Strabismus (squint)

When a person has a squint, their eyes are not looking in the same direction. One eye “moves away”, trying to fix on an object or another person. If this happens, it’s a sign that the brain is no longer able to put the images together correctly. As a result of squinting, the affected person experiences what is known as “double vision.” 

Contrary to popular opinion, squinting (medically called strabismus) is neither harmless nor cute, nor simply a flaw in someone’s appearance. Strabismus is a visual impairment that causes disorders of binocular and three-dimensional vision and requires consultation with an ophthalmologist. 

Acute double vision or acute squinting are ophthalmological emergencies.  


A characteristic of squinting is the misalignment of the eyes, which causes them to look in different directions. The origin of strabismus is usually the deviation of one of the two eye axes from the normal position (i.e. parallel position) inwards or outwards. 

However, a squint is often accompanied by other symptoms: 

  • Burning or shaking eyes 
  • Concentration problems 
  • Increased sensitivity to light 
  • Headaches 
  • Misalignment of the head 
  • Blinking or winking 
  • Literacy problems 
  • Clumsiness when going to grasp objects 

The individual symptoms depend on the respective form of squinting: 

Latent squint (known as heterophoria): In this case, the eye muscles are not balanced and the human brain is able to compensate for this imbalance and cause the two visual impressions to merge, which in turn leads to normal vision. This form of squinting is often imperceptible or only becomes visible when the affected person is exposed to factors that promote squinting – such as fatigue, stress, alcohol, mental stress or a general illness. Accompanying symptoms include headaches, rapid fatigue, blurred vision or double vision.

Concomitant or non-paralytic squint (concomitant strabismus): With this type of squinting, it is not possible for the affected person to independently compensate for the imbalance of the eye muscles, which means that the visual axes of both eyes cannot automatically point towards the same object, which is why the different viewing angles remain even when the eyes move. A mild form of farsightedness is often seen with this type of squint. If the patient only squints with one eye, this eye may also suffer from amblyopia (a lazy eye). Accompanying symptoms may also include head misalignment or shaking eyes. 

Incomitant or paralytic squint (incomitant strabismus): Here, one or more of the outer eye muscles fail completely, with the squinting angle also changing depending on the direction of vision. Paralytic squint is a sudden onset condition, which is why the symptoms are perceived all of a sudden. It is typical to see double vision, with those affected also reporting dizziness and nausea. In addition, many patients try to compensate for the double vision by holding their head crooked.   

If you suddenly develop a squint, you should see a doctor immediately.  


As a squint can be both congenital and acquired, the condition has many causes. On the one hand, it can be due to genetic factors, on the other hand to risk factors during pregnancy or childbirth, but it can also be due to refractive disorders of the eye (including refractive errors) or occur due to injuries.  

In the case of refractive disorders, the light rays penetrating the eye are not focused on the retina and thus cause a blurred vision. The shape of the eye or cornea or age-related stiffness of the lens may limit this focusing capacity of the eye. 

Specifically, the following factors may be responsible for squinting: 

  • The cause of the early childhood internal squint (a form of concomitant squint) has not yet been found. One risk factor is a history of strabismus in the family. Premature birth or uncorrected farsightedness can also cause a baby to squint. The same is true if the left and right eye has different vision problems. Insufficient oxygen during childbirth can be seen as a further risk factor for the early childhood internal squint. 
  • There are numerous possible causes of paralytic squinting, especially as it often occurs suddenly and can affect people of all ages. The trigger for nerve paralysis associated with the paralytic squint, which causes one or more eye muscles to no longer function properly, may be an inflammation or another serious illness such as a tumour or an aneurysm (a bulge in a blood vessel). A circulatory disorder in the area of the eye muscle nerves should also be considered, at least in cases where the patient suffers from diabetes or high blood pressure. 
  • Latent squinting is rarely noticeable, as it is normally compensated by binocular vision (the eyes look parallel at all times). In order to maintain this parallel positioning, the eyes perform barely noticeable compensatory movements (called vergence movements), which are controlled by the visual impression of the brain. If you are tired or have consumed alcohol, you may no longer be able to maintain this parallel state of the eyes, which leads to the affected person squinting and also to experience double vision. 
  • Furthermore, squinting can also develop as a result of an eye disease (known as secondary squinting). This may be due to weakness of the optic nerve, clouding of the lens of the eye or a prolonged inflammation of the interior of the eye. 


Strabismus is often already obvious: The typical eye position leads to a clear diagnosis. However, the accompanying symptoms mentioned above may also give the doctor an indication of strabismus. 

The first step is to check whether there is a secondary squint or a connection with another disease. If the ophthalmologist’s examination reveals no abnormalities – apart from the squint – the refractive power of the eyes is measured and glasses prescribed if necessary. 

