Hypertropia is a specific form of strabismus, or eye misalignment, characterized by an upward deviation of one eye relative to the other. This vertical misalignment prevents the visual axes of the two eyes from meeting at the same point, meaning they cannot work together as a coordinated pair. While strabismus usually involves eyes turning inward (esotropia) or outward (exotropia), hypertropia is the least common type. It can be present constantly or occur intermittently, often becoming noticeable when a person is tired or unwell.
Understanding Vertical Eye Misalignment
The inability of the eyes to align vertically can lead to several disruptive visual symptoms because the brain receives two different images. The most immediate consequence for many individuals is double vision, known medically as diplopia, where one image appears vertically displaced from the other. This separation of images makes everyday tasks difficult and causes visual confusion.
The eyes struggling to fuse the two images results in eye strain and headaches, especially during tasks that require sustained visual focus. To unconsciously correct for the double vision, many people, particularly children, adopt a habitual head tilt or turn to align the images and gain clearer vision. Over time, this compensatory head posture can become a permanent feature.
In children, the brain often attempts to cope with the visual confusion by suppressing the image from the misaligned eye, effectively ignoring the input from that eye. This visual suppression is a concern, as it can lead to amblyopia, or “lazy eye,” where the vision in the affected eye fails to develop properly. This results in long-term reduced visual acuity and difficulty with depth perception. Early detection is paramount to prevent the development of this permanent loss of binocular function.
Specific Muscle and Nerve Involvement
The precise movement of the eye is governed by six extraocular muscles surrounding the eyeball: four rectus muscles and two oblique muscles. These muscles are coordinated by three cranial nerves: the oculomotor nerve (III), the trochlear nerve (IV), and the abducens nerve (VI). Hypertropia arises when there is an imbalance in the strength or function of the muscles responsible for vertical movement.
A common cause of hypertropia is a weakness or paralysis of the superior oblique muscle, which is innervated by the fourth cranial nerve (trochlear nerve). Since the superior oblique muscle is responsible for moving the eye downward and inward, its impairment causes the eye to drift upward, leading to fourth cranial nerve palsy. This palsy can be congenital or acquired later due to trauma or neurological issues.
Hypertropia can also be caused by restriction or overaction of other vertical eye muscles. For example, in conditions like thyroid eye disease (Graves’ disease), the inferior rectus muscle can become inflamed and scarred. This restricts its ability to stretch, causing the eye to be pulled upward. Trauma, stroke, or other neurological disorders that damage the cranial nerves controlling eye movement can also result in acquired hypertropia.
Identifying and Treating Hypertropia
The identification of hypertropia involves a comprehensive eye examination, which includes specialized tests to measure the degree and nature of the eye misalignment. A common diagnostic tool is the cover/uncover test, where the examiner observes the movement of each eye as one is temporarily covered and then uncovered. This test helps to differentiate between a manifest deviation (hypertropia) and a latent one (hyperphoria).
To quantify the extent of the deviation, the angle of misalignment is often measured using prisms, typically in conjunction with the alternate cover test. The Parks-Bielschowski three-step test is also frequently employed to help pinpoint the specific paretic, or weakened, extraocular muscle causing the vertical misalignment. These diagnostic steps are crucial for determining whether the deviation is the same in all directions of gaze (comitant) or changes with eye position (incomitant).
Treatment for hypertropia is tailored to the individual’s age, the cause, and the severity of the eye turn, with the primary goals being to restore proper eye alignment and binocular vision. Non-surgical options often begin with prescription eyeglasses to correct any underlying refractive errors. For minor misalignments, prism lenses can be incorporated into glasses to bend light rays, shifting the image seen by the misaligned eye so that it fuses with the image from the straight eye, thereby eliminating double vision.
Vision therapy, which involves a customized program of eye exercises, may be used in conjunction with prisms to improve the eyes’ coordination and ability to work as a team. In young children with amblyopia, patching the stronger eye forces the brain to rely on and strengthen the weaker, misaligned eye. If non-surgical methods are insufficient, or for larger, constant deviations, eye muscle surgery may be recommended.
Surgical intervention involves adjusting the length or position of the extraocular muscles to restore proper alignment and balance. This procedure can involve weakening an overactive muscle or strengthening a weakened one, and it is beneficial for improving both the cosmetic appearance and the functional vision. Although surgery can offer a permanent solution, the goal is often to reduce the misalignment to a small, manageable angle that the patient’s visual system can easily control, sometimes still requiring a small amount of prism correction afterward.