© Fotograf: Alessandro Grandini

Crossed eyes (strabismus)

What is a strabismus?

Our vision is a complex interaction of several processes. Normally, each eye produces its own image, which is superimposed on that of the other eye, but is not a 100% match. The brain merges both images into a threedimensional visual impression. This finely tuned process can only work if both eyes are parallel and are thus looking in the same direction1.

The two images deviate from each other to such an extent that they can no longer be merged by the brain in a congruent manner.3 As a result, visual performance can be impaired, with double vision occurring (diplopia) or the brain suppressing visual information from one eye (amblyopia).1,3

In Germany, a strabismus is one of the most common eye disorders in children and adolescents. About 4% of all 0–17-year-olds are affected by this disorder.4

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What causes a strabismus?

Strabismus can occur independently, without another underlying disease (primary) or as a symptom of another eye disease (secondary).1 The possible causes are varied and cannot always be clearly identified. The most common causes include:1,3,5–7

  • Imbalance of the eye muscles
  • Genetic factors
  • Congenital visual impairment or eye disease
  • Lens opacities
  • Disorder of the eye muscles
  • Eye injury
  • Injury or disease of the brain

What are the different forms of strabismus?

Basically, a strabismus can be divided into latent strabismus, concomitant strabismus and paralytic strabismus.6 Microstrabismus is a strabismus with a small-angle deviation.1,3

Latent strabismus (heterophoria) is the most common type (affecting about 70% of the general population), where the eye muscles are not in balance with each other. Normally, the brain is able to compensate for this quite well. This strabismus only manifests itself in the event of overtiredness, exhaustion or alcohol consumption, for example. There are usually no symptoms.1

The term concomitant strabismus is used when there is a permanent eye misalignment which may affect only one or both eyes.8 The deviation of the squinting eye can occur in all directions of gaze:1,3

  • Convergent strabismus is an inward turning strabismus, or esotropia,
  • Divergent strabismus is an outward turning strabismus, or exotropia,
  • Vertical strabismus is an upward or downward strabismus, or hypertropia/hypotropia,
  • Intorsional strabismus the eye rotates around the visual axis.

Whereas the degree of deviation of the eye is always constant in the case of a concomitant strabismus, the deviation in the case of a paralytic strabismus varies depending on the direction of gaze; for example, it may be more pronounced when looking to the left than when looking to the right. A paralytic strabismus is due to paralysis of the eye muscles.1,6 It is typically characterised by the perception of double vision or head tilt.1

What is the significance of strabismus in new-borns and infants?

In the first weeks of life, a new-born baby cannot yet fully coordinate the movements of its eyes. Uncoordinated eye movements and associated intermittent strabismus may occur during this phase as part of normal development. In the course of their development, children learn to control their eye movements and strabismus should no longer occur.9,10 A constant strabismus should be examined by a doctor

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How does strabismus manifest?

In some people, the misalignment of the eyes is clearly visible, while in others it is only very slightly noticeable. Apart from the visible misalignment of the eyes, other symptoms may also be present:1,3,6,7,11,12

  • perception of double vision
  • disturbance of spatial vision
  • head malposition
  • eye trembling
  • head tilt
  • visual impairment
  • headache
  • concentration difficulties

How is strabismus diagnosed?

In both children and adults, any strabismus that appears beyond the first few weeks of life should be examined by a doctor. An eye specialist can diagnose an abnormal eye position using a wide range of examination methods.1,6,12,13

  • Eye mobility. The position and the mobility of the eyes are examined.
  • Cover test. Eye movements can be examined by covering the eyes unilaterally or alternately.
  • Corneal reflex images. With the help of a light source, the position of the light reflexes on the cornea can be determined, which is equal on both sides in healthy eyes.
  • Visual acuity measurement. Visual acuity measurement is possible from the age of two years.
  • Fixation test. The ability to look specifically at an object is examined through an ophthalmoscope. Children should be able to focus from the age of about 3 months.
  • Examination of the lens. The function and condition of the lens are tested.
  • Examination of the interaction of both eyes and the brain. The presence of double vision and the ability to see in three dimensions (from the age of 3 years) can be examined using various tests.

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How is strabismus treated?

The treatment of strabismus depends on its type and cause. A latent strabismus does not usually require any special treatment as long as there are no symptoms, whereas in the case of persistent (manifest) strabismus, early ophthalmological examination and treatment can prevent secondary symptoms such as visual impairment.1 The following therapeutic options are available:

  • Glasses. Prescribing glasses can compensate for a visual impairment.1,13
  • Occlusion. Covering the healthy eye can train the squinting eye.1,13
  • Surgery. Certain types of strabismus may require surgery. A possible side effect of surgery is a dry eye.1,13,14
  • Fusion exercises. Exercises to bring double images together are only recommended in individual cases.1

References

1. Grehn F. Augenheilkunde. Springer Berlin Heidelberg 2019.

2. Leydhecker W, Grehn F. Augenheilkunde. Springer Berlin Heidelberg 2013.

3. Lang GK. Augenheilkunde. Thieme 2014.

4. Schuster AK, Elflein HM, Pokora R, Urschitz MS. Kindlicher Strabismus in Deutschland: Prävalenz und Risikogruppen. Ergebnisse der KiGGS-Studie 2017.

5. Gräf M, Lorenz B. Strabismus. Monatsschr Kinderheilkd 2015; 163: 230-240.

6. Weber P, John R, Konrad K, Livonius B v., Ruple B, Schroeder A, Stock-Mühlnickel S, Karch D. S2k Leitlinie. Visuelle Wahrnehmungsstoerungen 2017.

7. Berufsverband der Augenärzte Deutschlands e.V. (BVA), Deutsche Ophthalmologische Gesellschaft (DOG). Leitlinien von BVA und DOG. Leitlinie Nr. 26 b, Nichtparetisches Schielen. http://augeninfo.de/leit/leit26b.htm, Zugriff: 30.03.2020.

8. Walter P, Plange N. Basiswissen Augenheilkunde. Springer Berlin Heidelberg 2016.

9. Baumann T, Adam O. Atlas der Entwicklungsdiagnostik. Vorsorgeuntersuchungen von U1 bis U10/J1 ; 52 Tabellen. 2., völlig überarb. und erw. Aufl. Thieme, Stuttgart 2007.

10. Flehmig I (Hrsg. 2007). Normale Entwicklung des Säuglings und ihre Abweichungen. 7. unveränderte Auflage Thieme, s.l., 2007.

11. Oestreicher E. HNO, Augenheilkunde, Dermatologie und Urologie für Pflegeberufe. Thieme 2003.

12. Berufsverband der Augenärzte Deutschlands e.V. (BVA), Deutsche Ophthalmologische Gesellschaft (DOG). Schielen und Amblyopie 2017.

13. Goebeler M, Walter P, Westhofen M. Augenheilkunde, Dermatologie, HNO in 5 Tagen. Springer Berlin Heidelberg 2018.

14. Berufsverband der Augenärzte Deutschlands e.V., Deutsche Ophthalmologische Gesellschaft e.V. Leitlinie Nr. 14a Uveitis anterior 2010.