-arrhythmia therapy is administered as needed. The core temperature of the patient can be lowered through a variety of measures including ice packs, cold water lavage, and cooled intravenous fluids as needed. Dantrolene may be administered after the crisis has passed to prevent recurrence of symptoms in the immediate postoperative period.

The best method to prevent a malignant hyperthermia crisis is to detect those individuals who may be susceptible prior to surgery. Patients with a family history of malignant hyperthermia or a history of a previous anesthetic complication suggestive of malignant hyperthermia should have a consultation with an anesthesiologist prior to surgery. These patients can undergo surgery without the use of triggering anesthetics. Typically surgery on at-risk patients is performed as the first surgical case of the day before volatile anesthetics are used in the anesthesia circuit.

28.3 Postoperative Nausea and Vomiting

One of the most common adverse effects of general anesthesia in patients undergoing strabismus surgery is postoperative nausea and vomiting. While postoperative nausea and vomiting is not an uncommon complaint following general anesthesia for any procedure, it is especially common following strabismus surgery. In fact, strabismus surgery is considered one of the known risk factors for development of postoperative nausea and vomiting following general anesthesia.

The feeling of nausea is a conscious recognition of excitation of the area in the medulla that is associated with the vomiting center. The vomiting center receives afferent input from the chemoreceptor trigger zone, the vestibular apparatus, the cerebellum, and other higher cortical and brain stem centers. These structures contain dopaminergic, muscarinic, serotoninergic, histaminic, and opioid receptors. The mechanism of action of antiemetic drugs may be partially due to blockade

Chapter 28

of these receptors. Because the chemoreceptor trigger zone is not protected by the blood–brain barrier, it can be activated by chemical stimuli received through both the systemic circula­ tion and the cerebrospinal fluid.

The incidence of postoperative nausea and vomiting varies according to individual susceptibility as well as certain risk factors. Risk factors include the type of surgery being performed, a previous history of postoperative nausea and vomiting, a history of motion sickness, and the duration of surgery and anesthesia. A history of previous postoperative nausea and vomiting can increase the risk of recurrence by two or three times. The increase in risk of postoperative nausea and vomiting with longer duration of surgery may be due to the greater accumulation of emetogenic anesthetic agents. One study calculated that the risk for nausea and vomiting increased by 59% for each 30-min increase in surgical duration [5, 6].

Intraoperative anesthetic agents may increase or decrease the incidence of postoperative nausea and vomiting. Nitrous oxide has been reported to produce a greater incidence of vomiting in some studies. Propofol has been shown to be associated with a lower incidence of nausea and vomiting when used for induction of anesthesia, but the same effect has not been shown when total intravenous anesthesia with propofol is used [7].

There are several important postoperative factors that may increase the incidence of nausea and vomiting. Pain may prolong gastric emptying time with a resultant increase in nausea and vomiting. In addition, opioids may be used to treat post­ operative pain and can increase the risk of nausea and vomiting by directly stimulating the chemoreceptor trigger zone, increasing vestibular sensitivity, and reducing motility of the digestive system. Various opioids will affect each individual differently. Therefore, opioid-induced nausea and vomiting may be either increased or decreased depending on the pharmacologic agent used in a given patient. Pain relief with a combination of systemic opioids, local anesthetics and nonsteroidal anti-inflammatory drugs may be helpful at both managing pain and reducing the incidence of postoperative nausea and vomiting.

Hypovolemia in the postoperative period can lead to dehydration, dizziness, and orthostatic hypotension which may increase nausea and vomiting. Thus, once the patient becomes dehydrated, postoperative nausea and vomiting may be exacerbated and thus hydration during and after surgery is important in reducing its incidence and severity. Goodarzi and coworkers [8] demonstrated that superhydration may further decrease the incidence of postoperative nausea and vomiting in children undergoing strabismus surgery.

