Management of Strabismus in Myopes.

Strabismus in myopes can be related to anisometropia, accommodation/convergence effects, and/or muscle path deviations. This review article highlights management considerations in myopic patients.

INTRODUCTION

Myopia describes the refractive state of the eye when light rays are focused in front of the retina and is typically associated with axial elongation. The prevalence of myopia from various population surveys ranges from 0.8% to 75%.12 Racial and ethnic variations exist with the highest prevalence in the Asian populations.3 Myopia may be associated with increased risk of cataract, glaucoma, macular degeneration, choroidal neovascularization, peripheral retinal degeneration and retinal detachment.3 Myopia may also be associated with abnormal ocular motility4567 and can make evaluation of strabismus clinically challenging.8 Horizontal strabismus associated with myopia can manifest in various forms ranging from infantile, intermittent, sensory, or constant alternating with a small component of vertical deviation. Additionally, acquired forms can occur, such as myopic strabismus fixus that can manifest in two forms, esotropia-hypotropia complex and exotropia-hypotropia complex. Specific conditions such as prematurity and associated retinopathy may predispose to strabismus in the presence of myopia.9

Due to the challenging nature of this strabismus associated with myopia, there are some special considerations prior to surgical corrections which include: (a) Accuracy of measurement of deviation; (b) effect of refractive correction on ocular alignment; (c) role of orbital imaging in specific situations of pathological myopia; (d) different surgical techniques; (e) effect of refractive surgery on ocular alignment and; (f) specific precautions while performing surgery on these eyes. This article focuses on the issues associated with various types of strabismus and its measurement, the role of imaging, and surgical techniques.

Classification of strabismus in patients with myopia

Strabismus associated with myopia can be classified into the following categories: The subsequent discussion provides an overview of each subtype.

Concomitant strabismus

Primary

Intermittent exotropia

Infantile exotropia

Infantile esotropia.

Secondary

Sensory strabismus

Myopia associated with prematurity and retinopathy of prematurity (ROP).

Incomitant strabismus

Myopic strabismus fixus

Esotropia-hypotropia complex

Exotropia-hypotropia complex.

Rare forms

Progressive esotropia with moderate and low myopia.

CONCOMITANT STRABISMUS

Primary

Intermittent exotropia

Intermittent exotropia is the most prevalent form of strabismus worldwide.10 The distribution of refractive errors in this subgroup has been widely reported. Initial studies by Donder et al.11 reported a high prevalence of high myopia in 70% patients with exotropia. However, subsequent studies by Schlossman and Boruchoff12 have shown refractive status to be similar in strabismic and nonstrabismic populations. Caltrider and Jampolsky13 reported a mean refractive error of 0.67 diopters (D) in 15 children with intermittent exotropia at a mean age of 6.9 years. Progression to myopia may occur in the natural course of intermittent exotropia. Ekdawi et al.10 have reported long-term refractive error changes in a series of 184 children with intermittent exotropia over a follow-up period of 20 years. The Kaplan-Meier survival rate of developing myopia was 7% at 5 years, 47% at 10 years, and 91% at 20 years.10 About 40% of children underwent surgical correction in this series.10 However, there was no significant difference in the rate of myopic progression between the operated and unoperated cohort.10 Surgical correction of intermittent exotropia during ages 6–15 years did not alter the rate of myopia progression, thereby suggesting that myopic refractive error might be very common in intermittent exotropia.1014

MYOPIA AND INTERMITTENT EXOTROPIA: CAUSE OR EFFECT?

Traditionally, it is believed that the presence of myopia may be associated with a decreased demand for accommodation and hence lower convergence. This may predispose to an increased risk of developing exotropia.11 Alternately, it is hypothesized that intermittent exotropia may lead to development of myopia due to increased accommodative demand15 and increased convergence may be necessary to control the exodeviation that can contribute to increased accommodation and myopia in intermittent distance exotropia.16 Other risk factors include ethnicity, female gender, younger baseline age at presentation and excessive near work.10

