The Hess Test
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Description/Overwiew
The Hess Test: A Diagnostic Tool for Incomitant Strabismus[1]
A Hess test or Hess chart is used to map the amount of underaction or overaction of the extraocular muscles. The Hess test is a valuable clinical tool used to assess and monitor incomitant strabismus, a condition where the deviation of the eyes varies depending on the direction of gaze. This condition often suggests an underlying issue with the extraocular muscles or their nerve supply, such as paresis, paralysis, or restriction. The test can be repeated over time to assess for change. The Lees chart is a similar test that uses double sided mirrors instead of red and green glasses.
Incomitant strabismus can arise from various causes, including:
- Nerve paresis: Affecting the third, fourth, or sixth cranial nerves, which control eye movements.
- Thyroid ophthalmopathy: An autoimmune condition affecting the eye muscles and surrounding tissues.
- Blow-out fractures: Orbital fractures that can entrap eye muscles.
- Myasthenia gravis: A neuromuscular disorder causing muscle weakness.
The Hess test helps identify imbalances in the extraocular muscles, highlighting underactions or overactions that contribute to the strabismus.
Principles
Principles of the Hess Test: Confusion vs. Diplopia[1]
The Hess test relies on foveal projection and maps the position of the non-fixating eye needed to superimpose the image seen by the non-fixating eye with that of the fixating eye.
The Hess test is based on the principle of confusion, which differs from diplopia.
- Diplopia is double vision, where two images of a single object are perceived due to misaligned eyes.
- Confusion occurs when two different objects are seen as overlapping, caused by stimulating corresponding retinal points in each eye, usually the foveae.
The Hess test artificially creates confusion using dissociative tools like red-green glasses, allowing for the assessment of ocular deviations.
Procedure/Methodology
The Hess test involves a specific setup and procedure to accurately assess ocular deviations.
Equipment and Exam Procedure[2]
- Hess Screen Setup: The Hess screen is a tangent screen featuring a checkerboard pattern on a dark gray background. It has twenty-five red lights individually controlled to represent the cardinal positions of gaze, outlining an inner field (15° from the primary position) and an outer field (30° from primary position). Each square on the grid equates to 5° of ocular rotation.
- Patient Positioning and Procedure: The patient stands 50 cm away from the screen with their head stabilized on a chin cup. Their eyes are aligned with the central point of the screen, which symbolizes the primary position. The examination is conducted in a dimly lit room. The patient wears reversible goggles with one red and one green lens. The eye with the red lens will see the red lights and the eye with the green lens will see the green lights. The red lens is placed in front of the fixating eye, and the green lens in front of the non-fixating eye.
- Testing Process: Red lights are illuminated on the screen by the examiner and the patient uses a green pointer to superimpose a green light over each red light. The test is measuring the amount of deviation of the non-fixating eye covered by the green glasses filter. The test is performed with each eye fixating by reversing the goggles. The test identifies the fixating and non-fixating eyes based on the position of the red filter. The examiner then connects the points where the patient projected the green light, creating the final graph. Left eye movements are charted on the left side of the graph, and right eye movements on the right.
Advancements in Technology[3]
Recent technological advancements offer alternatives that are less dependent on patient and examiner factors, such as eye tracker devices, for more accurate assessments.
Interpretation
Interpretation of Hess Chart Findings[2][4]
The Hess chart provides a graphical representation of ocular muscle function and deviations in different gaze positions. The Hess chart maps a foveal projection and deviations indicated on the results reflect the direction of misalignment of the eye, e.g. the map for an eye that is hypertropic and exotropic would be shifted up and out. Understanding the key elements of interpretation is crucial for accurate diagnosis and management of incomitant strabismus.
Starting with the Primary Position
Begin by observing the deviation in the primary position, which is the central point on the chart. This provides an initial assessment of the type and magnitude of strabismus.
Identifying the Affected Eye and Muscle
- Smaller Chart: Indicates the eye with the paretic muscle.
- Larger Chart: Indicates the eye with the overacting yoke muscle. The yoke muscle is the contralateral synergist, which overacts in response to the primary muscle's underaction.
Analyzing the Charts
- Maximum Restriction: The smaller chart shows the greatest restriction in the direction of action of the paretic muscle.
- Maximal Expansion: The larger chart shows the greatest expansion in the direction of action of the overacting yoke muscle.
- Inner Field: A smaller than normal inner field indicates underaction of a muscle.