Antiemetic drugs may be used to either prevent or treat postoperative nausea and vomiting. Several classes of drugs exist that are frequently used for these purposes (>Table 6.2). Droperidol inhibits dopaminergic receptors in the chemoreceptor trigger zone. It has a short-lived anti-nausea affect and its common side-effects include sedation and drowsiness. Metoclopramide is a dopamine antagonist that can be used to treat opioid-induced nausea and vomiting. It may help to reverse the gastric stasis that occurs with morphine use. Scopolamine is a muscarinic receptor antagonist. Application of a scopolamine patch prior to anesthesia may reduce postoperative nausea

and vomiting. Side-effects of scopolamine include sedation, dry mouth, and visual disturbances because of the effect this agent has on accommodation. Ondansetron, granisetron, tropisetron, and dolasetron are 5-HT3 receptor antagonists. These agents have been shown to be effective in the prevention and treatment of postoperative nausea and vomiting [9–11].

Glucocorticosteroids also have an antiemetic affect. Their mechanism of action in this regard is unclear. Dexamethasone has been shown to have antiemetic effects that are similar to conventional antiemetic agents. Its antiemetic effects are enhanced when it is used in combination with another antiemetic medication [12].

There are multiple studies showing the effectiveness of a large number of other pharmacologic agents in the prevention and treatment of postoperative nausea and vomiting. Given the large number of studies demonstrating the usefulness of each individual medication, it is not surprising to find that no single agent and no single approach has been shown to be the best pharmacologic agent for this purpose. Many anesthesiologists use a combination of agents, with different mechanisms of action, to increase the efficacy of drug therapy.

The management of postoperative nausea and vomiting is important not only because of its effect on patient comfort and safety, but because of its cost value. Prolonged nausea and vomiting in the postoperative period may result in a delay of discharge from the ambulatory surgery setting, requires the utilization of additional staff and material resources and tends to reduce the overall efficiency of the operating room. The fact that many of the medications used to prevent or treat postoperative nausea and vomiting have become very expensive counterbalances these savings to some degree, and several of the agents used to prevent and treat postoperative nausea and vomiting may produce drowsiness, which may also delay discharge. Further study and justification of their use will continue to be an issue.

28.4 Unintended Intraoperative Awareness

Unintended intraoperative awareness, also called anesthesia awareness, occurs when a patient who is under general anesthesia becomes aware of some or all events that are occurring during surgery and is able to recall those events after surgery. Because neuromuscular blocking agents are routinely used in many patients undergoing general anesthesia, the patient may be unable to communicate with the surgical team when anesthesia awareness is happening. The frequency of anesthesia awareness ranges between 0.1% and 0.2% [13]. Patients may experience auditory recollections, sensations of not being able to breath and/or pain [13]. Over half of affected patients later report experiencing mental distress following surgery, which may include post-traumatic stress syndrome [14].

The incidence of anesthesia awareness is greater in patients to whom administration of the general anesthetic agent is minimized. This may occur when lower doses of general anesthetic are given to reduce potential side-effects, when anesthesia is delivered intravenously or if premature emergence from anesthesia occurs at the end of the procedure.

Although the patient undergoing general anesthesia is extensively monitored, recognition of anesthesia awareness is difficult. Typical indicators of physiologic and motor response, such as an increase in blood pressure, heart rate, and body movement, may be masked by the use of pharmacologic agents. Methods are being developed that measure brain activity rather than physiological responses and may provide a method to prevent and/or detect anesthesia awareness. These methods are thus far less applicable to pediatric anesthesia than to adult anesthesia.

In order to reduce the risk of anesthesia awareness, certain measures have been recommended. These include premedication with amnesic drugs, administration of more than a “sleep dose” of induction agents if they are to be followed immediately by endotracheal intubation, and avoidance of muscle paralysis unless absolutely necessary [15].

