Strabismus – Causes, Types, Diagnosis and Treatment
Table of Contents
- Introduction
- Definition and Terminology
- Classification of Strabismus
- Epidemiology
- Etiology and Risk Factors
- Physiology and Pathogenic Mechanisms
- Clinical Manifestations
- Diagnosis
- Treatment
- Complications
- Prognosis
- Conclusions
- Bibliography
- Introduction
Strabismus represents one of the most common ophthalmic conditions, affecting millions of people worldwide. Characterized by an imbalance in the alignment of the eyeballs, strabismus is not merely an aesthetic issue but a complex disorder with significant visual, functional, and psychosocial consequences. Although often associated with childhood, strabismus can occur at any age, and its impact on quality of life can be profound, affecting both three-dimensional visual perception and the patient’s self-esteem and social interactions.
This article aims to provide a comprehensive overview of strabismus, covering the definition, classification, epidemiology, etiology, clinical manifestations, diagnostic methods, and available therapeutic options, with an emphasis on the importance of early diagnosis and interdisciplinary management.
- Definition and Terminology
Strabismus (from the Greek strabismos, meaning „an eye looking obliquely”) defines the ocular alignment disorder characterized by the abnormal positioning of one or both eyes relative to the visual axis. Normally, when a person looks at an object with the head in the primary position, the two eyes are aligned and simultaneously focus the image onto the central fovea of the retina. This alignment is essential for achieving binocular vision, the process by which images from each eye are integrated at the cortical level to produce a three-dimensional perception of space.
In cases of strabismus, one of the eyes deviates from the visual axis, causing a loss of ocular parallelism and impairing binocular fusion mechanisms. Specific terms used to describe strabismus include:
· Eso- – nasal deviation (convergent) of the eye
· Exo- – temporal deviation (divergent) of the eye
· Hyper- – superior deviation
· Hypo- – inferior deviation
Additionally, a distinction is made between:
· Tropia – manifest deviation, detectable with both eyes open
· Phoria – latent deviation, detectable only when one eye is covered
- Classification of Strabismus
3.1. According to the Direction of Deviation
Depending on the direction in which the eye deviates, strabismus can be:
· Esotropia – convergent deviation (the eye turns inward toward the nose), commonly known as „cross-eyed”
· Exotropia – divergent deviation (the eye turns outward toward the temple), commonly known as „wall-eyed”
· Hypertropia – vertical upward deviation
· Hypotropia – vertical downward deviation
3.2. According to the Character of Deviation
Concomitant (comitant) strabismus – the angle of deviation is constant in all gaze positions. This form represents the majority of childhood strabismus cases and is considered a neurodevelopmental disorder of the visual system.
Incomitant strabismus – the angle of deviation varies depending on the direction of gaze. This form is often paralytic or restrictive in nature and may indicate underlying neurological or orbital diseases.
3.3. According to the Time of Onset
· Congenital strabismus – present at birth or within the first months of life
· Acquired strabismus – appears later during life
3.4. According to Constancy
· Constant strabismus – the deviation is always present
· Intermittent strabismus – the deviation appears only under certain conditions (fatigue, inattention, distance viewing)
3.5. Pseudostrabismus
In some cases, the eyes appear to deviate, but alignment is actually normal, due to anatomical features such as prominent epicanthal folds. This condition does not require ophthalmic treatment.
- Epidemiology
Strabismus is the most common ocular condition in the pediatric population, affecting approximately 2–4% of children. Globally, the prevalence of horizontal strabismus is estimated at 1.81%, corresponding to approximately 138.5 million people, of whom 60 million have esotropia and 87.5 million have exotropia.
In the United States, strabismus occurs in about 3% of children. In Romania and Europe, epidemiological data are similar, with an estimated prevalence of 2–5% among children. Studies indicate that, in the absence of treatment, approximately half of children with strabismus develop some degree of vision loss due to amblyopia.
Regarding distribution by type, studies show that exotropia prevalence is higher among Asian populations, while esotropia is more common in older children, those with moderate anisometropia, and those with moderate hyperopia.
- Etiology and Risk Factors
5.1. General Causes
Strabismus can occur as a result of multiple etiological mechanisms:
· Refractive errors – uncorrected hyperopia is frequently associated with accommodative esotropia
· Binocular fusion abnormalities – the brain’s inability to fuse images from both eyes
· Neuromuscular dysfunctions – impairment of the extraocular muscles or the cranial nerves that innervate them
· Anatomical anomalies – structural issues of the orbit or globe
5.2. Histopathological Changes
Histopathological studies of the musculature of patients with strabismus have revealed important structural changes in extraocular muscle fibers. Compared to normal muscles, which show orderly aligned fibers with clear striations and intact sarcomeric architecture, patients with strabismus exhibit:
· Disorganized and atrophied muscle fibers
· Edema and myofibrillar degeneration processes
· Intracellular vacuolization
· Lipid droplet accumulations
· Mitochondrial aggregations
In patients with strabismus and Down syndrome, the changes are even more pronounced, including extensive vacuoles, myofibril disintegration, and intracellular and extracellular deposits of collagen fibrils.