Full correction and/or over correction of any myopic refractive error may help to improve control and alignment in intermittent exotropia.17 Caltrider and Jampolsky13 proposed the use of overcorrecting minus lens therapy to promote fusion in intermittent exotropia by stimulating accommodative convergence. The minimum minus lens power, which reduces the angle of deviation for distance and improves control for distant and near vision is prescribed, which ranges from 1 to 4 D. They reported that good control is maintained even after discontinuation of therapy beyond 1 year.13 Studies by Rowe et al. and Kushner suggest that there is little risk of inducing myopia with this form of therapy.1819 Another form of the use of myopic correction may be in the near segment of patients with convergence insufficiency type of intermittent exotropia. Minus lenses tend to improve convergence and hence reduce excess deviation for near. On average, minus lens therapy may help treat small deviations such as intermittent exotropia/residual exotropia under corrections up to 12 prism diopters (PD) postoperatively.13

Various factors influence the surgical outcomes of intermittent exotropia with success rates varying from 42% to 81%.20 These factors include the magnitude of the preoperative deviation, refractive error, anisometropia, presence of amblyopia, the degree of lateral and vertical incomitance and distance-near disparity. An inverse relation exists between the myopic spherical equivalent and surgical response.21 Patients with higher myopic refractive error tend to respond less favorably to surgery due to the false estimation of the magnitude of deviation through high power minus lens.22 Pineles et al.20 have analyzed the risk factors for failure of surgery in a cohort of 197 intermittent exotropia patients. The risk of reoperation was higher in cases with anisometropia, postoperative undercorrection and lateral incomitance.

In summary, myopia may contribute to the development of exotropia. Intermittent exotropes show progression of myopia with increasing age. A good cycloplegic refraction with correction of myopic refractive error or overcorrection may help establish better ocular alignment. If adequate control cannot be achieved, surgical dosages should be optimized accounting for the preexisting refractive error [Figure 1].

Figure 1

Preoperative (upper panel) and postoperative (lower panel) photographs of a child with intermittent exotropia and myopia of 3.75 diopters. Surgical dosage was decreased by 10% in this case

Infantile exotropia

The prevalence of various refractive errors in infants with exotropia resembles that of the normal age-matched pediatric population. However, a few cases of associated high myopia have been reported previously.23 Similar to intermittent exotropia, the presence of high myopia may influence the accuracy of the measurement of squint. Amblyopia is more commonly strabismic than anisometropic and varies in incidence from 0% to 25%.24 Primary infantile exotropia is a stable constant misalignment. Although spectacle prescription and amblyopia therapy play an important role, surgical alignment (preferably before 24 months of age) is necessary to provide better motor and sensory outcomes. It is preferable to aim for minimal overcorrection (about 5–10 PD) in primary infantile exotropia, as there is a higher incidence of repeat surgery. However, in the presence of associated neurological deficit with possible constricted visual fields, undercorrection of exotropia may be preferable.

Infantile esotropia and myopia

Less than 10% of children diagnosed with essential infantile esotropia can present with myopia.25 Myopia occurs in 3–5% of nonaccommodative esotropia. The patients with high AC/A ratio in this subset require bifocal spectacle correction to control the near esodeviation that is a result of correcting the distance correction of myopia leading to excess accommodative convergence and esodeviation for near.26

Secondary

Sensory strabismus

Another type of strabismus that may be associated with high myopia is sensory strabismus in children with anisomyopia. The differential focus of retinal images can disrupt the fusion mechanism, preclude suppression and cause sensory strabismus. Contrary to the popular belief that anisometropia leads predominantly to esotropia in children, anisometropia is known to cause esotropia or exotropia under the age of 5 years irrespective of the type of refractive error and predominantly exotropia in the older age group.27 Various issues that concern the management of this type of strabismus include timing of surgery, unilateral versus bilateral surgery and risk of recurrence of the deviation. Surgery for sensory exotropia due to anisomyopia or aniso-astigmatism is better restricted to the amblyopic or nondominant eye and involves lateral rectus (LR) recession with medial rectus (MR) resection for exotropia.28 When the deviation is <25 PD single muscle recession may be performed in the presence of myopia. MR resection can correct approximately 3 PD/mm and LR recession can correct 2.8 PD/mm of recession.29 For large angle sensory exotropia >75 PD (rare in anisometropia) in order to avoid surgery on the dominant eye, supramaximal resection of the MR (9–11) can be combined with supramaximal recession of the LR (12–14) without significant limitation of abduction or excessive adduction and palpebral fissure changes.30 Due to the absence of fusion, it is not uncommon for the deviation to recur. Attempts to minimize recurrence include intermittent injection of botulinum toxin, adjustable suture surgery, and planned overcorrection. Park et al.31 evaluated 34 patients, including patient with anisomyopia, and reported success rates of 75% and 88% respectively in the adjustable and nonadjustable groups. An exodrift of 5 PD in the immediate postoperative period is known to occur in these cases. Park et al. recommend aiming for postoperative alignment of orthotropia or 5 PD of esotropia to counter this exodrift that can lead to recurrence of deviation on follow-up. Rayner and Jampolsky32 suggested another approach involving large recession of the LR (to the equator) combined with recession of temporal contracted conjunctiva or T closure to release restrictions along with maximal resection of MR up to 14 mm in cases of large angle sensory exotropia. The authors postulate that the induced postoperative limitation in abduction helps prevent recurrence of the deviation.