28.5 Local Anesthesia

In an effort to reduce the risk of systemic complications, improve postoperative patient comfort, and reduce the postoperative nausea and vomiting associated with general anesthesia, many strabismus surgeons will perform surgery under local anesthesia when possible. Although many of the potentially more serious complications may be avoided with local anesthesia, this approach is not risk free and some risks not present with general anesthesia are introduced. The most common complication of local anesthesia is inadequate anesthesia. An inadequate sensory or motor block may make even the most routine procedure significantly more complicated. The region that is being blocked often dictates the effect obtained. For example, a retrobulbar injection of an anesthetic agent may not effectively block the eyelids. The frontal nerve, which carries sensation from the upper lid, passes through the superior orbital fissure and not through the intraconal space. Supplementation of the initial block through a follow-up injection or through use of a different injection technique may be required in some cases to obtain anesthesia sufficient to safely and comfortably carry out surgery.

28.6 Retrobulbar and Peribulbar Injection

Atkinson provided the classic description of retrobulbar anesthesia for ophthalmic surgery in 1956 [16]. He used a 23-gauge needle which was inserted at the inferior/temporal margin of the orbit. He had the patient look up and away from the injection site. The needle was advanced posteriorly and above the orbital floor until it passed the globe and then it was directed toward the apex. Unsold and coworkers performed a study using computed tomography scans of a cadaver orbit [17]. They investigated the anatomy of the globe and optic nerve when the injection was given as described by Atkinson as well as when the patient was asked to look down and out during the injection. They suggested that having the patient look up and in placed the optic nerve in the direct path of the needle and also

stretched the optic nerve potentially making it easier to penetrate with the needle and/or to enter the subarachnoid space. Based upon their findings, they suggested that the eye be maintained in the primary position during retrobulbar injection.

Peribulbar anesthesia was first described in 1986 by Davis and Mandel [18]. Their original technique was designed not to enter the muscle cone with the needle tip. Given the fact that they used a 31-mm needle, it is likely that they did enter the muscle cone during at least some of their injections. The use of shorter needles with more recent techniques may have helped lower the risk of entering the muscle cone during peribulbar injection along with its associated increased risk of globe and optic nerve damage [19].

Potential complications of both retrobulbar and peribulbar anesthesia are similar. These include globe perforation and injection into the central nervous system via the subarachnoid space surrounding the optic nerve. There has been considerable variability in the documented incidence of globe perforation using either of these injection techniques. In addition, some cases of perforation of the globe probably go unnoticed. In theory, peribulbar anesthesia should be associated with a lower incidence of complications if the muscle cone is not entered during injection. The risk of perforation is higher if the globe is more than 26 mm in length [20]. If the needle is placed into the optic nerve it may cause direct injury to the nerve itself or may allow the anesthetic agent to spread into the central nervous system. Signs of central nervous system spread of anesthetic agent include shivering, drowsiness, apnea, respiratory depression, seizure activity, cardiac arrest, and contralateral decreased vision. It is for this reason, among others, that patients undergoing strabismus surgery utilizing retrobulbar or peribulbar anesthesia should have personnel trained in airway support and cardiopulmonary resuscitation available. Retrobulbar hemorrhage may also occur during the injection (Chap. 24). In most cases, surgery should be postponed until a later date if a significant retrobulbar hemorrhage occurs.

The literature contains many case reports and case series about trauma to the extraocular muscles resulting from retrobulbar and peribulbar injections for cataract surgery. Many of these cases have led to the development of iatrogenic strabismus. Inferior rectus muscle trauma with a resultant restrictive strabismus and large postoperative hypotropia has been well described [21]. Any of the extraocular muscles can be damaged during retrobulbar injection. Care should be taken to avoid injecting in the region of any of the rectus muscles to avoid this complication, though it is probably not completely preventable even with the most meticulous technique.

