Generality
Strabismus (or heterotropy) is a defect of convergence of the visual axes of the two eyes; for example, in the cross-eyed subject one eye can look straight and the other deviate inward (convergent strabismus or esotropia), outward (divergent strabismus or exotropia) or upward or downward (vertical strabismus, respectively hypertropia and hypotropia).
Causes
In normal conditions, to align both eyes in a coordinated way and focus them on a single objective, the correct functioning of:
- Eye muscles;
- Cranial nerves (nerve pathways that transmit information to the muscles);
- Upper brain centers (part of the brain that controls eye movement).
The lack of alignment of one or both eyes with respect to the visual axes can be the result of a problem in one of these three levels of the visual system. For example, if the eye muscles are not coordinated due to an unequal traction on one side of the eye or paralysis, a convergence defect may occur.
The exact cause of a squint is not always known. Some children are born with a congenital strabismus, while other patients develop the condition later ( acquired strabismus ).
Refraction errors
The acquired form often arises when an eye attempts to overcome a refractive error (such as myopia, hyperopia and astigmatism). If a child is suffering from an optical defect, the visual stimulus to maintain the correct alignment may be weak (for example, due to blurred or confused vision), so the eye can deviate inwards or outwards while trying to focus. Strabismus caused by refractive errors usually develops in children two or more years of age and tends to be more common in hyperopic patients.
Amblyopia . When the eyes are oriented in different directions, the brain receives two different images; as a result, the brain can ignore the image from the misaligned eye to avoid double vision, resulting in poor development of the visual system of the affected part (cross-eyed amblyopia).
Other causes
If there are parents or siblings with strabismus, a lazy eye (amblyopia) or other ocular pathologies, the risk that the patient may develop this condition, even belatedly, increases. Convergence (or divergence) defects are particularly common among children with conditions such as Down syndrome, cerebral palsy and hydrocephalus. Because of their nature, in fact, these conditions can affect muscle movement and body coordination. Stroke is the leading cause of strabismus in adults.
Trauma and neurological problems are other conditions commonly underlying the onset of the disorder. Strabismus can be caused by a trauma due to: 1) brain damage that alters the control of eye movement, 2) damage to the nerves that control eye movement and / or 3) damage to the eye muscles, directly or secondarily to a trauma to the eye socket.
Although most cases are congenital or caused by refractive errors, strabismus is rarely the result of:
- Tumor, eye injury or other eye diseases (cataract, retinoblastoma etc.);
- Damage to the central nervous system of the fetus;
- Premature birth;
- Graves' disease (overproduction of thyroid hormone);
- In adults: brain tumor, thyroid disease, diabetes, myasthenia gravis and other neurological diseases.
Symptoms
To learn more: Strabismus symptoms
The most obvious sign of strabismus is the uncoordinated movement of the eyes (in practice, they look in different directions). Very often, one eye is directed inwards (convergent strabismus) or the outside (divergent strabismus), while in more rare cases strabismus can be vertical, ie the eyes can deviate upwards (hypertropia) or downward (ipotropia).
The symptoms of strabismus are:
- Poor depth perception;
- Reduced vision;
- In children: compensatory head inclination or chin uplift to minimize diplopia and allow binocular vision;
- In adults: eyestrain, headache, double or blurred vision.
Strabismus can be constant, periodic (appears from time to time), unilateral (always deviates and only the same eye) or alternating (alternately affects the two eyes).
Misalignment of the eyes may induce other visual disturbances, such as:
- lazy eye (amblyopia): when the eyes look in different directions the brain receives two images. To avoid double or confused vision, the brain can therefore ignore the image coming from the non-dominant eye, with consequent poor development of part of the visual apparatus. The correct alignment of the eyes is essential for a good perception of depth and to prevent the onset of sight problems in a squinting eye.
- Reduction of motor skills (in particular, in manual dexterity tasks that require speed and precision);
- Developmental delay (as in walking and speaking).
Diagnosis
Strabismus can be diagnosed during one of several routine eye tests performed at certain stages of child development.
The eye examination, in general, involves the following tests or procedures:
- Patient's clinical history : the doctor will ask the patient for information on symptoms, health problems, drugs in use, environmental factors and any other possible causes that may affect the general health conditions.
- Visual acuity : measures how clearly the patient is able to see. In general, the patient is asked to recognize some letters arranged at a precise distance.
- Refraction : an ophthalmologist will use a refraction test to determine the most appropriate lens characteristics to correct a refractive error (such as myopia, hyperopia or astigmatism).
