bubble_chart Overview

There are no obvious organic sexually transmitted disease changes in the eyes, or there are organic changes and refractive errors, but vision decline that does not correspond to the pathology and cannot be continuously corrected, or corrected vision below 0.9, is considered amblyopia, which can occur in one or both eyes. The most important type of amblyopia is strabismic amblyopia, with more than half of amblyopia cases related to strabismus. Symptomatically, strabismus is an abnormal eye position, while amblyopia is an abnormal vision. Their relationship is like the two wheels of a carriage, with refractive errors acting as the axle, connecting the two wheels. Amblyopia can lead to strabismus, and strabismus can cause amblyopia. In addition to strabismic amblyopia related to strabismus, amblyopia can also be caused by refractive errors and anisometropia. For those with refractive errors, vision often cannot be improved even with increased lighting or enhanced contrast of the fixation target if the condition is not corrected.

bubble_chart Etiology

1. Strabismic Amblyopia

Occurs in one eye, where the child has or had strabismus, commonly seen in patients with constant unilateral strabismus onset before the age of four. Due to the cerebral cortex actively inhibiting visual impulses from the deviating eye, prolonged inhibition leads to amblyopia. Visual suppression and amblyopia differ only in degree; suppression can generally be lifted when the deviating eye is injected, whereas amblyopia involves persistent visual impairment. The earlier the onset of strabismus, the faster the suppression occurs, and the more severe the amblyopia.

2. Anisometropic Amblyopia

Results from unequal vision between the two eyes, where retinal image size and clarity differ. The eye with higher refractive error produces a larger and blurrier macular image, leading to insufficient fusion reflex stimulation and failure to achieve binocular single vision, thus causing passive suppression. A refractive difference of 3.00D or more often results in amblyopia and strabismus in the eye with higher refractive error, leading to both passive and active suppression. The depth of amblyopia is not necessarily related to the degree of anisometropia but is associated with fixation type, with eccentric fixation causing more severe amblyopia. This type of amblyopia is similar to strabismic amblyopia—functional and reversible. Clinically, it can be difficult to determine whether amblyopia is primarily due to anisometropia or secondary to strabismus. Early detection and timely corrective glasses can prevent this condition.

3. Refractive Amblyopia

Usually bilateral, occurring in children or adults with uncorrected high myopia, hyperopia, or astigmatism. Most cases involve myopia ≥6.00D, hyperopia ≥5.00D, or astigmatism ≥2.00D (or combined astigmatism). Both eyes have equal or similar vision without binocular fusion dysfunction, so no macular functional suppression occurs. With timely and appropriate glasses, vision can gradually improve.

4. Deprivation Amblyopia (Form-Deprivation Amblyopia)

In infancy, due to ptosis, corneal opacity, congenital cataracts, or prolonged postoperative eyelid occlusion, light stimulation cannot enter the eye, obstructing or blocking macular form stimulation, leading to amblyopia. Hence, it is also called stimulus-deprivation amblyopia.

5. Congenital or Organic Amblyopia

Caused by macular hemorrhage at birth, resulting in irregular cone cell arrangement, occurring before binocular vision develops in infancy, leading to a poor prognosis. Some cases show no detectable retinal or central nervous system abnormalities but are still considered organic due to limitations in current diagnostic methods. This type is permanent and untreatable.

bubble_chart Clinical Manifestations

1. Vision and Refractive Anomalies

The boundary between amblyopic eyes and normal eyes is not very clear. Some patients complain of decreased vision, but objective examination shows vision remains at 1.0 or 1.2. This may be due to the patient comparing their current vision with their previous vision. Additionally, there may be some impairment in the visual cells of the fovea or the subsequent conduction system, with very small central scotomas, where the patient perceives visual impairment but no objective findings can be detected.

If the amblyopic eye has no organic changes and its vision is above 0.01 but below 0.2, it is often accompanied by fixation anomalies.

The relationship between amblyopia and refractive anomalies shows that hyperopia is more prevalent. Grade I hyperopia of +2.00D accounts for 37.7% of amblyopia cases, while myopia is more likely to present with Grade I amblyopia. Thus, amblyopia is closely related to higher degrees of hyperopia.

In strabismic amblyopia, Grade III amblyopia is more common in esotropia than exotropia, possibly because esotropia tends to develop earlier than exotropia.

2. Crowding Phenomenon

Also known as the crowding effect.

When testing vision with the same optotype, illumination, and distance, the measured values differ depending on the spacing of the optotypes. The crowding phenomenon is a characteristic feature of amblyopia.

Crowding refers to the amblyopic eye's ability to recognize isolated optotypes better than clustered or crowded optotypes. For example, the ability to discern single letters (like the letter E) on a vision chart is stronger than recognizing letters in a line.

There are various theories about the cause of crowding: one suggests that prolonged strabismus leads to localized axial changes in cone cell clusters, causing the optotype to appear distorted or overlapping with adjacent optotypes.