Myopia associated with prematurity including retinopathy of prematurity

Myopia and strabismus associated with ROP are increasing and an important consideration for pediatric ophthalmologists currently in practice. Strabismus has been reported in 14.7% preterm infants with birth weight <1251 g in the 1st year of life.33 Infants with ROP have increased rates of myopia and strabismus compared to term born infants. Sahni et al.34 reported a 50% incidence of strabismus in stage 3 ROP. The severity of ROP, small gestational age, and low birth weight are risk factors. Etiology of strabismus can be attributed to the presence of ROP, refractive errors, anisometropia, birth weight and neurodevelopmental anomalies including the severity of intraventricular hemorrhage.35 Both esotropia and exotropia have been reported in various studies; however, esotropia is the more common presentation.36 There are three types of myopia in premature infants. Physiological myopia, myopia of prematurity due to arrested anterior segment growth and myopia secondary to severe ROP. Postcryotherapy and laser, the incidence of myopia increases from 11% at 6 months to 28% at 36 months. Sahni et al.34 reported a positive correlation between strabismus and anisometropia. Cicatricial ROP with or without retinal ablative procedures is associated with higher rates of myopia.37

Severe ROP with macular ectopia results in pseudoexotropia due to positive angle kappa. Preterm infants should be assessed at regular intervals to detect and treat significant refractive errors, evaluate ocular alignment and provide aggressive amblyopia therapy for anisometropia and strabismus.

INCOMITANT STRABISMUS

Horizontal tropia with associated vertical component in myopia

Vertical deviation has been described to co-exist with exotropia in up to 50% of patients.38 These can include small angle deviations associated with horizontal strabismus. Small angle vertical tropia can be present in patients with intermittent exotropia, consecutive exotropia and constant exotropia. These small angle deviations associated with horizontal strabismus may not have any peculiar association with refractive errors. The horizontal muscle offset or small recession of the vertical muscle can take care of these small angle vertical deviations.

However, large-angle vertical deviation associated with horizontal tropia has been described in rare subtypes of strabismus in association with high myopia. These include the esotropia-hypotropia complex and exotropia-hypotropia complex.

Esotropia-hypotropia complex (myopic strabismus fixus)

High myopia can be associated with restrictive strabismus where the globe is fixed in esotropic and hypotropic position is termed as myopic strabismus fixus.39 Various theories have been proposed regarding its pathogenesis-including myopic myopathy, displacement of the muscle paths, etc., Downward displacement of the LR muscle was a mechanism proposed by many authors. However, this could not explain the failure of both abduction and elevation classically noted in these cases. Herzau and Ioannakis40 first demonstrated the deviation of the LR muscle paths intraoperatively. Subsequently pioneering orbital imaging studies by Yokoyama et al.4 and Krzizok and Schroeder.6 have confirmed anomalous muscle paths. According to both groups,46 there is superotemporal dislocation of the posterior portion of the elongated globe from the muscle cone due to increased axial length, which causes inferior displacement of the LR leading to limitation of abduction and nasal displacement of the superior rectus (SR) leading to a limitation of elevation. The inferior displacement of LR leads to weakening of its abducting force and converts it into a depressor whereas nasalization of the SR weakens its elevating force and converts it into an adductor thereby leading to the eye fixed in an esotropic and hypotropic position. Hence, the patient develops a progressive esotropia and hypotropia. Other postulated mechanisms include rectus muscle pulley instability and LR-SR muscle band degeneration in response to globe elongation.41

Notably, this type of strabismus is associated with high axial myopia. The majority of patients tend to have myopia >15 D and axial length >31 mm. This magnitude of myopia tends to be associated with a greater risk of prolapse of the superotemporal portion of the globe.5 According to the personal experience of the authors, a few cases of moderate myopia may rarely present with this type of strabismus complex.