Chapter 28

Tenon’s space and does not leak out through the incision site. We have not found this to be an absolute requirement. In fact, we have not found it necessary to use a cannula specifically designed for sub-Tenon’s infusion and we have used sub-Ten- on’s anesthesia in cases where a limbal incision has first been made under topical anesthesia and has proven sufficient for surgery. The fluid introduced into the sub-Tenon’s space will often cause some localized swelling of the tissues being operated. The operating surgeon should anticipate this because this tissue distortion can make surgical landmarks more difficult to visualize. We have not found this to be a significant drawback of this anesthetic technique. While the risks of many of the complications seen with retrobulbar and peribulbar injection are reduced with sub-Tenon’s anesthesia infusion, retrobulbar hemorrhage has been reported [22, 23] (Chap. 24). We treated a patient who experienced a retrobulbar hemorrhage during sub-Tenon’s infusion of anesthetic. The vortex vein in the region of the injection was clearly identified and was not the cause of this intraoperative hemorrhage causing us to theorize that the hemorrhage occurred secondary to a ruptured short ciliary vessel following distention of the orbital contents during anesthetic infusion [22].

28.8 Topical Anesthesia

Topical anesthesia has become popular for adult cataract surgery. It provides excellent surface anesthesia. However, true topical anesthesia provides little to no anesthesia to the deeper tissues and produces no akinesia. Infusion of the topical anesthetic into the sub-Tenon’s space may provide some degree of deeper anesthesia and some degree of akinesia. The greatest potential risk associated with the use of topical anesthesia for strabismus surgery is loss of control of the operative situation. If the patient feels pain and/or moves, the possibility of scleral perforation and other complications is increased. The strabismus surgeon who is well versed in the use of topical anesthesia will recognize this potential limitation and prepare both himself/herself and the patient for the steps of surgery where movement by the patient may be particularly problematic.

Each form of anesthesia has its own potential risks and benefits. Knowing the limitations and potential complications of each form of anesthesia will allow the strabismus surgeon to choose the most appropriate form of anesthesia for his/her patient.

In an effort to further reduce the risk of globe perforation with a sharp needle, local anesthesia can be delivered directly into the sub-Tenon’s space using a blunt-tipped cannula. It has been recommended that a small, flexible cannula be utilized to allow for deeper placement of the anesthetic agent into the space. It has also been suggested that a tight seal is essential to make certain that the anesthetic agent remains in the sub-

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The evaluation and management of strabismus, including strabismus surgery, is not an exact science. Underand overcorrection, other unanticipated postoperative alignment difficulties, and recurrence following strabismus surgery are not uncommon. There is as much art in the practice of strabismus surgery as there is science, perhaps more. In many cases, the surgeon may elect to intentionally undercorrect or overcorrect a deviation, anticipating slow postoperative drift of the patient’s ocular alignment. The surgeon must be able to recognize when postoperative alignment is favorable or unfavorable and should be able to advise patients on the expected postoperative course. In this chapter, we will review evaluation and management of the patient whose alignment is not satisfactory following strabismus surgery. The chapter is divided into sections based on possible etiologies of unanticipated postoperative ocular misalignment.

29.1 Prism Problems

Accurate measurement of strabismus requires proper use of the prisms used to quantify the deviation. Failure to correctly use this important tool may lead to unexpected postoperative alignment. Errors in the use of prism are covered in the chapter on preoperative management errors (Chap. 18).

29.2 Unsuspected Myasthenia Gravis

Myasthenia gravis is a condition that can mimic almost any ocular motility disturbance. The diagnosis should be suspected when there is a history of variable strabismus and/or variable ptosis, especially if the problem is worse toward the end of the day. Infrequently, patients will also report systemic weakness, though most patients we have treated with myasthenia gravis have had ocular signs and symptoms only. Myasthenia gravis can present as stable, unchanging, constant angle strabismus without associated ptosis. In this setting, it is virtually impossible for the strabismus surgeon to make the correct diagnosis unless the patient is already known to have the disease.

