Strabismus: Description
Normally, both eyes are always moved together in the same direction. This ensures that a three-dimensional image is created in the brain. However, this balance can be disturbed so that the visual axes deviate from each other, even though the focus is actually on something specific. This is known as strabismus.
A manifest strabismus (heterotropia) is present if the strabismus is permanent. In latent strabismus (heterophoria), on the other hand, the affected person only squints occasionally. In both cases, different strabismus directions are possible. Strabismus can also be divided into concomitant strabismus and paralytic strabismus according to how it develops.
Manifest strabismus (heterotropia)
A distinction is made depending on how the visual axis is displaced:
- Strabismus convergens (esotropia): manifest inward strabismus (internal strabismus) – the visual axis of the squinting eye deviates inwards.
- Strabismus divergens (exotropia): manifest outward strabismus (external strabismus) – the visual axis of the squinting eye deviates outwards.
- Cyclotropia: manifested strabismus – the squinting eye “rolls” inwards (incyclotropia) or outwards (excyclotropia) around the visual axis.
Latent strabismus (heterophoria)
Latent strabismus occurs, for example, when the affected person is tired or when one eye is covered. Similar to manifest strabismus, a distinction is also made here between the above-mentioned strabismus directions: latent outward (exophoria) or inward strabismus (esophoria), latent elevation (hyperophoria) or lowering of one eye (hypophoria) and latent strabismus (cyclophoria).
You can read more about the symptoms and treatment of latent strabismus in the article Heterophoria.
Concomitant strabismus
In concomitant strabismus, also known as strabismus concomitans, the squint angle remains constant during all eye movements, i.e. one eye “accompanies” the other. Spatial vision is not possible and the visual acuity of the squinting eye is usually weaker. Most cases of strabismus occur in children.
There are various forms of concomitant strabismus. The most common is early childhood strabismus syndrome, which occurs within the first six months of life – i.e. before a baby has learned to see with both eyes (binocular vision). It accounts for the majority of manifest strabismus.
Another form of concomitant strabismus is microstrabismus. In this case, the squint angle is less than five percent, which is why the squint is often only discovered late.
Paralytic strabismus
In paralytic strabismus, also known as strabismus paralyticus or strabismus incomitans, a muscle or a nerve supplying the eye muscles fails. This means that the eye can no longer move fully, resulting in a misalignment.
Unlike strabismus incomitans, strabismus incomitans affects all age groups. It usually occurs as a sudden strabismus without any warning signs. Typical characteristics are double vision and incorrect spatial judgment. If the head is held at an angle to the side, strabismus can often be minimized as the neck muscles bring the whole head into an oblique position so that the eye looks straight ahead, although it looks sideways out of the eye socket.
Strabismus in children
Strabismus: Symptoms
Strabismus in itself merely describes two deviating visual axes and is therefore a symptom. Those affected sometimes have poor spatial vision or perceive double vision.
It is often not so easy to determine whether someone really has strabismus. One possible misinterpretation of strabismus in babies is due to the often low-set eyelids at the transition to the nose (epicanthus). This can give the false impression of deviating visual axes, although the visual axes of both eyes are the same. This is particularly common in Asian babies. This phenomenon is also known as pseudostrabismus. It has no pathological value as no squint angle can be measured.
If vision is lost in one eye, outward strabismus slowly develops over several years. Some people only have outward strabismus when they look into the distance. This is called intermittent outward strabismus.
Symptoms of strabismus
The squint angle depends on the direction of gaze. In some directions of gaze, strabismus is not noticeable, as usually only one specific muscle is affected by the underlying paralysis and not all eye muscles are always involved in all eye movements.
Strabismus: causes and risk factors
Strabismus can have many causes. If strabismus occurs suddenly, nerve damage, infections, tumors or bleeding must be ruled out.
Causes of concomitant strabismus
Corneal injuries and changes to the retina can trigger strabismus concomitans. If vision is lost in one eye, outward strabismus slowly develops over several years.