Various surgical approaches have been attempted, ranging from MR tenotomy, disinsertion, recession-resection, Faden's operation, to a partial Jensen procedure. However, these procedures result in suboptimal surgical outcomes.42 These procedures have resulted in residual horizontal strabismus without effect on the vertical deviation. Since, the description of anomalous muscle paths by Yokoyama et al.4 and Krizozk and Schroeder,6 the procedure of choice has been the loop myopexy of SR and LR muscles. The procedure proposed by Yamaguchi et al.43 unites the LR and SR muscle with nonabsorbable suture 15 mm behind the limbus. This procedure normalizes the vector force of these muscles and eliminates the mechanical disturbance in eye movement. It can be combined with MR muscle recession in cases of contracture as demonstrated by a positive forced duction test. The disadvantage of suture myopexy includes muscle strangulation, which may affect anterior ciliary circulation and may rarely cause cheese-wiring of the muscle.

A novel modification of loop myopexy was proposed by senior author (Kekunnaya)44 using a 240 silicone band that is passed through a scleral tunnel and a sleeve to unite the muscles without using anchoring sutures on the sclera [video 1, supplement]. Wong et al.45 reported a similar procedure in 1 of 2 patients where only silicone band loop myopexy without sclera anchorage was performed with good outcomes. However, this procedure risks migration of the band. Hence, the authors recommend fixation of the silicone band to the sclera. Shenoy et al.44 studied the outcomes of silicone band loop myopexy in 26 eyes of 15 patients with myopic strabismus fixus with a mean follow-up of 7.9 months. In this series, 11 patients underwent bilateral loop myopexy whereas 4 patients underwent unilateral loop myopexy. Totally, 16 eyes underwent additional MR muscle recession. At last follow-up, mean limitation of abduction improved to 1.5 ± 1.3 from -2.9 ± 1.2 (P = 0.0000) (on a scale of 0–4, where 0 means no limitation and 4 means inability to abduct/elevate the eyeball beyond the midline), mean limitation in elevation improved to 1.2 ± 0.9 from − 2. 8 ± 1.1, (P = 0.0000), mean esotropia improved from 79.3 ± 32.3 PD to 16.9 ± 17.4 PD (P = 0.0000) and success (deviation ≤ 20 PD) was achieved in 73% (95% confidence interval: 48–89%) [Figure 2a and b]. Mean preoperative hypotropia improved from 8.9 ± 10.1 PD (range, 0–25 PD) to 0.6 ± 1.3 PD postoperatively (range, 0–4 PD), (P = 0.007). In the authors’ experience, silicone band loop myopexy alone effectively corrects approximately 40 PD of esotropia and can be combined with MR recession in cases of tight MR muscle. This procedure eliminates the risk of anterior segment ischemia and has the advantage of being potentially reversible compared to suture loop myopexy. However, there can be a mild risk of foreign body granuloma and extrusion of the silicone sleeve.

Figure 2

Preoperative (a) and postoperative (b) photographs of a female who underwent silicone band loop myopexy with medial rectus recession for myopic strabismus fixus

Exotropia-hypotropia complex

Exotropia associated with hypotropia in high myopia is a rare entity, [Figure 3a and b] initially described on orbital imaging by Krzizok and Schroeder.6 In their series, they reported 2 cases with exotropia-hypotropia with a caudal dislocation of MR muscles. Monga et al.7 reported the largest series of exotropia and hypotropia in 15 patients with progressive strabismus and high myopia with anisometropic amblyopia. Significant hypotropia of >5 PD was noted in 80% cases.7 The mean axial length of the deviated eye was 29.43 + 1.51 mm, which was smaller than the degree of myopia in esotropia-hypotropia complex.7 The mean preoperative exodeviation and hypodeviation were 37 ± 9 PD and 13 ± 6 PD respectively.7 However, normal extraocular muscle paths were noted on imaging.7 An elevation defect was noted in 6 patients (40%).7 Eight cases (53%) were surgically managed with a median follow-up of 7 months.7

