Approach
All patients with manifest strabismus, and some with intermittent strabismus, will experience a decrease in binocular visual functions (e.g., depth perception and stereopsis) that lead to a characteristic history and clinical findings. Although evaluation usually requires only noninvasive tests that can be done in the physician's office, further investigations may be required to evaluate underlying causes.[2][17]
If strabismus occurs acutely, urgently refer the patient to an ophthalmologist for thorough evaluation.[2] Paretic forms of strabismus may have neurologic causes, including intracranial mass lesion, aneurysm, elevated intracranial pressure, or central nervous system infarction or inflammation. Children with craniosynostosis and craniofacial syndromes also require evaluation by an ophthalmologist to detect possible strabismus. Although adults and children may share the same underlying pathologies, there can be subtle differences in presentation.[18][19]
Clinical history
A comprehensive history should be taken, including relevant presenting features, medical and surgical history, and family and social history.[18][19]
Presenting features:
Parents and other close contacts may note eye misalignment, or abnormal eye movements.
Although the eyes of full-term babies may show occasional misalignment within the first 2 months of life, constant misalignment is abnormal.
Diplopia is often absent in children with strabismus due to suppression.
Some children (particularly those with intermittent exotropia) intermittently close one eye, especially when outside in the sunlight.
Adult patients usually complain of diplopia and strabismus, and may complain of ocular discomfort with "eye strain" or headaches in intermittent or latent strabismus. They may also note a "pulling sensation" and be aware of when the eyes are misaligned.
Past medical history:
Associated conditions, such as Graves disease, orbital fractures, craniofacial syndromes, cranial dysinnervation syndromes, craniosynostosis, myasthenia gravis, or supranuclear palsies.
A history of partial or successful treatment for strabismus may also be present. In adults, check for a history of chronicity and review past clinical, surgical, and imaging records.
As preterm birth and retinopathy of prematurity increase the risk of developing strabismus, a detailed birth history should be taken.
Family and social history:
Approximately 30% of patients with strabismus have a relative who is affected, and most families are concordant for the type of strabismus (e.g., esotropia or exotropia).[13]
The social history may uncover learning difficulties, behavior problems, or issues with social interactions.
Adults and older children may describe more specific psychosocial issues, such as being uncertain where to look or being concerned that other people do not know where they are looking.
Clinical exam
Every patient with strabismus requires a complete eye exam by an ophthalmologist.[18][19] The referring physician should perform a basic exam, which in children, includes assessing the red reflex by direct ophthalmoscopy.[20]
Inspection of the eyes:
This may reveal manifest strabismus, but the result should be interpreted with caution in patients with epicanthal folds, a wide nasal bridge, or eyelid abnormalities because these features may falsely suggest ocular misalignment, particularly in young children and in the Asian population.
In adults, inspect the ocular surface for conjunctival scars and scleral exposure behind anatomic insertions (e.g., possible muscle recession). Also inspect the interpalpebral fissures for evidence of prior vertical or horizontal rectus muscle resection (smaller fissure) or recession (larger fissure).
Sensory testing or anatomic evidence of torsion during indirect ophthalmoscopy can indicate torsion, particularly with vertical strabismus.
Evaluation of diplopia:
In strabismus, the diplopia is binocular (i.e., present only when both eyes are open).
If diplopia is present when one eye is closed, it is most likely not caused by strabismus.
Horizontal diplopia is indicative of esotropia or exotropia, while vertical diplopia is indicative of hypertropia or hypotropia.
May be absent in children with strabismus due to suppression.
Assess fusional control of exodeviations at both distance and near fixation.
Assessment of visual acuity:
Assess manifest refraction to identify barriers to binocular alignment or fusion.[21]
Strabismus in children may cause suppression (the brain ignores the visual input of the non-fixating eye) that leads to amblyopia (decreased vision in an anatomically normal eye) and irreversible loss of vision if not adequately treated.
Although amblyopia generally develops in children below the age of 7 years, there is no universally accepted upper age limit, and it can occur at any age prior to the development of visual maturity.
Amblyopia does not usually occur in visually mature patients.
Accommodative esotropia is frequently related to hyperopia (farsightedness).
Strabismus may be secondary to poor vision (sensory strabismus).
Neurologic exam:
A thorough neurologic exam, including a detailed assessment of the cranial nerves, should be undertaken in patients suspected of having acquired paralytic strabismus secondary to a cranial nerve or supranuclear palsy, myasthenia gravis, or acute-onset strabismus due to an intracranial process (e.g., a mass lesion, elevated intracranial pressure, CNS infarction, or CNS inflammation affecting the cranial nerves).
Paralytic strabismus secondary to oculomotor nerve (cranial nerve III) palsy is characterized by exotropia, hypotropia, ptosis, and possibly mydriasis.
Paralytic strabismus secondary to abducens nerve (cranial nerve VI) palsy is characterized by esotropia and decreased abduction of the involved eye.
Paralytic strabismus secondary to trochlear nerve (cranial nerve IV) palsy is characterized by hypertropia, elevation in adduction, and possible excyclotorsion of the involved eye.
