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.
internuclear ophthalmoplegia
A supranuclear disorder caused by a lesion in the medial longitudinal fasciculus (MLF) of the brainstem resulting in impaired adduction of the ipsilateral eye and nystagmus of the contralateral abducting eye during horizontal gaze. It may be unilateral or bilateral and is often associated with other brainstem or neurological signs depending on the location of the MLF lesion. INO helps localize the site of dysfunction to the MLF pathway and differentiates from other causes of horizontal gaze palsy like abducens nerve palsy.
intorsion (incyclodeviation)
The inward wheel-like rotation or torsional movement of the eye around its anteroposterior axis, causing the superior pole of the vertical meridian to rotate nasally.