3. Amblyopia Only Occurs in Young Children

Binocular amblyopia develops gradually from birth to age 9. During this developmental period, factors such as strabismus or form deprivation can lead to amblyopia. After age 9, even with these factors, amblyopia does not develop.

4. Amblyopia Only Occurs in Monocular Patients

If both eyes are used alternately, amblyopia does not occur.

5. Fixation Anomalies

In severe amblyopia, due to poor macular fixation ability, the parafoveal retina is often used instead of the macula for fixation. Eccentric fixation refers to fixation outside the fovea. There are many theories about its formation, but its manifestations include parafoveal fixation, peripheral fixation, paramacular fixation, and wandering fixation.

bubble_chart Treatment Measures

Depending on the degree of amblyopia and the nature of fixation, different treatment methods can be selected as follows:

(1) Central Fixation Amblyopia

It is generally agreed to adopt the dominant eye full occlusion method, i.e., conventional occlusion, to force the amblyopic eye to fixate. Patients are also instructed to perform fine visual tasks at home, such as tracing, threading needles, or stringing beads, to promote visual improvement.

(2) Eccentric Fixation Amblyopia

Opinions vary among experts. Some scholars advocate the continued use of the dominant eye occlusion method, believing that forcing the amblyopic eye to act as the fixing eye can automatically shift eccentric fixation to central fixation and improve vision. Others suggest that eccentric fixation should first be converted to central fixation before surgery to maintain proper alignment postoperatively. The methods used include:

1. Pleoptic Therapy: A flashing device (e.g., a haploscope) is used to project a strong beam onto the retina outside the macula (pseudomacula). Before the flash stimulation, a black dot with a diameter of 3 or 5 degrees in the lens covers the macular area to prevent strong light exposure. The patient is then asked to focus on the center of a black "+" on a white screen. A negative afterimage is produced, where a bright center is surrounded by a dark ring, training normal macular fixation. After multiple treatments, eccentric fixation is converted to foveal fixation to enhance vision. During treatment, the amblyopic eye must be occluded until foveal fixation is reestablished. Once vision improves to 0.6–0.7, binocular vision training with a synoptophore or surgical correction of eye alignment can be performed.

2. Grating Stimulation Therapy: Black-and-white stripes of varying spatial frequencies or post-surgical alignment training are used with a stimulation therapy machine, which most effectively activates visual nerve axons.

The stimulation therapy machine features a rotating grating with a transparent pattern board placed above it. The child covers the healthy eye and traces interesting patterns while receiving subliminal grating stimulation in the background, exciting vision and achieving therapeutic effects. After treatment, the healthy eye does not need to be occluded.

(3) Red Filter Therapy

Since cone cells in the macular area are more sensitive to red light, a red filter film (wavelength 620–760 nm) is added to the amblyopic eye’s glasses while the dominant eye is occluded. This encourages the shift from eccentric fixation to foveal fixation. Once fixation properties change, the red filter can be removed, and conventional occlusion therapy continues.

This method is particularly suitable for wandering and paramacular fixation eyes.

(4) Penalization Therapy

Introduced by Psandl and Pouliquen in 1958, the principle of this therapy is to weaken the dominant eye’s vision using optical and pharmacological methods while enhancing the amblyopic eye’s function. Since occlusion is not required, children are more receptive. Common methods include:

1. Near Penalization of the Dominant Eye: The dominant eye is instilled with 1% atropine daily and wears corrective lenses, while the amblyopic eye wears an overcorrected +2.00D lens, forcing it to perform near tasks like reading. This is suitable for severe amblyopia with or without eccentric fixation.

2. Distance Penalization of the Dominant Eye: The dominant eye is instilled with 1% atropine daily and wears an overcorrected +3.00D lens, limiting it to near vision, while the amblyopic eye wears corrective lenses, forcing it to focus on distance. This is suitable for Grade I amblyopia or to prevent recurrence and treat abnormal retinal correspondence.

3. Selective Penalization: The dominant eye is treated with atropine and corrective lenses, while the amblyopic eye is overcorrected with +2.00D bifocal lenses to encourage near vision and eliminate accommodative esotropia. This is indicated for cases where amblyopia is mostly corrected but accommodative convergence causes esotropia during near tasks.

4. Alternating suppression: The method involves using two pairs of glasses, one overcorrecting the right eye by +3.00D and the other overcorrecting the left eye by +3.00D, without using atropine eye drops. The glasses are alternated daily. This is indicated for cases where the visual acuity of both eyes is already roughly equal, but abnormal retinal correspondence has not yet been corrected; or for those who cannot undergo surgery for certain reasons. For young children in the initial stage of strabismus, this can prevent amblyopia and the development of abnormal retinal correspondence.

Evaluation Criteria for Amblyopia Treatment Efficacy

1. Ineffective: Includes cases where vision deteriorates, remains unchanged, or improves by only one line.

2. Improvement: Vision increases by two lines or more.

3. Basic Recovery: Vision is restored to ≥0.9.

4. Full Recovery: Vision remains normal after three years of follow-up.

Note: If conditions permit, other visual function training may be undertaken simultaneously to restore binocular single vision.