Amblyopie.
The Greek word "amblyopia" means vagueness or dullness of vision (ambly αμβλυσ = dull and ops ωψ = vision), and the condition has been defined in various ways in the literature. Amblyopia is reduced vision in one or both eyes due to abnormal visual development during infancy or early childhood. In the early years of life, the brain must learn to see and interpret the images from the eyes. In amblyopia, the brain receives a poor image from one (or in rare cases, both) eye(s), causing the brain to suppress the image it is being presented with.
In addition to reduced visual acuity, amblyopia also causes other issues, such as functional imbalance between the eyes and insufficient binocular information input in the primary visual cortex.
Classically defined as a reduction in visual acuity (which is clinically easier to assess), reduced contrast sensitivity at high spatial frequencies, and binocular vision impairment, amblyopia also affects the development of a wide range of neural, sensory, oculomotor, and perceptual functions of vision.
Several visual functions are not fully developed at birth; their complete development depends on three fundamental conditions during the critical period of visual development in childhood: adequate stimuli received from each eye, corresponding images, and the integrity of the visual pathways.
Disruptions in the input of stimuli received by the visual cortex during this plastic and unstable phase of visual development prevent proper use of input from the affected eye, resulting in amblyopia. The impact on the visual system is closely related to the timing of the visual disturbance, as well as its intensity, type, and duration.
When a visual stimulus disruption occurs early, is severe, goes undetected, and is not reversed in the first months or years of life, it can lead to significant structural modifications in the visual neural circuitry, resulting in permanent morphological changes in the cortical structures of the lateral geniculate nucleus (LGN) and visual cortex, ultimately causing permanent changes in the visual function of amblyopic eyes.
When the visual stimulus disruption occurs later and with less intensity, the normal anatomical construction of the system remains intact, but active inhibition of neurons from the normal eye over neurons from the affected eye can still occur, leading to functional amblyopia. This neurological mechanism suppresses the image from the affected eye in an attempt to avoid interfering with the processing of the normal eye.
Although known since ancient times, many neural, physiological, and psychological aspects of amblyopia are still not fully understood.
The first mention of "amblyopia" dates back to ancient Greek civilization in the fifth century BC. At that time, Hippocrates used the word amblyopia to refer to conditions that resulted in reduced visual acuity in seemingly healthy eyes. The ancient Greeks also defined "strabismus" as a disorder of eye movement and position. More than a thousand years later, in the Byzantine Empire around 900 AD, the first historical record of occlusion therapy for the treatment of strabismus appeared. In the 13th century, the very first eye hospital was established in France, and in the 16th century, the German physician Bartisch designed special caps to treat strabismus.
German physician Georg Bartisch (1535–1607) was both ahead of his time and a product of his time. On one hand, he was a skilled surgeon who closely observed the human body; on the other hand, he believed that illness was a punishment from God.
To cure strabismus, Bartisch suggests training the eye by wearing a corrective mask, as shown in the illustration. The mask varies depending on whether the patient turns their eyes outward toward the ears or inward toward the nose.
It is believed that the actual start of ophthalmology as a distinct field took place in 1765 in Paris, when the first chair in ophthalmology was established. A few decades earlier, in 1722, the French ophthalmologist Charles de Saint-Yves wrote in his book that by covering the "straight eye," the "deviating" eye would immediately look straight ahead—something we now recognize as the cover test. He also suggested covering the "straight eye" to restore the correct eye position in the deviating eye. A few years later, in 1743, George Comte de Buffon, another French physician, described that the weak eye would regain all its strength by covering the good eye, which is why he is considered a pioneer of occlusion therapy for amblyopia.
The first idea to treat strabismus (and amblyopia) surgically came in the 18th century from John Taylor, though his surgical techniques on eye muscles were later deemed fraudulent. About a hundred years later, the Germans Stromeyer and Dieffenbach, along with the Belgian Cunier, were the first to successfully perform surgeries to treat strabismus. It was later noted by Ludwig Boehm that surgery to correct strabismus also improved amblyopia.
Occlusion and fusion exercises for the treatment of amblyopia
The first treatment for amblyopia and strabismus was occlusion. In the 18th and 19th centuries, various doctors developed so-called new fusion exercises. Erasmus Darwin in England prescribed a type of fusion exercise in which he separated the two visual fields to improve vision. Later, Louis Emile Javal, who was a strong opponent of surgical treatment, devised certain orthoptic exercises to restore binocular vision. Even later, Claud Worth used occlusion and introduced the use of atropine — what we now know as pharmacological penalization — as an alternative to occlusion. He also invented the active training stimulation device known as the amblyoscope. Ernest Edmund Maddox later invented various orthoptic devices and dedicated his career, along with his daughter, to orthoptic training, including the use of the previously mentioned Worth amblyoscope. His daughter Mary later founded the first orthoptic clinic in London.
Modern occlusion is (re)introduced
Later, at the beginning of the 20th century, occlusion therapy was extensively reintroduced into clinical practice for amblyopia. Sattler began using an eye patch similar to what we know today, although he suggested occlusion for at least several consecutive days—unlike what is recommended today. However, he was absolutely correct in noting that occlusion is most effective in children up to 6-8 years old. Around the same time, Weckert proposed using a spectacle-mounted occluder instead of an eye patch, and this technique was generally more accepted by children than Sattler's patch. In the following decades, there was extensive debate about the benefits and duration of occlusion, and even the top professionals of that time could not fully agree on whether amblyopia was caused by strabismus or vice versa. They also began to recognize the potential side effects of prolonged occlusion (occlusion amblyopia, increased strabismus, and loss of binocularity), and thus began experimenting with applying patches for only a few hours a day.
Modern approaches to amblyopia treatment
During the 20th century, occlusion therapy became increasingly sophisticated, particularly in determining the most effective duration of occlusion times. Perhaps even more importantly, ophthalmologists also concluded that robust refractive correction is just as crucial as occlusion itself, especially in the anisometropic etiology of amblyopia. Improved surgical techniques made strabismus correction more effective and much safer as well. An alternative to occlusion, known as "penalization," was introduced into clinical practice and remains an appealing option to this day, particularly for those who cannot cooperate with conventional occlusion. Finally, increasingly powerful computers and ongoing digitization enabled the development of various new active treatment approaches, including, but not limited to, pleoptic training, perceptual learning, and binocular dichoptic training.
Sources: Loudon S. E. en Simonsz H. J. (2005). The history of the treatment of amblyopia. Strabismus 13, 93-106.
