Hypertropia is a specific type of strabismus, the general term for eye misalignment. This condition is characterized by a vertical deviation where the visual axis of one eye is physically higher than the other, causing a noticeable upward turn. Hypertropia can manifest constantly or intermittently, often appearing when a person is tired or under stress. The disorder affects the ability of both eyes to work together, which is necessary for clear, single vision.
Understanding the Upward Deviation
Hypertropia is defined by the upward deviation of one eye relative to its partner, making it a form of vertical strabismus. It is distinct from hypotropia, the opposite vertical misalignment where one eye turns downward. These vertical deviations contrast with the more commonly known horizontal misalignments, such as esotropia (inward turn) and exotropia (outward turn).
The mechanical nature of hypertropia involves an imbalance in the six extraocular muscles that control eye movement. These muscles, including the superior rectus, inferior rectus, superior oblique, and inferior oblique, dictate the eye’s vertical trajectory. When these muscles are too weak, too tight, or overactive, they fail to maintain parallel alignment, leading to the upward drift.
When a person with hypertropia attempts to look straight ahead, the eye with the stronger vertical muscle may drift upward. If one eye has hypertropia, the other eye is simultaneously considered to have hypotropia, as it is lower in comparison. Eye care professionals typically refer to the condition by the eye that is deviating upward.
Root Causes and Visual Disturbances
The underlying causes of hypertropia are diverse, stemming from issues with the extraocular muscles or the cranial nerves that control them. The fourth cranial nerve (trochlear nerve) is often implicated because it innervates the superior oblique muscle, which helps move the eye down and in. Weakness or paralysis of this nerve, known as fourth nerve palsy, is a common cause, leading to an unopposed upward movement of the eye.
Hypertropia is classified as either congenital (present from birth due to developmental abnormality) or acquired later in life. Acquired causes include Graves’ disease, which can cause fibrosis and restriction of the inferior rectus muscle, or neurological events such as a stroke, which can damage the controlling nerves. Trauma to the eye or orbit can also result in muscle or nerve damage leading to vertical misalignment.
The misalignment disrupts binocular vision, forcing the brain to receive two slightly different images, a primary symptom known as diplopia, or double vision. To compensate and fuse the images, many individuals, particularly children, instinctively adopt a compensatory head posture, often tilting their head to one side. This characteristic head tilt, or torticollis, is an attempt to align the eyes and eliminate the double image.
Other common visual disturbances include asthenopia (eye strain or fatigue) and frequent headaches, as the visual system struggles to maintain focus. In young children, the brain may suppress the image from the misaligned eye to avoid diplopia, which can lead to strabismic amblyopia (lazy eye). This suppression can result in impaired depth perception and long-term vision problems if not addressed early.
Testing and Treatment Pathways
Diagnosing hypertropia begins with a comprehensive eye examination that includes a detailed medical history. A fundamental diagnostic tool is the cover/uncover test, where the clinician covers one eye and observes the movement of the other eye to assess for a manifest deviation. If the uncovered eye moves downward to take up fixation, it confirms the presence of an upward deviation.
Specialized testing, such as prism tests, measures the precise degree of the vertical misalignment, typically quantified in prism diopters. The clinician uses prisms of increasing strength to neutralize the deviation, allowing for an accurate assessment across different directions of gaze. In some cases, neuroimaging, such as an MRI, may be ordered to investigate nerve compression or other neurological issues.
Management of hypertropia varies based on the cause and severity, starting with non-surgical options. Corrective lenses are often prescribed, sometimes with a ground-in prism that bends light to fuse the two images without physically moving the eye. Prism glasses are effective for small deviations, typically under 10 prism diopters, and can provide significant relief from double vision.
Another non-surgical option is vision therapy, a structured program of eye exercises designed to improve the eye-brain connection and strengthen coordination. Temporary treatment using botulinum toxin injections can be employed to weaken an overacting eye muscle. For significant or structurally caused misalignments, surgical intervention on the extraocular muscles is often the most effective route.
Surgical correction aims to adjust the length or position of the muscles to restore alignment. This is achieved through recession (reattaching a strong muscle further back to weaken its pull) or resection (shortening a weak muscle to strengthen it). Surgery improves visual function, such as depth perception, and cosmetic appearance, offering a permanent solution for many patients.