Certainly it is not unreasonable to perform strabismus surgery on patients with disabling diplopia due to myasthenia gra-

vis [1, 2] and we do so routinely when the deviation appears stable. Others have reported successful surgery for patients with unstable diplopia due to myasthenia gravis [3]. We always assume that the operative outcome and stability of ocular alignment will be less stable in the long term in patients with myasthenia gravis and patients are so advised prior to surgery. On the other hand, we have operated on several patients who had exhibited, both historically and on repeated examinations, stable horizontal and/or vertical strabismus with no signs or symptoms suggestive of myasthenia gravis. Following strabismus surgery, unusual postoperative alignment was noted which included no response to surgery in one patient and a complete reversal of the deviation in another patient despite a correct surgical plan. Further diagnostic testing ultimately disclosed the true diagnosis as myasthenia gravis in these patients. While we routinely ask questions to elicit a history suggestive of myasthenia gravis in patients with strabismus, the surgeon should not be lulled into believing he/she cannot overlook a diagnosis of myasthenia gravis preoperatively.

29.3 Postoperative Duction Limitation

The appearance of a duction limitation following surgery often prompts concern that a slipped or lost muscle has occurred. A mild to moderate temporary duction limitation is not uncommon in several important clinical situations and should be recognized as normal. It is not uncommon, for example, for a patient who has undergone a large recession/resection of the agonist/antagonist pair in an eye to have a duction limitation when looking toward the side of the recessed muscle postoperatively. Recession of a rectus muscle in the face of a large resection of the antagonist produces an obvious tendency for ductions in the involved eye to be limited. With time, the resected antagonist will usually gradually loosen and the duction limitation will resolve or markedly improve. The patient may experience diplopia in the field of restricted gaze and this can be of concern to patients unless its cause is explained.

As stated above, small duction limitations are not uncommon during the first week or so after strabismus surgery, particularly those who have undergone resection procedures. Typically, mild limitation of ductions will be seen in the involved eye and the patient may complain of pain or discomfort

Fig. 29.1. Limited down gaze producing diplopia in the reading position following large recession of the right inferior rectus muscle. (Courtesy of Richard A. Saunders, MD)

with large eye movements. Such apparent duction limitation in this setting is produced by discomfort and the patient’s unwillingness to stress the uncomfortable gaze position. It will usually resolve in a period of days or weeks.

Treatment of strabismus in the setting of thyroid ophthalmopathy often requires very large recessions that seem out of proportion to the size of deviation. The patient in Fig. 29.1, for example, underwent a 9-mm right inferior rectus muscle recession to treat a 16 prism diopter right hypotropia. This resulted in excellent alignment in the primary position and she has achieved comfortable single vision. The trade-off was development of a moderate limitation of down gaze in the right eye with diplopia in the reading position. This can be addressed by additional strabismus surgery consisting of a posterior fixation suture to the contralateral inferior rectus muscle in an effort to balance the down gaze restrictive forces and/or the use of prism in the bifocal segment of her glasses. We typically will recommend a slab-off to induce prism in the reading position in this situation when the reading position deviation is small, and have found this to be an effective solution for many patients.

Unexpected duction limitation due to a slipped, lost, or over recessed muscle or unwanted restrictive forces should be suspected when a duction limitation does not resolve within a few weeks of surgery or when it is large or atypical. Recognition and management of a slipped or lost muscle and of unwanted restrictive forces is covered in detail in Chaps. 23 and 25, respectively.

children with cerebral palsy, developmental delay, mental retardation, and other neurologic problems are more likely to have residual strabismus following surgery. For children who have significant neurologic disabilities, overcorrections are of particular concern. It is not uncommon for us to recommend a slight, empiric reduction in the surgical dose for patients with profound cerebral palsy/mental retardation to reduce this risk.

In the adult population, concurrent neurological diseases that most commonly alter the results of strabismus surgery include myasthenia gravis, Parkinson disease, and multiple sclerosis, though any significant neurological condition can impart a less favorable outcome following strabismus surgery. Despite our impression that patients with neurologic disease do not fare as well following strabismus surgery, strabismus surgery can and should be offered when indicated. We recommend advising patients of their increased risk for bothersome postoperative diplopia, recurrence, and possibly a greater need for additional treatments such as prism and/or additional surgery. Adjustable sutures may be helpful in improving the immediate postoperative alignment in some patients.