5.3. Genetic Factors
Strabismus has both genetic and non-genetic risk factors. Although strabismus has been known since antiquity to have a familial tendency, its hereditary patterns are complex, and the precise genetic mechanisms have not yet been fully elucidated.
Genome-wide association studies (GWAS) have identified two risk loci and three copy number variants in Caucasian populations. A common genetic variant within the NPLOC4-TSPAN10-PDE6G gene cluster is associated with the risk of strabismus. Mendelian randomization studies have also provided genetic evidence supporting maternal smoking as a causal risk factor for strabismus in offspring.
Causal genes have been identified in congenital cranial dysinnervation disorders, syndromes in which eye movement is limited or paralyzed.
5.4. Non-Genetic Risk Factors
Risk factors associated with strabismus include:
· Family history of strabismus
· Prematurity and low birth weight
· Maternal smoking during pregnancy
· Intrauterine exposure to medications or substances (including alcohol)
· Advanced maternal age
· Refractive errors – high hyperopia and anisometropia
· Retinopathy of prematurity
· Congenital or acquired vision loss
· Neurological conditions
· Genetic syndromes (e.g., Down syndrome)
· Congenital ocular malformations
· Cerebral palsy
- Physiology and Pathogenic Mechanisms
To understand strabismus, it is essential to understand the normal mechanisms of binocular vision. The human eye is positioned anteriorly, allowing gaze in the same direction. Normally, the eyes move in a coordinated manner so that the object being viewed is centered in each eye. Because the eyes are placed at a slight distance from each other, the image from each eye is slightly different. The brain fuses these images to produce a three-dimensional, depth-perceiving image – a process known as stereoscopic or binocular vision.
To maintain alignment, the eyes must move in a coordinated fashion, a process involving twelve different muscles (six in each eye):
· Four rectus muscles – move the eye up, down, right, and left
· Two oblique muscles – perform more complex actions, aiding downward and medial gaze, or upward and medial gaze
Three different cranial nerves are involved in the contraction and relaxation of these muscles, with the main coordination center located in the brain.
In strabismus, the eyes look at different objects, thus sending different images to the brain. Depending on the individual, the brain may handle these images in different ways:
- Visual confusion – rarely, the person sees two different objects in the same place
- Diplopia (double vision) – more common in acute strabismus, the person sees two images
- Suppression – in long-standing strabismus, the image from the deviated eye is ignored or suppressed by the brain
In all these situations, the person has a reduced perception of depth.
- Clinical Manifestations
7.1. Main Signs and Symptoms
The clinical manifestations of strabismus vary depending on the patient’s age, the type, and the duration of the deviation:
· Eye misalignment – the most visible sign, with one eye oriented in a different direction than the other
· Diplopia (double vision) – common in acquired strabismus in adults
· Amblyopia – reduced visual acuity of the deviated eye
· Abnormal head posture – patients may tilt or rotate their head to compensate for the deviation and reduce diplopia
· Frequent blinking
· Intermittent eye closure – to avoid double vision
· Asthenopia – eye strain
· Photophobia – light sensitivity
· Lacrimation and ocular redness
· Reading difficulties – tendency to skip lines
In young children, signs may be more subtle and may include frequent eye rubbing, closing one eye to see better, or turning the head in a particular position to look.
7.2. Psychosocial Impact
Strabismus is not only a medical problem but also a significant psychosocial one. Patients, especially children, may experience:
· Decreased self-esteem
· Difficulties in social interactions
· Occupational and social discrimination
· Negative impact on quality of life
- Diagnosis
The diagnosis of strabismus is primarily clinical and involves a complete ophthalmic evaluation.
8.1. History Taking
The evaluation begins with a detailed history, which includes:
· Complete ocular history
· Age of onset of the deviation
· Frequency and constancy of the deviation
· Triggering factors (fatigue, distance viewing, etc.)
· Family history of strabismus
· Perinatal history (prematurity, birth weight)
· Associated symptoms (diplopia, headache, eye strain)
8.2. Clinical Examination
Head posture assessment – the examination begins with evaluating head posture, as patients may adopt compensatory positions.
Hirschberg test (corneal reflex test) – observation of the light reflex on the corneas. Normally, the reflex is centered on both corneas. In strabismus, the reflex is off-center on the deviated eye.
Cover test – this is the primary diagnostic investigation. It includes:
· Uncover test – uncovering the covered eye and observing the refixation movement
· Cover test – covering the fixating eye and observing the movement of the deviated eye
· Alternate cover test – alternating eye coverage to reveal the total deviation
· Simultaneous prism and cover test (SPCT) – for precise measurement of the angle of deviation
Ocular motility testing – evaluation of eye movements in all gaze directions (version testing, duction testing).
8.3. Additional Investigations
Binocular vision assessment – includes tests such as:
· Lang test
· Bagolini test
· Red glass test
· Maddox rod test
· TNO test for stereoscopic discrimination
Visual acuity measurement – for each eye individually, to detect amblyopia.
