Introductory Chapter: Why, When, and How to Treat Eye Motility Disorders

1. Introduction

Our everyday activities are more and more demanding, and we need not only perfect visual acuity but perfect stereoacuity as well to assess a clear image that is three-dimensional. Our eyes are positioned slightly apart from each other. This position enables our visual system to form two slightly different images of the same target on each eye. These images then travel into the visual part of the brain, where they are united in one single picture with the perception of depth. In order to create this image that is as close as possible to perfect, many conditions have to be fulfilled. A first condition for good visual acuity and depth perception is the formation of a clear image in each eye. The eye, with its refractive structures, has to be perfectly transparent with no refractive error in its structure. If there is a refractive error that is unrecognized and not corrected, the image of the eye will be blurred. In case one of the images is blurred, the brain cannot unite the distorted image from one eye with the image formed in the other good eye. In addition, if one eye is squinting, the visual target of the squinting eye will be different. Instead of two slightly different pictures that can be united in one perfect image, our brain will receive two different images from each eye, which will result in confusion or double vision. The capability of our visual system to perceive two slightly different images with each eye simultaneously is called binocularity [1]. Our brain then unites these two images in one single vision. This process is called fusion. However, even if we unite two slightly different images in one picture, there is still one more step to make this image three-dimensional. Uniting two images and forming a single one with depth is a function of the visual cortex and is called stereoacuity. It is already mentioned that our eyes have to be aligned to be able to perform one single vision of two slightly different pictures. For this precise alignment of the eyes, ocular muscles are responsible. There are six extraocular muscles that are responsible for eye movement. All of the muscles are innervated with cranial nerves. It is important to bear in mind that any process in cranium can affect cranial nerves, their nuclei, or structures adjacent to the nerves. Any of these can cause paresis or paralysis of the ocular muscle. Therefore, when treating patients with strabismus, we have to consider imaging and collaboration with neurologists, internal medicine specialists, or ENT specialists.

Eye motility problems can start in any age group. Sometimes, babies are born with squinting eyes. In such cases, early ophthalmological examination is obligatory in order to exclude anatomical reasons for squinting, such as congenital cataract or retinoblastoma, as well as some of the intrauterine infections. The main goal in the treatment of children with strabismus is to enable normal development of the eye. When children are born, they do not see well; their eyes, visual pathways, and visual cortex are not fully developed. In the cases when the eye cannot produce a good image of the target or cannot transfer it to the brain, the visual cortex of the affected eye will not develop normally. In children with squinting eyes, even if both of the eyes are healthy, the problem arises because the eyes are not aligned and the brain is receiving two different images, so to avoid double vision, the child’s brain involuntary excludes one image and concentrates on the better one. With time, the squinting eye is less in use, and the lazy eye develops [2]. Since the plasticity of the brain and the ability to develop vision is limited to the early years, it is very important to enable normal visual input during the time when the visual system is developing [3]. When we have a small child with a squinting eye, our main goal is to enable proper development of both eyes. For that reason, surgery is not the only method of choice, and we have to prescribe glasses and patch the better eye. In the cases when squinting eye is not treated in childhood, it is possible to perform surgery later in life, but the result of the surgery will be only esthetic because the development of vision has already finished and cannot be improved anymore. The treatment of eye motility disorders is therefore obligatory in children. If an adult person has squinting eye that is not treated, or not successfully treated during childhood, treatment is possible, but it is only cosmetic. On the other hand, in the cases of newly developed eye motility disorders in older age, subjective symptoms of strabismus are usually much more dramatic because the brain of adults cannot adjust to the changes, so one of the most prominent signs is double vision. The line of diagnostics and treatment is different than in children. Excluding the cases when strabismus was present from an early age, every new eye motility problem in adult age has to be meticulously evaluated since the reason can be decompensated strabismus form earlier, but usually, it is some kind of vascular disease, compression, trauma, or, in some cases, disorder of hormonal or autoimmune system [4]. It is important to bear in mind that, in some cases, the reason behind strabismus can be life-threatening. After treatment of underlying disorder, if the eyes are not aligned, surgery is a method of choice. However, even in older age squinting can resolve spontaneously (e.g., recovery of the cranial nerve paresis), so it is recommended to wait for the surgery at least for 6 months. The treatment of eye motility disorders can be conservative or surgical.

2. Conservative treatment

2.1 Treatment by glasses

The first step in the treatment of a patient with strabismus of any age is the correction of refractive error. Refractive correction plays an important role in creating a sharp image on the retina, therefore promoting foveolar fixation. It also enables a proper balance between accommodation and convergence. Recommendations of prescription depend on the type of strabismus, age of the patient, and refractive error.

Patients with strabismus more often require the prescription of a full cycloplegic amount of refractive error, which can create difficulties in the acceptance of new glasses. The most common complaint is blurred vision at a distance in cases where a full amount of hypermetropia is prescribed because the patient is unable to relax accommodation. After several days, accommodation usually relaxes, and the patient, especially a young child, will accept new corrections. Another problem is that in children with strabismus, glasses have to be prescribed even when refractive error is physiological and visual acuity good, which might be difficult for both the parents and the child to accept. It is necessary to explain to parents why correction of the full amount of refractive error is needed. In the cases of accommodative (refractive) strabismus, the change in the deviating angle with and without glasses is dramatic [5].

