Haidinger’s brushes
An entoptic phenomenon where the viewer perceives a faint, yellowish brushstroke pattern when viewing a polarized light source. This pattern arises from the radially arranged yellow pigment fibers in the macula lutea. Haidinger’s brushes are used to assess fixation in amblyopia. If fixation is foveal, the brushstroke pattern moves in synchrony with eye movements. However, if fixation is eccentric, the brushes appear displaced from the object being viewed.
Harada-Ito procedure
A surgical technique used to treat torsional diplopia and excyclotorsion resulting from superior oblique palsy where the superior oblique tendon is divided lengthwise into anterior and posterior halves. The anterior one-third of the tendon fibers are disinserted and transposed anterolaterally to be sutured near the superior border of the lateral rectus muscle insertion. This enhances the incyclotorsional effect of the superior oblique to correct the excyclotorsion, without affecting the vertical position.
Hering’s law of equal innervation
For any intended eye movement, the neural signals (innervation) sent from the brain to the extraocular muscles are equal for both eyes. This results in conjugate, yoked movements where the two eyes move symmetrically in the same direction.
heterophoria
Also known as dissociated phoria, heterophoria is a latent condition where the eyes tend to deviate from the orthophoric position when binocular fusion is disrupted. It requires fusional vergence to maintain bifixation and alignment. Heterophoria can be exophoric (outward deviation) or esophoric (inward deviation). It is measured using dissociating tests that disrupt binocular fusion, like the cover test or Maddox rod. Excessive heterophoria can cause symptoms like eyestrain, diplopia, and asthenopia, but differs from strabismus, which is a manifest misalignment.
Hirschberg test
A test to estimate the angle of ocular misalignment or strabismus. The examiner notes the position of the corneal light reflex relative to the pupil center. A temporally displaced reflex indicates exotropia (outward deviation), while a nasally displaced reflex indicates esotropia (inward deviation).
horror fusionis
A rare binocular vision anomaly characterized by an active avoidance or suppression of bifoveal fusion. Patients experience constant diplopia but are unable to fuse or tolerate the double images. It often presents in childhood with long-standing strabismus or amblyopia and may involve abnormal cortical wiring or sensory adaptations. Treatment is difficult as prisms, occlusion, or surgery typically cannot eliminate the diplopia. Over time, patients learn to ignore or suppress one of the images.
Hummelsheim procedure
A transposition surgery used to treat complete lateral rectus palsy or sixth nerve palsy that involves disinserting the lateral halves of the superior and inferior rectus muscle tendons and transposing them to be sutured adjacent to the lateral rectus muscle insertion. This augments the lateral rectus function by adding vector forces from the transposed vertical rectus muscles, improving abduction and reducing esotropia caused by the palsy.
hyperphoria
A binocular vision condition where one eye has a tendency to be vertically higher than the other when fusion is disrupted.
hypertropia
A type of vertical strabismus where the visual axes are vertically misaligned, with the hypertropic eye appearing higher compared to the fellow eye. The deviation may be constant or intermittent, comitant or incomitant.
hypophoria
Hypophoria is a binocular vision condition where one eye has a tendency to be vertically lower than the other when fusion is disrupted.
hypotropia
A type of vertical strabismus where the visual axes are vertically misaligned, with the hypotropic eye appearing lower or more depressed compared to the fellow eye. The deviation may be constant or intermittent, comitant or incomitant.
iatrogenic
Conditions or diseases that are inadvertently caused by medical treatment, procedures, or advice from a healthcare provider. Common iatrogenic conditions include dry eye from certain medications, corneal complications from surgery, and lens issues from intraocular procedures.
idiopathic
A condition or disease with an unknown or indeterminate cause, where the origin or underlying mechanism cannot be identified or explained despite investigation. It comes from the Greek roots “idio” meaning one’s own and “pathos” meaning suffering.
incomitant (strabismus)
A form of strabismus where the angle of ocular misalignment varies depending on the direction of gaze. It results from restriction or paresis of the extraocular muscles, contrasting with concomitant strabismus where the deviation remains constant in all gaze positions. The deviation increases when looking in the direction of the paretic or restricted muscle and decreases in the opposite direction.
infacility of accommodation
A condition characterized by difficulty or sluggishness in changing the accommodative response from one viewing distance to another. It involves the accommodative system’s ability to rapidly increase or relax ciliary muscle contraction to adjust focus. Infacility can affect both increasing accommodation for near vision and relaxing accommodation to focus at distance. It is often associated with aging and reduced amplitude of accommodation, causing symptoms like blurred vision, eye strain, and headaches when changing focus between distances.
inferior oblique overaction
An overaction or excessive contraction of the inferior oblique muscle, resulting in elevation or upturning of the eye when it adducts (looks inward). It can be primary/idiopathic or secondary to other conditions like superior oblique palsy.
infraduction
The downward movement of the eye, controlled by the inferior rectus and superior oblique muscles. Limitation in infraduction can indicate dysfunction of these muscles or their innervating cranial nerves (oculomotor and trochlear nerves).
infranuclear
Infranuclear refers to disorders below the level of the oculomotor, trochlear, and abducens nuclei in the brainstem. It includes conditions like myasthenia gravis, thyroid eye disease, and myopathies.
infraversion
The conjugate movement of both eyes downward, a type of version. It occurs due to simultaneous contraction of the inferior rectus muscles in each eye.
intermittent strabismus
A condition where the ocular misalignment is not constant, with periods of proper eye alignment (orthotropia) alternating with periods of strabismus or eye deviation. Patients often experience difficulty maintaining fusion and diplopia during the periods of misalignment. Factors like fatigue, illness, and distance/near fixation can influence the intermittent nature. Intermittent exotropia is more prevalent than intermittent esotropia. Treatment focuses on improving fusional control and vergence ranges.