Refraction – determination of refractive errors.
Imaging investigations – in cases of incomitant strabismus or suspected neurological involvement, the following may be necessary:
· Computed tomography (CT) or magnetic resonance imaging (MRI) of the orbit
· Brain MRI
· Chest CT
8.4. Differential Diagnosis
Strabismus must be differentiated from:
· Pseudostrabismus – appearance of deviation due to anatomical features (epicanthal folds, wide nasal bridge)
· Nystagmus – involuntary oscillatory eye movements
· Oculomotor palsies – inability to move the eye in a certain direction
- Treatment
Treatment of strabismus aims to achieve multiple objectives:
· Restoring and maintaining ocular alignment
· Eliminating diplopia or visual confusion
· Allowing binocular vision
· Restoring a normal appearance
· Preventing amblyopia
9.1. Medical Treatment
Optical correction – the first step in managing strabismus is evaluating and correcting refractive errors. Proper optical correction is essential, especially in children. Glasses may completely correct some forms of strabismus (especially accommodative esotropia).
Amblyopia treatment – to prevent permanent vision loss in the deviated eye, the following are recommended:
· Occlusion therapy (patching) – covering the healthy eye to force the use of the amblyopic eye
· Pharmacological penalization – eye drops to temporarily blur vision in the healthy eye
· Orthoptic exercises – to strengthen the ocular muscles and improve coordination
9.2. Orthoptic Exercises
Vision therapy consists of eye exercises designed to strengthen the eye muscles and improve coordination between the two eyes. These exercises combine:
· Focusing exercises
· Tracking exercises
· Convergence exercises
9.3. Surgical Treatment
Surgical intervention is indicated when medical treatment fails to correct the deviation or when the strabismus is not accommodative.
Principles of strabismus surgery:
· The procedure is performed under general anesthesia
· The goal is to strengthen or weaken the extraocular muscles to realign the eyes
· Surgical techniques include recession (weakening) and resection (strengthening) of muscles
Success rates – the success rate of strabismus surgery varies in the literature between 35.6% and 93.3%, with the success rate for horizontal strabismus being approximately 60–80%. In some cohorts, 87% of cases were evaluated as having „excellent” results.
Importance of postoperative glasses wearing – after surgery, it is mandatory to continue wearing glasses and performing exercises, as surgery serves both aesthetic and biological purposes.
9.4. Therapeutic Approach According to Age
In children – treatment of the congenital form should be performed within the first 6 months of life to allow normal binocular vision development. Early diagnosis and treatment are essential for a favorable prognosis.
In adults – treatment primarily aims to eliminate diplopia and improve aesthetic appearance.
- Complications
In the absence of treatment, strabismus can lead to multiple complications:
Amblyopia – functional reduction of visual acuity in one eye caused by its disuse during the visual development period. Strabismus is often the symptom that leads to the discovery of amblyopia. In patients with strabismic amblyopia, visual acuity is selectively affected in the central part of the visual field of the deviated eye.
Loss of binocular vision – inability to fuse images from the two eyes, leading to a lack of depth perception.
Persistent diplopia – double vision, especially in acquired strabismus in adults.
Abnormal head posture – developed as a compensatory mechanism to reduce diplopia.
Psychosocial impact – decreased self-esteem, social and occupational discrimination.
- Prognosis
The prognosis for strabismus is excellent if diagnosed and treated early. Factors influencing the prognosis include:
· Age at diagnosis – the earlier the diagnosis, the better the prognosis
· Type of strabismus – accommodative strabismus responds well to optical correction
· Presence of amblyopia – early amblyopia treatment improves prognosis
· Underlying cause – strabismus secondary to neurological conditions may have a reserved prognosis
Without treatment, approximately half of children with strabismus develop vision loss due to amblyopia.
- Conclusions
Strabismus represents a complex ophthalmic condition with significant visual, functional, and psychosocial implications. Although it primarily affects the pediatric population, it can occur at any age, and its impact on quality of life is considerable.
The etiology of strabismus is multifactorial, involving both genetic and environmental factors, with structural changes in the extraocular muscles and anomalies of neuromuscular control. Early diagnosis through clinical examination and specific tests is essential for preventing complications such as amblyopia and loss of binocular vision.
Therapeutic options are varied and include optical correction, occlusion therapy, orthoptic exercises, and surgery, with the therapeutic approach being personalized according to the type of strabismus, the patient’s age, and the presence of complications. An interdisciplinary approach involving ophthalmologists, orthoptists, optometrists, pediatricians, and neurologists is crucial for optimizing therapeutic outcomes.
In the context of contemporary medicine, strabismus is no longer viewed merely as an aesthetic problem but as a condition requiring complex and coordinated management, with an emphasis on early diagnosis and individualized treatment, to ensure patients have normal binocular vision and an optimal quality of life.
- Bibliography
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Article prepared based on current specialized literature and international clinical guidelines.
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