Specific eye tests
There are a number of specific clinical tests (light reflex tests: Hirschberg and Krimsky, cover/uncover tests, and tests with dissimilar targets) undertaken by the ophthalmologist to diagnose and evaluate strabismus.[2][18][19]
The initial test of choice is the cover test. As this test requires a high degree of cooperation from the patient, it may not be appropriate in young children.
As this test requires a high degree of cooperation from the patient, it may not be appropriate in young children.
Both eyes fixate an object and one eye is covered. If the fellow eye makes a re-fixation movement, manifest strabismus is present, but if the fellow eye makes no movement, no manifest strabismus is present. The direction of the re-fixation movement is opposite to the direction of the deviation (e.g., if the fellow eye makes an outward movement when the other eye is covered, it is deviated inwards, and an esotropia is diagnosed).
A positive cover test denotes the presence of a manifest strabismus and a negative cover test denotes the absence of a manifest strabismus.
If a manifest strabismus is detected on the cover test, a simultaneous prism and cover test (SPCT) is carried out to measure the angle of the manifest strabismus.
A prism is introduced in front of the deviating eye and, simultaneously, the fixating eye is covered.
If the prism neutralizes the strabismus, no re-fixation movement of the deviating eye is noted.
The strabismus is described according to its quality (e.g., esotropia or hypertropia) and size (prism diopters in the US and degrees in Europe).
If the cover test is negative (i.e., absence of a manifest strabismus), the presence of a latent strabismus is indicated by a positive uncover test (UCT).
First the cover test is performed and, if this is negative, the cover is removed. If the uncovered eye makes a re-fixation movement, latent strabismus (also known as phoria) is present.
Following the cover test and UCT, the alternate prism cover test (APCT) is carried out to measure the angle of the manifest plus latent strabismus.
A cover is held in front of one eye and left there for several seconds while the patient fixates an object (e.g., reads letters on a chart). The cover is then moved swiftly to the fellow eye and left there for several seconds.
Re-fixation movements of the uncovered eye are noted and can be neutralized with prisms.
In patients unable to cooperate with cover testing, the Hirschberg test can be carried out.
A light is shone on the eyes from an arm's length away while the patient looks into the light (fixates the light target).
If the reflection of the light is symmetrically centered on the cornea in each eye, no manifest strabismus is present. If it is decentered in one eye, manifest strabismus is likely.
False-positive results occur if the visual axis does not coincide with the pupillary axis (e.g., in ectopia of the macula).
False-negative results can be caused by lack of fixation.
This test can be used to estimate the angle of the strabismus. The angle of the strabismus is approximately 7 degrees or 15 prism diopters for each 1 mm displacement of the light reflex relative to the corneal center.
Following the Hirschberg test, the Krimsky test is carried out to measure the angle of misalignment.
If the reflection of a fixation light is decentered on the cornea of one eye (i.e., the deviating eye), a prism is held over the fixating eye. This will induce a conjugate movement of both eyes (version) in the direction of the apex of the prism.
The correct prism strength is reached when the position of the corneal light reflex is symmetric between both the eyes.
Centering of the corneal light reflex with a prism over the fixating eye measures the angle of strabismus.
Following the above tests, another 3 tests (version, duction, and forced duction tests) are carried out to look for restrictive or paralytic strabismus.
Version testing examines conjugate eye movements (versions). A target is moved in front of the eyes and the patient is instructed to follow it with both eyes. The eyes are observed for any incomplete excursions (e.g., incomplete abduction in abducens nerve [cranial nerve VI] palsy). Double vision noted by the patient when looking in a certain direction suggests misalignment.
Duction testing, which examines the movements of each eye individually, is then carried out. If the excursions of one eye are incomplete on version testing, the fellow eye is covered and the excursions are again examined. If the abnormal finding persists, restrictive or paralytic strabismus is likely.
Forced duction testing allows differentiation between mechanical and innervational (i.e., restrictive and paralytic strabismus) causes of abnormal duction. In cooperative patients, this test can be undertaken in the office, otherwise anesthesia is necessary as it is uncomfortable and painful. First, the ocular surface is anesthetized with a topical anesthetic. The eye is then grasped with forceps or a cotton-tip applicator (the latter method is more comfortable, but less precise) and pushed in the direction of gaze being tested. If the patient is awake, they are instructed to look in this direction and the examiner feels for any restriction, noting its intensity and location.
Investigation of underlying cause
Neuroimaging (MRI or CT of the brain and orbits) may be required for the evaluation of acquired paralytic and restrictive strabismus to identify the lesion and help define the underlying mechanism.[17]
In strabismus secondary to cranial nerve palsy, a mass lesion must be ruled out with an MRI of the brain, depending on the associated clinical findings. If oculomotor nerve palsy is diagnosed, an MRI/MRA (magnetic resonance angiogram) is needed to rule out an aneurysm of the posterior communicating artery (sometimes, a CT angiogram can be necessary).
In orbital trauma or Graves disease, a CT scan of the orbit helps identify the (e.g., entrapment of an extraocular muscle or perimuscular orbital tissue in an orbital fracture, enlarged extraocular muscles in Graves disease).
In myasthenia gravis, a thymoma should be ruled out by chest CT.
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