Binocular Vision Anomalies II - Incomitant Strabismus

This set of lecture notes focuses on incomitant strabismus, particularly the distinctions between innervational and restrictive strabismus, which is crucial for accurate diagnosis and treatment planning. Understanding the underlying causes of strabismus involves differentiating between issues arising from nerve damage (paresis or paralysis) and those resulting from mechanical restrictions in the eye movements.

1. Strabismus Overview

In strabismus, when one eye fails to execute a full range of duction, it is essential to determine whether the limitation is due to neural causes or mechanical issues. This determination is critical for effective management and treatment.

2. Tests for Assessing Eye Movement Limitations

2.1 Forced Duction Test (FDT)

The Forced Duction Test (FDT) assesses eye movement by evaluating the passive movement of extraocular muscles (EOM). It can discern whether limited eye movement results from a neurological issue or a mechanical restriction.

Procedure:

The conjunctiva is anesthetized using proparacaine, tetracaine, or 4% lidocaine.
The patient is instructed to look in the direction where movement is limited.
The examiner stabilizes the eye and attempts to move it passively.

Results:

Negative FDT: Free passive ductions, indicating paresis or paralysis.
Positive FDT: Restricted ductions, suggesting mechanical obstruction.

2.2 Forced Generation Test (FGT)

The Forced Generation Test (FGT) is carried out once a mechanical restriction is established. It evaluates the potential function of a muscle by analyzing the active movement of the EOM.

Procedure:

The conjunctiva is again anesthetized.
The patient looks in the opposite direction of the limited duction.
The examiner assesses the pull or tug felt by the examiner while the patient looks towards the affected muscle.

Results:

A positive FGT indicates muscle activity.
Observation of saccadic velocity is noted to further assess muscle function.

3. Types of Strabismus

3.1 Paralytic Strabismus

Paralytic strabismus occurs when one or more extraocular muscles are paralyzed, leading to motor imbalance. Identifying the involved muscles is vital for successful treatment.

Diagnosis is based on key indicators:

Presence of motor deficiency in the affected muscle's field of action.
Diplopia and increased deviation when the patient looks with the paralyzed eye.
Compensatory head posture anomalies.

3.2 Differential Diagnosis

A differential diagnosis must be conducted to distinguish between congenital and acquired paralysis. Results from Hering's law indicate how innervation differs between eyes, affecting deviations:

Primary Deviation: Measured with the sound eye fixating.
Secondary Deviation: Assessed with the paretic eye fixed, where excessive innervation is required to maintain fixation.

4. Examination Techniques

4.1 Ductions and Versions

Examinations of duction and version are critical in the evaluation of recent paralysis, allowing detect of affected muscles in diagnostic positions of gaze.

4.2 Cover Test

The cover test can qualitatively diagnose which muscle is affected in cases of longstanding paresis. The procedure involves alternating eye cover to observe muscle responses and deviations.

4.3 Bielschowsky Head Tilt Test

This test identifies oblique muscle paresis related to vertical deviations.

Steps:

Identify type of hypertropia.
Monitor vertical deviation changes on right or left head tilting.
Determine the paretic muscle based on deviations observed.

5. Vertical Anomalies

Several vertical anomalies can occur, such as:

Double Elevator Palsy: Weakness in both the inferior oblique and superior rectus muscles, limiting elevation.
Double Depressor Paralysis: Rare, affecting the depressor muscles, leading to an inability to depress the affected eye from primary position.
Dissociated Vertical Deviation (DVD): Characterized by an upward drift in fixation, often confused with other forms of strabismus.

6. Cyclovertical Deviations

Cyclovertical deviations may manifest as vertical strabismus or isolated cyclotropia, often with compensatory head postures. Diagnosis involves identifying overacting or underacting muscles during diagnostic gaze positions and using the Forced Duction Test to determine the nature of the strabismus.

This comprehensive understanding of strabismus types and diagnoses aids clinicians in developing effective treatment strategies and improving patient outcomes.

Incomitant Strabismus Overview and Assessment

Incomitant Strabismus Overview and Assessment