Binocular Vision Lecture 3

Binocular Vision Lecture 3 - Classification of Eye Deviations

• Students will be able to define orthophoria, heterophoria, and heterotropia by the end of the session.
• Students will also be able to classify heterophoria and heterotropia.

Motor and Sensory Fusion

• Binocular single vision is dependent on both motor and sensory fusion.
• Motor fusion is the fusional vergence used to move the eyes from a dissociated position to an active position, ensuring the image falls on corresponding retinal points.
• Sensory fusion requires higher level processing once the image is located on the corresponding retinal points to achieve a perceptual fusion of the two retinal images.

Achieving Dissociation

• There are various methods to dissociate the eyes, including covering an eye, using a septum, or distorting the image.
• When dissociated, the eyes assume passive positions determined by physiological and anatomical features like tonic vergence, postural and fixation reflexes, and proximal and accommodative convergence.

Orthophoria (No Deviation)

• Orthophoria is present when the active and passive positions of the eyes are the same.
• Fusional vergence is not required for bifoveal fixation.
• When one eye is covered, the other eye does not move out of position.
• A perfect oculomotor balance (OMB) is present.
• Orthophoria is not very common.

Heterophoria

• Heterophoria is where the dissociating and active positions collide.
• A latent deviation becomes manifest when the eyes are dissociated.
• Heterophoria is present only if the eye under the cover moves.
• Fusional vergence is required for bifoveal fixation.
• The motor fusion reflex eliminates the deviation when the obstacle to sensory fusion (cover) is removed.
• Heterophoria does not affect binocular vision and is considered a physiological condition.

Heterotropia

• Heterotropia is where fusion is not possible.
• The fusion reflex may have failed to develop or is unable to function.
• The visual axes are misaligned.
• This deviation is manifest even without any attempt to dissociate the eyes.
• Binocular vision is impaired and considered pathological.

Classification of Heterophoria - Direction

• The classification of heterophoria includes direction, magnitude, fixation distance, and compensation.
• Direction: includes esophoria (SOP, Eso), exophoria (XOP, Exo), and R/L (or L/R) hyperphoria.
• The visual axes may converge or diverge, or be vertically misaligned.
• Diagrams for various types of heterophoria are provided.

Classification of Heterophoria - Magnitude

• Magnitude is measured in prism dioptres (△) or degrees (°).
• Horizontal and vertical heterophorias are measured and methods, including using a synoptophore and completing a prism bar cover test are described.

Classification of Heterophoria - Fixation Distance

• Fixation distance (D or N) is mentioned.
• A typical range from 24 prism diopters (SOP) to 44 prism dioptrees (XOP) for distance vision is provided as an example.
• A related number of prism dioptres for near-vision is also noted.
• Based on the varied distance for measurement, other classifications may be possible.

Classification of Heterophoria - Compensation

• Compensation is important, as it can be classified as compensated (adequate fusional reserves, no symptoms) or uncompensated (inadequate fusional reserves, symptoms like asthenopia).

Classification of Heterotropia

• The classification of heterotropia includes comitancy, frequency, laterality, direction, magnitude, fixation distance, age of onset, and influence of accommodation considerations.

Classification of Heterotropia - Comitancy

• Comitancy: Constant deviation in all directions of gaze (versus incomitant deviation that varies by direction of gaze).

Classification of Heterotropia - Frequency

• Constant: Tropia is always present.
• Intermittent: Tropia is present some of the time. Motor fusion may control it with latent (heterophoria state).
• Cyclic: Uncommon condition; 48 hour cycle of normal vision and manifest tropia; tends towards constant over time.

Classification of Heterotropia - Laterality

• Unilateral: Patient constantly fixates with the same eye (R or L); reduced visual acuity in the deviating eye.
• Alternating: Either eye can fixate; visual acuity usually approximately equal.

Classification of Heterotropia - Direction

• Provides examples, like esotropia and exotropia (eyes deviating towards or away from the nose), while also illustrating hypertropia (deviation of the eyes vertically), incyclotropia (rotation of the eye about the visual axis), and excyclotropia (rotation of the eye temporally).

Classification of Heterotropia - Magnitude

• Units of measurement include prism dioptres (△), degrees (°), and cyclophorias. These are for horizontal/vertical phorias.
• Methods for determining magnitude are outlined; including educated guess, Hirschberg's method, using a synoptophore, and prism bar cover test.

Classification of Heterotropia - Fixation Distance

• Fixation distance measurement (D or N) is discussed.
• Variable assessment distance of heterotropia may lead to further classification.
• Examples on how basic heterophoria may be different at near or at distance; and these are for the differences on convergence insufficiency, convergence excess, divergence insufficiency, and divergence excess.

Classification of Heterotropia - Age of Onset

• Congenital: Developed during the first few months of life.
• Acquired: Onset may occur during childhood or later.

Classification of Heterotropia - Influence of Accommodation

• Non-accommodative: Magnitude of tropia is independent of accommodation.
• Accommodative: Magnitude of tropia changes based on accommodation; may be fully or partially accommodative. This relates to accommodation convergence relationship.