- Outer Field: A larger than normal outer field suggests overaction of a muscle.
- V- and A-Patterns: The inward or outward slope of the fields on the chart reveals V- and A-patterns in strabismus.
Additional Considerations
- Homolateral Antagonist: In an eye with a paretic muscle, the homolateral antagonist to the paretic muscle (the muscle that opposes the action of the agonist) may show overaction. This occurs due to the unopposed action of the antagonist in the absence of normal function of the paretic muscle.
- Contralateral Antagonist: The contralateral agonist is the muscle in the eye contralateral to the agonist that moves the eye opposite the direction of the agonist (e.g., for a right lateral rectus, the contralateral agonist is the left lateral rectus). An overacting contralateral synergist (yoke muscle) to a paretic agonist can be associated with underaction of the contralateral antagonist. This is a secondary effect, as the overaction of the yoke muscle inhibits the action of its contralateral antagonist. When this interaction exists, it implies a long-standing palsy.
Muscle Sequelae
Muscle sequelae are a pattern of muscle overaction and underaction seen in incomitant strabismus, which sequentially develops after the onset of the condition. These sequelae follow Sherrington's law of reciprocal innervation, which states that when a muscle receives increased innervation, decreased innervation is directed to its ipsilateral antagonist.
The order of muscle sequelae development as follows:
- Overaction of the contralateral synergist (Hering's law)
- Overaction of ipsilateral antagonist (Sherrington's law)
- Underaction of contralateral antagonist (Hering's law and Sherrington's law)
The absence of muscle sequelae suggests a recent palsy, while their presence indicates a long-standing condition. The greater the extent of muscle sequelae, the more chronic the palsy is likely to be.
By carefully analyzing the size, shape, and slope of the Hess charts, along with considering the interplay between agonist and antagonist muscles in both eyes, ophthalmologists can understand the complex nature of incomitant strabismus.
Limitations
While the Hess test is a valuable tool in strabismus evaluation, it's essential to be aware of its limitations:[2][4]
- Patient Collaboration and Attention: The test's accuracy depends heavily on the patient's ability to understand and follow instructions, maintain concentration, and provide reliable responses.
- Time-Consuming: Administering the Hess test can be time-consuming, requiring patience from both the examiner and the patient.
- Requirement for Normal Binocular Vision: The Hess test is most informative in patients with binocular vision. It may not be suitable for those with suppression (where the brain ignores one eye's image) or abnormal retinal correspondence (where non-corresponding retinal points are interpreted as a single image).
- Color Vision: Patients with red-green color blindness cannot perform the test accurately.
- Limitations with Large Deviations: In cases of very large deviations, the perceived point may fall outside the Hess chart's field, limiting the assessment's accuracy.
- Difficulty Assessing Torsional Deviations: The Hess chart's gridded scale primarily measures horizontal and vertical deviations, making it challenging to evaluate torsional deviations (rotations of the eye around its visual axis).
- Old Paralysis Complications: In long-standing paralysis cases, secondary motor adaptations can develop, potentially obscuring the initial diagnosis.
- Manifest vs. Latent Deviations: The Hess test does not differentiate between manifest deviations (tropias, always present) and latent deviations (phorias, only present when binocular fusion is disrupted).
Understanding these limitations helps clinicians interpret Hess test results accurately and use the test appropriately in conjunction with other diagnostic methods for comprehensive strabismus evaluation.
Example
Video explanation by Connie Koklanis
https://youtu.be/4E_L2YKI4N0?si=fCqGxaCWjr_gV9om
References
- ↑ 1.0 1.1 American Academy of Ophtalmology, Basic and Clinical Science Course, section 6 – Pediatric Ophtalmology and strabismus, Chapter 7.
- ↑ 2.0 2.1 2.2 Sociedade Portuguesa de Oftalmologia, Meios complementares de diagnóstico, Chapter 2.6, Avaliação da visão binocular
- ↑ Orduna-Hospital E, Maurain-Orera L, Lopez-de-la-Fuente C, Sanchez-Cano A. Hess Lancaster Screen Test with Eye Tracker: An Objective Method for the Measurement of Binocular Gaze Direction. Life. 2023; 13(3):668. https://doi.org/10.3390/life13030668
- ↑ 4.0 4.1 Von Noorden, G.K. and Campos, E.C. (2002) Binocular vision and ocular motility. 6th Edition, CV Mosby, St. Louis
