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.

Jensen procedure

A transposition surgery used to treat lateral rectus palsy or complete sixth nerve palsy. It involves splitting the muscle bellies of the lateral, superior, and inferior rectus muscles behind their insertions. The lateral halves of the superior and inferior rectus muscles are then sutured to the lateral rectus muscle belly without disinserting any of the tendons, augmenting lateral rectus function by adding vector forces from the vertical rectus muscles.

Knapp procedure

A surgical technique used to treat double elevator palsy by full tendon width transposition of the medial and lateral rectus muscles to the insertion of the superior rectus muscle. This allows the horizontal rectus muscles to act as elevators, improving elevation and reducing hypotropia.

Knapp’s law

A correcting spectacle lens placed at the anterior focal plane of an axially ametropic eye will form a retinal image equal in size to that formed in a standard emmetropic eye. Knapp’s law allows calculation of retinal image size and is useful for predicting aniseikonia, the image size difference between the two eyes.

Krimsky test

A method for objectively measuring the angle of strabismus. The clinician positions their eye directly above a light source that the patient fixates on. Prisms are placed in front of the fixating eye until the corneal light reflex appears centered in the deviated eye.

lag of accommodation

The dioptric difference between the accommodative stimulus and the accommodative response, where the eye under-accommodates for a near target. A normal lag is between 0.50 to 0.75 D, while a lag >1.00 D is considered abnormal and may cause symptoms like eyestrain, headaches, and blurred vision at near.

Lag of accommodation can be measured objectively using dynamic retinoscopy techniques such as MEM or Nott retinoscopy. It may occur due to reduced accommodative ability or uncorrected hyperopia. Treatment options include prescribing plus lenses at near or vision therapy to improve accommodative function.

latent nystagmus

A conjugate, horizontal jerk nystagmus that manifests only under monocular viewing conditions. The fast phase always beats toward the fixating eye, while the slow phase drifts nasally.

Latent nystagmus is associated with infantile strabismus, amblyopia, and dissociated vertical deviation, and is thought to arise from maldevelopment of cortical binocular connections. It may convert to manifest latent nystagmus if binocularity is disrupted.

lead of accommodation

The dioptric difference between the accommodative stimulus and the accommodative response, where the eye over-accommodates for a near target. It may cause blurred vision at near due to excessive accommodation.

Potential causes include accommodative spasm and uncorrected myopia. Treatment options involve reducing accommodative demand through minus lenses at near or vision therapy to improve accommodative control.

levoversion

The conjugate movement of both eyes to the left, a type of version. It occurs due to simultaneous contraction of the left lateral rectus and right medial rectus muscles.

low vision

Significant, permanent vision impairment that cannot be fully corrected with glasses, contact lenses, medications, or surgery. It results from eye diseases, congenital abnormalities, or injuries that damage the visual system, preventing satisfactory vision even when refractive errors are corrected. Low vision patients experience reduced visual acuity, contrast sensitivity, or visual fields that impair daily functioning like reading, mobility, and face recognition, yet some usable vision remains.

M-unit (print size)

The M-unit is a standardized unit used to specify the size of printed text or optotypes for visual acuity testing independent of testing distance. By definition, the detail of 1M print subtends a visual angle of 1 minute of arc (1/60 of a degree) at a viewing distance of 1 meter. The overall 1M letter or optotype subtends 5 minutes of arc at this same 1 meter viewing distance. Likewise, the detail of 2M print subtends 2 minutes of arc at 1 meter and the letter subtends 1o minutes of arc at 1 meter.

mechanically restrictive deviation

An incomitant strabismus resulting from physical impedance of ocular rotations, rather than extraocular muscle paresis. The deviation varies in magnitude in different positions of gaze. Etiologies include extraocular muscle enlargement, as in thyroid eye disease; orbital scarring after trauma or surgery; and anomalous orbital structures.

Microtropia

Monofixation syndrome, alsos known as microtropia, is a small angle strabismus, typically an esotropia, measuring less than 5-8 prism diopters. It is often difficult to detect on cover testing due to the subtle deviation. A central suppression scotoma is usually present in the deviating eye. Microtropia can occur as a primary condition or secondary to treatment of a larger angle strabismus.

motor fusion (fusional reserves)

Motor fusion, also known as fusional vergence or fusional reserves, is the maximum amount the eyes can converge or diverge while still maintaining binocular single vision. Positive fusional reserves, measured with base-out prisms, represent the eyes’ maximum convergence ability. Negative fusional reserves, measured with base-in prisms, represent maximum divergence.

myectomy

An extraocular muscle surgery where a segment of the muscle belly is excised to weaken the muscle’s pulling force. It is commonly performed on the inferior oblique muscle to treat persistent inferior oblique overaction.

negative relative accommodation

The maximum amount of accommodation that can be relaxed while maintaining clear, single binocular vision at a fixed distance, typically 40 cm. It is measured clinically by adding plus lenses binocularly until the patient reports the first sustained blur. NRA assesses the flexibility of the accommodative system relative to vergence demand. Low NRA findings may indicate accommodative excess or latent hyperopia, while high values suggest uncorrected hyperopia or over-minused refraction.

neutral density filter test

This test differentiates functional (reversible) from organic (pathological) amblyopia. Visual acuity is measured with and without neutral density filters that reduce luminance equally across wavelengths. In organic amblyopia, acuity significantly decreases with filters, while in functional amblyopia, acuity is minimally affected. This test helps determine if reduced acuity is due to retinal/optic nerve pathology or suppression. Filters of increasing density are used until acuity diminishes.

nystagmus

An involuntary, repetitive, and rhythmic movement of the eyes characterized by alternating slow and fast phases. It can occur in a horizontal, vertical, rotary, or combined pattern. Nystagmus may be congenital, appearing in infancy, or acquired later in life due to vestibular disorders, neurological conditions, medications, or toxins. It can affect visual acuity, depth perception, and balance. Treatment depends on the underlying cause and may include corrective lenses, medications, or surgery in some cases.

occlusion amblyopia

A form of stimulus deprivation amblyopia that develops when an eye is constantly occluded for an extended period, particularly during the critical period of visual development. The obstruction of visual stimuli to the retina leads to abnormal development of the visual cortex, resulting in reduced visual acuity and contrast sensitivity in the affected eye. Occlusion amblyopia can occur due to various causes, such as congenital ptosis or media opacities.