Adult strabismus is a condition where the eyes do not align properly, causing one eye to turn inward, outward, upward, or downward while the other remains focused straight ahead. This misalignment is not limited to childhood and can either develop anew during adult life or represent the recurrence of a childhood condition. The brain relies on both eyes working together to create a single, three-dimensional image, and when the eyes are misaligned, this process is disrupted.
Defining Adult Strabismus
Strabismus occurs when the six muscles controlling the movement of each eye fail to work in coordination, resulting in a loss of parallelism between the visual axes. The misalignment is described by the direction of the deviating eye. When an eye turns inward toward the nose, the condition is called esotropia; an outward deviation toward the temple is known as exotropia.
Vertical misalignments are termed hypertropia when the eye turns upward and hypotropia when it turns downward. The brain’s main challenge is fusing two disparate images into one coherent view. When the eyes are misaligned, the brain receives two distinct images that it cannot easily reconcile.
The misalignment may be constant, meaning the deviation is always present, or it may be intermittent, appearing only during certain activities, such as reading, or when the person is tired. Intermittent strabismus is often more challenging for the brain because the visual system constantly struggles to maintain proper alignment. When the eye muscles are unable to consistently keep the eyes straight, the visual system fails to achieve binocular fusion, which is the ability to use both eyes together.
Why Strabismus Develops in Adults
Adult strabismus is frequently acquired due to underlying medical conditions or neurological events, distinguishing it from most cases of childhood strabismus. One common cause is the decompensation or recurrence of a mild strabismus that was present but controlled during childhood, where the eye muscles weaken over time and lose their ability to maintain alignment. This re-emergence can be triggered by factors like illness, stress, or age-related changes in muscle tone.
Systemic diseases often affect the delicate nerves and muscles that control eye movement, leading to strabismus. Graves’ disease, an autoimmune condition that affects the thyroid, can cause inflammation and swelling of the extraocular muscles, physically restricting eye movement and causing misalignment. Uncontrolled diabetes and high blood pressure can also damage the small blood vessels supplying the cranial nerves responsible for eye movement, resulting in sudden muscle weakness or paralysis.
Sudden-onset strabismus requires immediate medical investigation because it can signal a serious neurological event. Strokes, brain tumors, or cranial nerve palsies (involving the 3rd, 4th, or 6th nerves) can directly impair the signals to the eye muscles, leading to a sudden misalignment. Physical trauma to the head or orbit, or complications following previous eye surgeries, can also injure or disrupt the coordination of the eye muscles.
Recognizing the Visual and Physical Symptoms
The most common symptom for adults with newly acquired strabismus is diplopia, or double vision, where the person sees two images of a single object. Unlike children, whose developing brains often suppress the image from the misaligned eye, adult brains struggle to ignore the double image. This results in two overlapping or entirely separate images, significantly affecting daily activities.
The lack of proper eye coordination also impairs stereopsis, which is the brain’s ability to process the slightly different images from each eye into a single perception of depth. This loss of depth perception makes judging distances difficult, impacting tasks like driving or walking down stairs. To reduce the severity of the double image, many adults develop compensatory head postures, such as tilting or turning their head to the side.
The constant effort to control the misaligned eyes or to suppress the second image can lead to significant eye fatigue and strain, often accompanied by frequent headaches. Beyond the functional symptoms, the visible misalignment of the eyes can cause self-consciousness and affect social interactions.
Management and Treatment Options
The initial step in managing adult strabismus is to address any identified underlying systemic or neurological cause. If the misalignment is a result of diabetes, strict control of blood sugar levels may help stabilize the condition and improve the eye muscle function. Similarly, managing thyroid levels for Graves’ disease is a necessary precursor to treating the resulting eye muscle changes.
Non-surgical interventions are often the first line of treatment. Prism glasses utilize special lenses that bend the light entering the eye, shifting the image to align with the position of the misaligned eye, thereby reducing or eliminating double vision. Vision therapy, which includes targeted eye exercises, can also be helpful for certain types of intermittent strabismus, particularly those involving convergence insufficiency.
In cases where non-surgical methods are insufficient, surgical correction is considered. This involves adjusting the length or position of the six extraocular muscles surrounding the eye to physically realign them. Strabismus surgery is a common and effective procedure that can restore binocular vision and depth perception, though sometimes multiple surgeries are required for optimal alignment.
Another treatment option is the injection of Botulinum toxin (Botox) directly into an overactive eye muscle to temporarily weaken it. This allows the opposing muscle to gain strength and pull the eye toward a more aligned position.