29.5Diplopia Associated

with the Chiari Malformation

Acute acquired esotropia and diplopia has been reported as a manifestation of Chiari I malformation [4]. A Chiari I malformation is defined as ectopia of the cerebella tonsils more than 5 mm below the foramen magnum [5]. While primary strabismus surgery can be successful, especially in patients with strabismus as an isolated finding [6], we have had several patients who developed rapid recurrence of their symptoms after strabismus surgery. Correction of a Chiari malformation may sometimes eliminate the need for strabismus surgery [4, 7]. The decision to perform neurosurgical decompression versus primary strabismus surgery should be made on a case-by-case basis in consultation with a neurosurgeon [8].

29.6Spectacle-Induced

Prism and Refractive Issues

Patients with uncorrected, latent hyperopia may exhibit residual esotropia following surgery, the result of accommodative convergence. This condition should be suspected when an intermittent esotropia is seen following surgery. Ideally, cycloplegic retinoscopy should be performed on patients for whom knowledge of the refractive error could alter the surgical procedure or surgical dose.

Patients with concurrent neurological disease often do not experience the same level of surgical success as patients who are not neurologically impaired. In the pediatric population,

Many patients with small angle strabismus are initially treated with prism ground into their spectacles. Many patients may be

unaware or may have forgotten about earlier attempts to treat their strabismus with prism. Patients presenting with or without a known history of strabismus that has been previously treated with optical correction should have their spectacles examined to look for any evidence of prism. Omission of this relatively simple task can result in undercorrection if measurements are made without this knowledge (Chap. 18).

29.6.2 Anisometropia

Patients with significant spectacle-corrected anisometropia often experience diplopia when viewing through eccentric portions of their spectacles, due to induced prism. Such patients may continue to complain of diplopia in the reading position despite achieving single vision in the primary position after surgery. The amount of induced prism can be calculated by using Prentice rule (PD = D × h, where D is the power of the lens in diopters, and h is the distance in centimeters from the optical center of the lens). This rule applies only to a single thin lens spectacle [9], but provides a reasonable estimate for more complex lenses. This problem occurs most commonly in adults who wear bifocals. The patient is often not aware of the problem prior to strabismus surgery because of the presence of diplopia in all positions of gaze. The distressed patient who has single vision in the primary position but vertical diplopia in the reading position can often be easily managed by utilization of a slab-off to eliminate or minimize the induced prism; assuming that the deviation in down gaze is relatively small.

29.7Unsuspected Torsion, Aniseikonia, and Central Disruption of Fusion

A patient with a small angle residual horizontal and/or vertical deviation who cannot fuse with correcting prism following surgery should be suspected of having one of several potential problems including torsion, aniseikonia, or central disruption of fusion.

29.7.1 Torsion

In patients with large angle strabismus, it may be difficult if not impossible to accurately assess torsion preoperatively. Inspection of the fundus for objective signs of torsion may be helpful, but are no guarantee that torsion will be accurately detected. Additionally, it is very difficult to plan a surgical procedure to address torsion that is noted objectively but cannot be accurately measured. Following strabismus surgery, torsion can often be easily measured when there is little or no residual horizontal and/or vertical strabismus. Detection of torsion greater than 9–10 degrees using a double Maddox rod or other appropriate test can help to explain why a patient with small angle residual horizontal or vertical strabismus still cannot fuse despite adequate fusional amplitudes or prism.

29.7.2 Aniseikonia

Aniseikonia is defined as a difference in the shape and/or size of images presented to the visual cortex by the two eyes. Aniseikonia is difficult to detect both historically and during clinical examination. When the degree of aniseikonia is large it can preclude fusion. Despite this, rarely will a patient volunteer that the image size and/or shape between the two eyes is different. The surgeon must specifically inquire if it is suspected and assess the patient for the presence of aniseikonia. An eikono­ meter is an instrument used to detect and measure aniseikonia. To our knowledge, eikonometers are no longer manufactured. Clinically, a simple printed direct comparison aniseikonia test and a computerized test [10, 11] is available to analyze aniseikonia. Treatment is complex and beyond the scope of this textbook. Some simple measures that can minimize aniseikonia in spectacle-wearing patients include prescription of contact lenses and modification of spectacle lenses [12].

