In general, strabismus (or tropia) is an eye deviation defined by frequency (intermittent or constant), laterality (right, left, or alternating), and direction (horizontal or vertical).
Horizontal strabismus is termed esotropia (inward turn of the eye) or exotropia (outward turn of the eye). Vertical strabismus is termed hypotropia (downward turn of the eye) or hypertropia (upward turn of the eye).
Therefore, hypotropia is a form of vertical strabismus where one eye deviates downwards in comparison to the other eye.
Symptoms of hypotropia can range from cosmetic to visual perceptual. A cosmetic symptom of hypotropia is one eye appearing to drift or stay in a downward gaze position. This can happen to either the right or the left eye. In the visual perceptual space of symptoms, the patient can experience double vision (diplopia) stemming from the misalignment between the two eyes. The double vision often appears as two images "stacked" on top of each other, although it may also appear as two images stacked but one may appear slightly askew to the other due to the vertical deviation along with a horizontal deviation.
Patients may also experience classic symptoms of binocular misalignment such as bumping into walls, reading difficulties, balance issues, headaches, blurred vision, and more. It is important that if a patient is experiencing symptoms such as this they undergo a complete ocular health and binocular vision assessment.
Those with Hypotropia may also be experiencing the following, whether they are aware of it or not.
Suppression is not perceived by the patient, but rather by the observer. Suppression occurs when the visual system ignores (suppresses) the image perceived by the deviated eye. This occurs to alleviate the patient from double vision. The amount of suppression can vary from small suppression scotomas in binocular fusion to large suppression areas on the affected side. Suppression often goes hand-in-hand with amblyopia (lazy eye) and depends on various factors such as the size of the strabismus and age of onset. In general, a younger patient that develops strabismus will suppress the doubled images, while an older patient will not be able to suppress due to a developed visual system.
Diplopia is the medical term for double vision. In the case of hypotropia, the diplopia is vertical; however, most patients will report a mild horizontal offset as well. This is due to a mild or moderate horizontal component affecting eye positioning rather than pure vertical strabismus.
Vision confusion occurs when two images are perceived in the same location, due to a misalignment of retinal correspondence points on the fovea. This symptom is a consequence of diplopia. You can imagine that seeing two of everything would be difficult to manage! Younger patients "learn" to deal with visual confusion by suppressing the image of one of the eyes. This occurs because a young patient's visual system is capable of altering neural inputs to adapt and suppress to avoid double vision. An older patient has a less adaptable neural framework and is unlikely to suppress diplopia. Diplopia for older patients can be debilitating due to visual confusion.
A light source located in front of the optometrist's eyes is directed at the patient's eyes, while the patient is asked to fixate the light source directly. The corneal light reflex is observed. The Hirschberg test is considered normal when the corneal light reflexes are slightly decentred nasally (about 5º, due to angle kappa). In the case of a hypotropia, the light reflex of the deviated eye is located above the light reflex of the fixing eye. The amount of deviation can be grossly estimated by multiplying the mm of deviation by 15PD.42,43
This test uses prisms to complement the Hirschberg test. Prisms are positioned in front of the deviating eye, base-up in the case of a hypotropia, and are progressively increased until a neutral Hirschberg test is obtained. It is particularly helpful in patients that don’t collaborate well in the former test, especially with low visions.
The cover/uncover test allows for the diagnosis of tropias, when performed correctly. In order to achieve this, the optometrist needs to briefly cover the eye that is fixing and see if there is a refixation movement of the fellow eye. In the case of a hypotropia, the non-fixing eye moves upward as it takes up fixation. If no refixation is observed the other eye may be the fixing one, in which case it is covered and the test is performed again. It is very important for the cover to be very brief, since a prolonged cover will break binocular fusion and provoke a possible phoria that can be misinterpreted as a -tropia. A pure -phoria won’t have a positive cover/uncover test, while a -tropia is also associated with positive alternate cover test.
A test that can be used to estimate the angle of deviation attributable to a -tropia. The amount of PD that have to be added in order to revoke refixation movements in the deviating eye, correspond to the deviation angle. This shouldn’t be confused with the alternating prism cover test for the correction of a -phoria component, in which case binocular fusion is interrupted. In the case of incomitant strabismus due to muscle paresis or restrictive syndromes, one prism is placed over the eye with limited ductions to measure the primary deviation and a second prism is placed in front of the good eye to measure secondary deviation. The deviation is always bigger when the eye with limited ductions is fixing (i.e. the prism is over the normal eye)
Allows for the diagnosis of diplopia and suppression. A different light filter is positioned in front of each eye, a green and a red light filter. The patient is asked to look at 4 different dots: 2 green dots on either side; one red dot on top; one white dot on the bottom, forming a cross. Green dots can only be seen by the eye with the green light filter and the red dot can only be appreciated by the eye with the red filter, while the white dot can be seen by both eyes. If the patient sees 5 lights instead of 4, a diplopia is present. If the lights seen by one eye are above the expected position, it means that eye is hypotropic (the image is projected on the superior retinal quadrants, which perceive the lower visual fields). If the patient sees less than 4 lights, suppression is present.
Same principle as the Worth dot test, but with only one light source and one light filter (red) in front of the eye to be examined. The patient seeing a pink light is a normal test result. If two lights are perceived, diplopia is present. If the patient only sees a white light, suppression of the eye with the red filter is present.
One Maddox rod is positioned in front of each eye, while the patient is asked to look at a light source. This way, each eye will only see one linear streak of light. In order to test for a vertical tropia, the Maddox rods have to be placed in order to create streaks at 180º degrees. If one streak is perceived above the other, a hypotropia/phoria is present. Since the Maddox rod test is highly dissociative, it doesn’t allow for a differential diagnosis between a -phoria and –tropia.
This test is very similar to the Maddox rod test, with the exception that Bagolini striated lenses allow for a better view of the peripheral visual field, giving more binocular clues. This way, less dissociation is present and a better distinction between a small suppression scotoma with peripheral fusion and a large suppression scotoma is possible.
Typically patients can start with Vision Therapy to help aid in binocular vision. From there, other options may be explored. Treatment can be complicated for patients with hypotropia and so, it requires commitment on the patient's behalf to begin and complete a vision therapy program.