Figure 3

Preoperative (a) and postoperative (b) photographs of a female child who underwent surgery for exotropia hypotropia complex

In contrast to esotropia-hypotropia complex, a single specific procedure is not described for this condition. In the study with the largest series that included the senior author (RK),7 horizontal rectus muscle recession/resection with upshift was performed in all patients. Two patients underwent additional IR recession, 1 patient with A pattern underwent additional SO tenotomy, 1 patient underwent additional IO myectomy. Intraoperatively, anomalous temporally displaced IR insertion was noted in 1 patient who underwent IR recession. Loop myopexy uniting the MR and SR was performed in 1 patient as repeat surgery for 16 PD of residual exotropia and 22 PD of hypotropia with successful outcomes.

Successful alignment (<8 PD of exotropia and <5PD of hypotropia) was achieved by surgery on the horizontal rectus muscle alone (37%) and combined with vertical muscles in 62% of cases. Although, the authors could not determine the etiology of this unique complex from clinical and imaging workup, they suggested the need to distinguish the incomitant deviation of high myopia from sensory deviations which can have significant oblique overaction and from cases with contralateral superior oblique palsy.

Rutar and Demer46 suggested that LR-SR band degeneration with advancing age could contribute to downward displacement of the LR muscle. It is possible that similar changes or instability of extraocular muscle pulley due to volume disproportion between orbit and globe and consequent orbital tissue fragility can contribute to the pathogenesis of this strabismus complex. High resolution and advanced imaging studies could aid in the understanding of the underlying connective tissue-muscle biomechanics. The success of the SR-MR loop myopexy in this series further highlights the need to confirm anomalies of extraocular muscle paths and possible use of this procedure as the primary surgical option in these cases.

RARE FORMS

Progressive esotropia with low to moderate myopia

Apart from the above-mentioned well-known entities, there are a few reports4748 of progressive myopia with moderate to low degrees of myopia. These entities probably reflect the varied clinical spectrum of strabismus seen in myopia. Although it is important to understand these conditions, their pathophysiology may not differ from constant exotropia.

Moderate myopia

Acquired progressive esotropia in the presence of moderate myopia was first described by Von Graefe49 and Bielschowsky.50 Webb and Lee.47 reported a series of 26 patients with myopia <10 D with progressive distance esotropia and horizontal diplopia. Ocular motility showed minimal restriction of abduction and normal orbital imaging. Treatment options included prisms (23%), botulinum toxin (27%) and surgical correction (65%) using unilateral LR recession and MR resection with satisfactory outcomes.

Progressive esotropia with low myopia

Progressive esotropia with low myopia is a rare association. Weir et al.48 have reported a case of a 20-year female with <4 D spherical equivalent and axial length averaging 24 mm who presented with progressive esotropia of 80 PD for distance and 70 PD for near. Mild limitation of abduction was reported with normal muscle paths on imaging.48 The patient was treated sequentially with botulinum toxin injection, prisms followed by horizontal rectus surgery with good surgical outcome.48

Refractive surgery for myopia in the presence of strabismus

In the current era, refractive surgery is common surgery with good safety and efficacy. In our practice, myopes with exotropia often present expressing a desire for correction of both conditions. As refractive surgery can affect the binocular fusion mechanism and alter ocular alignment, a detailed orthoptic evaluation is warranted in myopic exotropes. A well-controlled exophoria can decompensate postrefractive surgery.51 However, the effect of refractive surgery on ocular alignment has been variously reported. Godt et al.52 evaluated 13 patients, 7 with myopia, and found no significant difference in exodeviation following refractive correction. However, esodeviation decreased.52 Nemet et al.53 reported complete resolution of exotropia following Laser in situ Keratomileusis (LASIK) for myopic anisometropia - these patients had positive contact lens simulation. In addition, after refractive surgery the prismatic effect of the glasses on the measurement of deviation can also be obviated.

Hence, refractive surgery should ideally precede the strabismus correction and adequate time (about 2–3 months) should be allocated for the realignment mechanism to take control before planning any surgical intervention.54

The only difficulty with planning the refractive correction prior to strabismus surgery may be the torsion of the eye in the supine position that can be aggravated by any type of strabismus. This can usually be overcome by asking the patient look at the internal fixation target. In situations where strabismus surgery has to be performed prior to refractive correction, the fornix approach is preferred as it facilitates the application of an adequate suction force by the microkeratome head.