29.7.3 Central Disruption of Fusion

Central disruption of fusion should be considered in the patient who cannot fuse despite adequate ocular alignment when there is a history of serious neurologic disease or closed head injury. It has also been reported in patients with a history of prolonged obstruction of the visual axis due to cataracts [13]. In this relatively rare condition, the patient no longer has the ability to fuse even when the eyes are properly aligned. Following seemingly successful strabismus surgery, the patient may have a small angle horizontal or vertical strabismus that is well within standard fusional ranges. Despite this, the patient experiences intractable diplopia. Upon gradual introduction of correcting prism, the patient often notes the images approaching each other and then when the images are about to become superimposed, they separate again now in opposite directions, with crossed diplopia becoming uncrossed diplopia, for example. After ruling out the presence of unsuspected torsion and of unsuspected aniseikonia, such patients are best managed through careful discussion of the problem, and if needed with monocular occlusion or with graded filters [14]. One report suggested that treatment emphasizing awareness of single vision in the periphery can be helpful [15].

29.7.4 Underand Overcorrections

Underand overcorrections are the most common form of unexpected postoperative alignment faced by the strabismus surgeon. Although underand overcorrections occur in a sizable minority of patients, it is unfair to list these unfavorable results as „complications.“ Underand overcorrections are an inherent part of strabismus surgery. While careful preoperative and intraoperative measures can help to reduce the chance of needing repeat surgery, reoperations cannot be completely eliminated.

Medical treatment of an underor overcorrection may require the prescription of spectacles for an esodeviation in a patient with significant hyperopia, prism glasses for small deviations or temporary occlusion to eliminate diplopia. Surgical options may include the injection of botulinum toxin or additional standard incisional strabismus surgery. Assuming no other mitigating circumstances, we prefer to observe patients for a period of up to several months before making a decision to proceed with additional surgery. Often an initial overor undercorrection will significantly improve or resolve during this period of observation. An exception to this general rule might include cases where suspicion of a slipped, lost or stretched muscle exists. In such cases, early surgical intervention is warranted.

References

Chapter 29

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6.Kowal L, Yahalom C, Shuey NH (2006) Chiari 1 malformation presenting as strabismus. Binocul Vis Strabismus Q 21:18–26

7.Defoort-Dhellemmes S, Denion E, Arndt CF, Bouvet-Drumare I, Hache JC, Dhellemmes P (2002) Resolution of acute acquired comitant esotropia after suboccipital decompression for Chiari I malformation. Am J Ophthalmol 133:723–725

8.Biousse V, Newman NJ, Petermann SH, Lambert SR (2000) Isolated comitant esotropia and Chiari I malformation. Am J Ophthalmol 130:216–220

9.Remole A (1999) Determining exact prismatic deviations in spectacle corrections. Optom Vis Sci 76:783–795

10.de Wit GC (2003) Evaluation of a new direct-comparison aniseikonia test. Binocul Vis Strabismus Q 18:87–94; discussion 94

11.Corliss DA, Rutstein RP, Than TP, Hopkins KB, Edwards C (2005) Aniseikonia testing in an adult population using a new computerized test, “the Aniseikonia Inspector”. Binocul Vis Strabismus Q 20:205–215; discussion 216

gery in selected patients with stable myasthenia gravis. Bin Vis Eye Muscle Surg 9:283–290

without making a spectacle of yourself. Triad Scientific, Gainsville, pp 188–189

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3.Morris OC, O’Day J (2004) Strabismus surgery in the management of diplopia caused by myasthenia gravis. Br J Ophthalmol 88:832

4.Lewis AR, Kline LB, Sharpe JA (1996) Acquired esotropia due to Arnold-Chiari I malformation. J Neuroophthalmol 16:49–54

following cataract extraction. J Pediatr Ophthalmol Strabismus 31:391–393

14.Rutstein RP, Bessant B (1996) Horror fusionis: a report of five patients. J Am Optom Assoc 67:733–739