If, on the other hand, the doctor diagnoses a paralytic squint, magnetic resonance tomography (i.e. an MRI scan) is usually necessary. The underlying cause of the squint is then treated further by the relevant specialists. 

The ophthalmologist can also use various eye tests as well as orthoptic (ophthalmological) tests to diagnose the squint. A very important test is the cover test, which first covers one eye. The ophthalmologist then checks whether the eye position of the uncovered eye changes or remains the same. If the eye readjusts itself, this may be an indication of the presence of a concomitant squint. In the uncover test – as it is known – on the other hand, the uncovered eye attempts to merge the seen images by means of additional movements. In this case, there may be a latent squint.  

If you develop a squint suddenly, on the other hand, you have to see an ophthalmologist as soon as possible. The doctor can then carry out the appropriate tests and discuss suitable treatment.  


The good news is that squinting can generally be easily managed by initiating therapy at an early stage. 

If strabismus is detected in childhood or toddlers, conservative therapy is usually sufficient to treat the squint and the affected children therefore usually do not have to have surgery. For adults, too, conservative treatment is preferable to surgery. 

  • If children have a squint, they are prescribed both glasses and eye training exercises. As part of this eye training (also: fusion training), the children learn to correct the double vision by reuniting the split images. In addition, the doctor may also attempt to treat any incipient visual impairment using occlusion therapy. Both eyes are alternately covered with a patch, which remains stuck over the respective eye for a certain period of time. As a result, the visually impaired eye is “forced” to exert more effort, whereby the children learn to see well and learn to no longer squint. 
  • If an adult suffers from concomitant strabismus, they also prescribed therapy to prevent stereo vision and restore full visual acuity. In the case of double vision, a pair of glasses with prism lenses is usually used. These glasses have a matt film affixed to one of the lenses in order to alter the incidence of light rays and prevent stereo vision. Adults may also be prescribed fusion training. If, on the other hand, the patient has a paralytic or incomitant squint, the underlying cause is treated. In rare cases, this will resolve itself. Sometimes, however, the squint remains even after treatment, which is why glasses with prismatic lenses are the material of choice. 
  • For many years, strabismus has been treated worldwide either with eye muscle surgery or, more rarely, with an injection of botulinum toxin (known under the brand name Botox) into the eye muscles. The aim of both interventions is to make the eyes parallel again and reduce the squint angle. In a squint operation (also known as strabismus surgery), the surgeon alters the mechanism, mobility and position of the eyes by changing the outer eye muscles. Surgery – for children under general anaesthesia, for adults under local anaesthesia – can compensate for the squint, but relapse and the development of a new squint cannot be ruled out. 


Regular check-up appointments should be made to ensure that the progress of each treatment is documented. After an eye operation, it goes without saying that you need to attend the agreed follow-up appointments and strictly follow the instructions of your treating doctor. 


What other types of squint are there?

In medicine, squint refers to the persistent or recurrent misalignment of one eye (the monolateral strabismus) or both eyes (alternating strabismus). A characteristic of squinting is the misalignment of the eye or eyes, as the gaze always goes in different directions. 

Doctors also refer to squinting as “primary strabismus”, which means the squint occurs without any other eye disease. By contrast, the term “secondary strabismus” refers to squinting that is caused by another disease. For example, blindness of an eye – regardless of its cause – can lead to squinting because the blind eye no longer participates in the process of vision.  

Important information: If double vision occurs suddenly in conjunction with symptoms such as drooping corners of the mouth or one-sided paralysis, this may also indicate a stroke. 

Why do people squint in the first place?

People squint when there is an imbalance in the eye muscles. There are six external muscles of each eye that control the movement of the eyeball; in addition, there are two internal eye muscles that control the size of the pupil and vision at a distance. While squinting, the eye muscles are imbalanced.

With a squint, in which directions can the eyes be turned?

In medicine, a distinction is made between the inward squint (convergent squint or esotropia), where an eye is turned inward towards the nose, and the outward squint (divergent squint or exotropia). If the eye is turned up or down, this is called vertical strabismus. And when the eye is rotated around the visual axis, the specialist may describe this as a rolling squint. 

What are the clinical manifestations of squinting?

  • Manifest or concomitant squint (medical term: concomitant or non-paralytic strabismus) and paralytic squint (medical term: incomitant or paralytic strabismus) are characterised by the fact that the lines of sight of both eyes are constantly deviating from each other. These forms are treated with medication. Squinting often occurs as early as infancy or childhood. Paralytic strabismus is associated with paralysis of the eye muscle, which is usually caused by a previous infection or injury. 
  • Latent strabismus (medical term: heterophoria) often simply occurs under stress and therefore usually remains untreated, especially as the brain is still able to compensate for squinting by a slight impairment in both visual positions.  

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