However, many parents expect that strabismus will disappear completely after the glasses are worn for some time, so it has to be explained that glasses correct the position of the eyes and, by that, in children enable proper development of the visual system, but in cases when strabismus is not only refractive or accommodative, the eyes will squint even when the glasses are worn. However, in some patients with refractive accommodative strabismus, refractive correction can be the only needed therapy (followed by refractive surgery in adult age). Usually, it is only the first but necessary step in the treatment of strabismus, even if the surgery is eventually necessary. It has to be remembered that some studies have shown how full hypermetropic correction in a child’s age can interfere with the process of emmetropization of the eye and should not be carried out routinely unless it is necessary in the cases of strabismus.

A special form of glasses is needed in the cases of accommodative esotropia with convergence excess. In these cases, bifocal lenses are used to reduce the angle for near vision and enable binocularity to develop. Bifocals in child’s age should have the lower segment set high to the level of the pupilar axis. Instead of bifocal lenses, progressive lenses can be prescribed for older children.

Refractive correction can be used in some cases to blur the image in front of the fixating eye by over- or under-correction in order to change fixation preference and hide certain types of deviations like DVD.

In rare cases of insuperable diplopia, glasses or contact lenses can often be used to help patients ignore diplopia. The ability to ignore diplopia varies; in some cases, the reduction of vision can help the patient, but in others, it may require complete occlusion of the affected eye. If the reduction of vision is helpful, over- or under-correction in the glasses can be tried. In cases when complete occlusion is needed to ignore double vision, occlusive patches can be worn over the spectacles or painted lenses incorporated. Less obvious but effective can be neutral density filters. Occlusive correction can be worn in the form of contact lenses or occlusive intraocular lenses as well.

2.2 Treatment by prisms

Prism is a transparent, triangular refracting medium with a base and apex. A prism of one prism diopter (PD) power produces a displacement of 1 centimeter on an object that is situated 1 meter away. Light entering the prism will deviate toward its base, which will shift the image to the apex. On the other side, the prism has other unwanted optical effects, such as color dispersion and distortion of the image, which limits their clinical use. In patients with strabismus, prisms can be used for diagnostic or therapeutic purposes. Prescribing a prism can be a time-consuming and tiring task, but in the end, it can prove very helpful to the patient and worth all the time consumed. It is important to remember that before trying to help the patient with prism correction, all the other preconditions for good visual acuity and comfortable binocular vision have to be fulfilled (objective refractive error correction, exclusion of neurological or other medical conditions, assessment of binocularity). Criteria for the prescription of the prisms in the patient should be subjective problems (double vision, asthenopia, shift of binocular visual field); it is not recommended to prescribe prisms to the patient for cosmetic purposes. Cases that will most benefit are the ones with stress-dependent binocular problems (exo- or esophoria increasing during the day). The effect of the prisms has to be assessed in the office.

There are several types of prisms available.

Prisms can be incorporated into the glasses of the patient for continuous wearing, but care should be taken about the optical side effects of the prisms as well as the unsightly appearance and weight of such glasses. For these reasons, usually, not more than 5 PD in each eye is recommended.

Fresnel prisms are thin, narrow prisms that are arranged on a plastic sheet. In this kind of prism, the continuous surface of the prism is arranged as a series of steps. Fresnel foils are thin (1 mm) and flexible, and they can be cut into pieces that are applied to the back surface of the spectacles. Fresnel prisms are available in the power of 1PD-40 PD.

3. Surgical treatment

The last but not least option in the treatment of eye motility disorders is surgery. When the treatment of the underlying disease is completed, and conservative treatment is finished, surgery is the method of choice [6]. Although the reason for eye motility problems is sometimes situated in the visual areas of the brain or in cranial nerves, we can improve strabismus by performing surgery on extraocular muscles. Adjusting the muscle position and strength makes it possible to improve the position of the eyes and, in some cases, improve mobility. The most often performed surgeries are the weakening by recession (shifting backward tendon of the affected muscle) and strengthening with resection (shortening of the muscle-tendon). The amount of recession and resection is calculated based on the angle of deviation measured before surgery. Recession of 1 mm will correct 2 PD and a resection of 1 mm will correct 4 mm of squinting angle. In some specific cases, when the squinting angle is not the same for near and far distances, there is a possibility of performing some special type of surgery by splitting the muscle into two parts, which will change the lever arm of the operated muscle and correct only the near angle of deviation with much less effect at the far distance angle. To change action of the muscle, it is also possible to put an extra suture far behind the insertion to change the action of the operated muscle (posterior fixation or Faden surgery). In the cases of muscle palsy, it is possible to transpose adjacent well-functioning muscles or their parts next to the affected muscle and, by that, to some degree, improve the function of paretic or paralyzed muscle. There are several techniques available. In the cases when the muscle is too tight (dysthyroid orbitopathy, congenital fibrosis), a part of the affected muscle can be replaced with a prepared bovine pericard.

In conclusion, eye motility disorders can be congenital or acquired in any age group. They can be an isolated eye disease or connected with some systemic disorders or trauma. It is important to bear in mind the whole medical picture of the patient and have close cooperation with other medical specialties.

The treatment of high motility disorders is a demanding but interesting part of ophthalmology, with many conservative as well as surgical options for how to help our patients.