In children, defective vision in particular must be ruled out – for example in the case of strabismus divergens, as this causes outward strabismus. Birth defects and brain development disorders can also cause strabismus. Premature babies in particular are often affected by this: One in five children with a birth weight of 1250g or less will develop strabismus in later life.
Concomitant strabismus is less common in adults. The possible causes are also more varied here than in children – in the little ones, strabismus can often be attributed to the same causes depending on their age.
Causes of strabismus
Strabismus can develop at birth as a result of brain trauma or defective brain development. Paralysis of individual muscles is sometimes also caused by inflammation of the brain (encephalitis) or an infection during childhood. Measles viruses, for example, can penetrate the brain and cause major damage.
Strokes, tumors and blood clots can also disrupt a nerve pathway and lead to sudden paralytic strabismus. As the wiring of the visual pathway is very complicated and the location of possible damage is varied, detailed imaging (MRI) is often required to clarify the cause of strabismus.
Risk factors for strabismus
Untreated visual impairment, premature birth and lack of oxygen during birth can lead to strabismus. If a person goes blind in one eye during life, this eye no longer actively participates in the visual process, incorrect movements are no longer compensated for and within a few years the affected eye begins to squint.
There is also a family history of strabismus, which suggests a genetic cause.
Strabismus: examinations and diagnosis
During the initial consultation, the patient’s medical history is taken (anamnesis). The doctor may ask the following questions, among others (in the case of babies, the parents are asked):
- Which eye is affected?
- Is the same eye always affected?
- In which direction does the eye deviate?
- How large is the angle?
- Is the angle the same in all directions of vision?
- Do you see double vision?
- Do you have other visual complaints?
In some patients, strabismus is clearly recognizable as such, but in other cases it is not – for example because the squint angle is less than five degrees (microstrabismus). The same applies to the extremely rare strabismus in which one eye is rotated clockwise or anticlockwise around the visual axis.
In general, strabismus can be detected using the following methods:
Cover test
In the cover test, the patient must fixate the center of a cross (Maddox cross) on the wall with both eyes. The ophthalmologist then covers one eye and observes it. The squinting eye reveals itself by an adjusting movement in the direction of the fixed point.
Hirschberg method
From a distance of 30 centimeters, the ophthalmologist observes the light reflexes of his visiting lamp on the pupils of the infant or small child. If the reflexes are not in identical positions, there is a squint angle.
Treatment for strabismus
Strabismus in young children is treated in several stages. If there is an uncorrected visual defect (such as farsightedness), the child is fitted with glasses. In the case of one-sided visual impairment (e.g. clouding of the lens), the underlying disease must be treated accordingly. The ophthalmologist then observes for a few months whether the squint angle disappears.
If this is not the case, the eyes – starting with the weaker one – must be taped shut alternately (occlusion treatment). In this way, amblyopia (weak vision) can be prevented or, if necessary, reduced. This is because the brain is forced to use and train the weak eye despite strabismus. Occlusion treatment can take years – until the visual acuity of the weaker eye has improved sufficiently. The remaining squint angle can then be corrected surgically.
If the accompanying strabismus occurs after the age of six, occlusion treatment is no longer necessary. Otherwise, children, adolescents and adults receive the same treatment as small children.
Treatment for strabismus
In the case of strabismus, the cause must be treated as far as possible (e.g. the stroke). Sometimes a strabismus angle can also be corrected with prism glasses. However, this is rarely the case. Strabismus surgery is an option for some patients.
Strabismus: progression and prognosis
There is no generally applicable prognosis for strabismus. If someone has strabismus due to a one-sided loss of vision, this will not improve on its own. This is not the case with strabismus that occurs as a result of defective vision: if the defective vision is treated quickly, the strabismus can improve within a few months or a few years.
The progression of strabismus is therefore highly dependent on the cause. The better the trigger can be treated, the better the prognosis. The later and more sudden the strabismus occurs in life, the more difficult it is to treat. A prognosis must therefore be made individually by the treating doctor. An interdisciplinary approach involving neurologists, ophthalmologists, pediatricians, radiologists and internists is often required in order to cover all causes of strabismus.