- Alignment and focus : this exam pays particular attention to the way the eyes concentrate and move together to focus a visual stimulus.
- Eye health : this exam includes several useful procedures to exclude any defect in eye structures that may cause strabismus or contribute to its onset.
In most cases, the tests are performed without the use of eye drops, so that the ophthalmologist can evaluate how the eyes respond under normal circumstances. Once the test is completed, if a strabismus has been diagnosed, the doctor will be able to recommend the most appropriate treatment.
Therapeutic options
Early diagnosis increases the chances of success of therapeutic interventions. This means that when it is detected and treated early, strabismus can often be corrected; vice versa, if the disorder is neglected, sight problems are likely to worsen or become permanent.
Treatment is most effective in very young children.
The therapeutic management of strabismus has three main objectives:
- Improving eyesight;
- Get the correct eye alignment;
- Restore binocular vision;
Treatment options for strabismus include:
- Eyeglasses : constantly wearing corrective lenses helps correct vision disorders (refraction errors), which can cause strabismus, such as myopia, hyperopia and astigmatism;
- Ocular exercises : they consist of particular exercises that help improve the movement of the eye muscles and help the brain and eyes to work in a coordinated way;
- Botulinum toxin injections : represent a treatment option for some types of strabismus. Botulinum toxin can be injected into one of the muscles responsible for the abnormal movement of the eye. The injection weakens the treated area, allowing the eyes to realign temporarily. The effects of botulinum toxin usually last about three months; after this time, the eyes can remain in place or require further treatment. Botulinum toxin injections can cause transient side effects, such as: ptosis (drooping eyelid), involuntary eye movement and double vision.
To improve the associated amblyopia, the child may need to occlude the dominant eye with adhesive patches.
If these treatments are unsuccessful, surgery will most likely be necessary to correct strabismus. Sometimes, the strabismus of the eyes corrected during childhood can appear again in adulthood.
Surgery
Surgery can be used to improve eye alignment and help their coordinated work. During the surgical correction of strabismus, one or more eye muscles are strengthened, weakened or moved to a different position.
In children with strabismus, surgery may be recommended to improve the ability to restore or promote normal binocular vision. In adults, surgical alignment restores a normal aesthetic appearance, but there are many other advantages: the surgery improves depth perception and eliminates or minimizes double vision.
Pre-operative examinations
Preoperative tests help the doctor determine the surgical plan. For example, a specialized investigation called sensorimotor examination is performed before the operation; the test is able to highlight which muscles are contributing to strabismus and on which action is needed to improve eye alignment. Often, both eyes require corrective surgery, even if the strabismus is unilateral.
The procedure
The corrective action of strabismus requires a general or local anesthesia. In both cases, the patient must fast for about eight hours before the procedure.
The eyelids are held slightly open and firm with an eye retractor. The surgeon makes a small incision through the conjunctiva to access the muscles of the eye; these, depending on the operating purposes, are detached from the ocular wall and weakened, strengthened or repositioned with permanent or resorbable sutures. Most operations last less than 1-2 hours. After surgery, with the patient awake, the alignment can be reviewed and, if necessary, adjustments can be made to minimize the risk of over-correction or under-correction. These controls are generally performed the same day or as the day following the surgical procedure. In some cases, more interventions may be needed to correct strabismus as best as possible.
After surgery
Recovery after corrective surgery can take several weeks. Children are usually able to resume their normal activities within a few days. Adults should not drive on the day of surgery or the next. Pain can be managed with analgesics, such as ibuprofen and acetaminophen or with cold packs. After surgery to correct strabismus, it is important not to swim for at least two weeks. The eye will remain red for one or two weeks, rarely longer, especially if it is a re-intervention.
The potential risks
The possibility that a serious complication may occur following surgery may compromise eyesight or eye health is extremely low. However, as with any other type of operation, there is a risk that certain disorders will arise. The risks associated with strabismus correction surgery are:
- Ache;
- Redness;
- Residual misalignment;
- Double vision;
- Infection;
- Bleeding;
- Corneal Abrasion;
- Decreased vision;
- Retinal detachment;
- Complications related to anesthesia.
Results
Surgical treatment of strabismus is a common procedure and most patients will experience a marked improvement in their condition. If the strabismus is severe, sometimes further surgery is needed to optimally align the eyes. Furthermore, it is not excluded that strabismus may recur.
Each clinical case is unique and must be discussed with the ophthalmologist to understand the goals and expectations of surgical treatment. In most cases, early management of strabismus can correct the problem and greatly reduce the patient's suffering.