The basic guideline is to perform a comprehensive strabismus evaluation in all heterophoric patients undergoing refractive surgery. Strabismus surgery if required can follow refractive stabilization and appropriate preoperative counseling should address both issues.

OPTIMIZING THE SURGICAL OUTCOMES

Recording ocular deviation in myopes: Obviating the prismatic effect of spectacle lens

High power spectacle lenses can have a significant prismatic effect [Figure 4]. “Plus lens decrease” and “minus lens increase” the amount of measured exodeviation. This “base in” prismatic effect of minus lens becomes clinically significant with corrective lens power above minus 5 D.22 The deviation thus measured can lead to surgical overcorrection in high myopes. Scattergood et al.22 proposed a simplified optical model to predict the true deviation from the measured deviation. This model approximates the measured deviation to be equal to true deviation increased by 2.5%. Table 1 describes the method to predict the actual amount of deviation as provided by the authors.

Figure 4

The prismatic effect of concave lens spectacle in myopes

Table 1

Guidelines for estimating the amount of the deviation after taking into consideration the amount of the spectacle power

Another recommendation for planning the undercorrection according to the refractive error is based on Hansen's data from 1983 which is presented in Table 2.8

Table 2

Guidelines for planning the undercorrection at the time of exotropia surgery based on the spectacle power of the patient*

Another novel way of more accurately estimating the magnitude of the deviation can be done by replacing the spectacles with contact lenses and then measuring the deviation [Video 2, supplement]. This will obviate the prismatic effect of spectacles and yield accurate readings.

In our institute, we compared the mean deviation obtained in 15 exotropes with refractive error of >3 D myopia (range 2.25–18 D) who underwent a strabismus evaluation with both spectacles and contact lenses (unpublished data). The mean magnitude of deviation was 41 ± 16.6 PD and 40.3 ± 16.8 PD using spectacles for distance and near respectively. The mean deviation using appropriate contact lens correction was 34.2 ± 12.1 PD and 33.1 ± 12.2 PD for distance and near respectively. The mean difference in deviation measured with spectacles and contact lens was 7.8 ± 5.7 PD for distance and 7.2 ± 6.35 PD for near (higher reading obtained with spectacles). Owing to the small sample size, a possible regression analysis could not be performed but the data above suggest a simple method for improving the accuracy of measurements in these patients.

SPECIAL CONSIDERATION WHILE PLANNING SURGERY IN HIGH MYOPES

Myopic eyes are anatomically predisposed to the risk of scleral penetration and perforation due to the thin sclera, which is a consequence of axial elongation of the globe. In addition, high myopes can have pathological changes such as staphyloma, which can further increase the risk. The incidence of inadvertent scleral perforation following strabismus surgery according to recent reports is approximately 0.4%.55 The incidence has reduced due to improvement in surgical techniques and instrumentation. Park et al.56 noted an incidence of 1.77% in their series of 453 eyes. All eyes56 had varying severity of myopia. Horizontal rectus recessions and transposition were the surgical procedures.56 Awad et al.57 reported a 3 times higher incidence of perforation in myopes > 6 PD (P = 0.5) in their series of 4886 procedures.

Keeping the spatulated needle tip under constant view while passing the scleral bite is imperative to avoid scleral perforation. In the event of scleral perforation, a prompt dilated fundus examination should be performed to confirm/rule out choroidal or retinal entry, and adequate retinal ablation with laser or cryotherapy should be considered if indicated.

Anomalous muscle paths and insertion on imaging studies can be visualized intraoperatively in high myopes with the strabismus fixus pattern. Additionally, variations in muscle insertion may contribute to small vertical deviation in intermittent and constant exotropes. Generally, however, there is not enough data to support anatomical variations in extraocular muscle position is higher in myopes.

CONCLUSION

Myopia may affect various aspects of the strabismus evaluation and management. To mitigate overcorrections, appropriate care is required during the measurement of the deviation with respect to the refractive error. Appropriate correction of myopia may help improve fusion and control the deviation. However, moderate and high myopia may be associated with some specific forms of the deviation that require appropriate imaging and specific surgical procedures (loop myopexy) for good surgical outcomes. Lastly, surgeons shall need to exercise more care while operating on these patients to minimize the risk of complications.