15. Birnbaum MH (1976) Management of intractable diplopia in small angle, non-fusing squint. Am J Optom Physiol Opt 53:424–430

Strabismus surgeons who operate on older children or adults are aware that changes in vision following surgery occur frequently. We surveyed 108 consecutive adult patients undergoing strabismus surgery and asked them to rate the subjective degree of blurred vision they experienced in the first week after surgery on a scale of 1 to 10. The average patient rated their vision as blurred to a level of 3.6 on this scale, with a range of 1 to 10. It is notable that 25% of the patients in our study rated their vision as moderately to severely impaired (>5 on a 10-point scale) in the immediate postoperative period. In many cases, we believed that the blurred vision reported was due to temporary alterations of the tear film. The vision of all of our patients returned to its preoperative level, most within 1 week after surgery. This finding has significant potential implications for many patients who wish to drive and work in the immediate postoperative period and should be discussed with patients prior to surgery.

There are many potential causes of a change in vision following strabismus surgery. Vision may be temporarily altered as a normal and ordinary consequence of surgery as noted above or may be altered due to a surgical complication. Both recognized and unrecognized complications may be responsible for alteration of vision following surgery. In most cases, examination of the patient will allow rapid and accurate diagnosis of the problem.

Complications that may occur during and after strabismus surgery are covered in detail in other chapters, but are briefly reviewed with respect to visual implications in this chapter. Each of the following surgical complications can produce a reduction of visual acuity or other alteration of vision. Obvious complications involving iatrogenic trauma to the anterior segment of the eye may include corneal abrasion, hyphema and corneal or lens perforation. Retinal detachment and vitreous hemorrhage may occur following perforation of the globe, though this is rare. A low lying detachment involving the macula may be more difficult to detect in the early postoperative period in a young child. Endophthalmitis following strabismus surgery is rare. However, patients with endophthalmitis may initially present with altered vision. We have seen two cases of endophthalmitis that occurred after strabismus surgery that presented with mild vitreous inflammation with little or no pain. The complaint of floaters shortly after surgery was the initial presenting symptom in both of these cases. Cases of endophthalmitis reported in the literature have been diagnosed

between 3 and 30 days after surgery. (Chap. 22) Any patient experiencing a sudden, unexpected change in vision during the immediate postoperative period should be evaluated without delay.

30.1 Anterior Segment Ischemia

Anterior segment ischemia can also present in the early postoperative period with reduced vision. Reduced vision may be due to uveitis, cataract formation or a maculopathy related to hypotony of the globe. Ocular discomfort, often with accompanying photophobia, may be present. Anterior segment ischemia is more likely to occur in older patients undergoing simultaneous surgery on multiple rectus muscles. However, it can occasionally occur in susceptible younger patients and in cases where only two muscles have undergone surgery [1, 2]. Slit lamp examination of the anterior segment will often reveal anterior chamber inflammation and pupillary changes consistent with this diagnosis, including anterior chamber cell and flare and ectopia of the pupil with a poor papillary response to light. More severe cases of anterior segment ischemia may present with hyphema, hypotony, and corneal edema. The topic of anterior segment ischemia is covered in detail in Chap. 20.

30.2 Cystoid Macular Edema

Cystoid macular edema has been reported following strabismus surgery in adults. Mohney and Agarwal [3] reported a case of cystoid macular edema in a 75-year-old phakic woman with no history of diabetes. Cystoid macular edema developed 2 weeks following a strabismus procedure that consisted of a recession of the lateral and superior rectus muscles in one eye. The initial complaint was a “film” that appeared to cover her vision in the involved eye. There was no indication of scleral perforation at the time of surgery or on subsequent examination by a retina specialist. Following treatment with topical and sub-Tenon’s corticosteroids, the patient’s vision returned to its measured preoperative level. The authors did not suggest an etiology for the development of this complication.