The Hess test is an examination used to record the degree of underaction or overaction of the extraocular muscles. It is suitable for evaluating and monitoring incomitant strabismus, where the deviation changes with gaze direction.
The main significance of the Hess chart and visual field test is to “record” the degree of ocular motility disorder and diplopia. Unlike other ophthalmic tests, this test rarely detects abnormalities for the first time or leads to a definitive diagnosis. Although inspection of eye movements by visual observation provides more information, the Hess test is superior in objectively recording the degree of impairment and tracking changes over time.
Paralytic ocular motility disorders and diplopia: when recording the degree and temporal changes of the disorder
Orbital floor fracture, abducens nerve palsy, thyroid eye disease: recording the degree of ocular motility disorder (in orbital floor fractures, the Hess area ratio has been reported as a predictive indicator for residual postoperative diplopia) [1]
Pre- and post-operative paralytic strabismus: evaluation of changes due to treatment
Myasthenia gravis: “drift” on the Hess chart during sustained gaze has been reported to be useful for diagnosing myasthenia gravis with high specificity (100%) [5]
Incomitant strabismus occurs due to the following causes.
Nerve palsy: disorders of cranial nerves III (oculomotor), IV (trochlear), and VI (abducens)
Thyroid eye disease: an autoimmune disease affecting the extraocular muscles and surrounding tissues
Blowout fracture: entrapment of the extraocular muscle due to orbital fracture
Myasthenia gravis: a neuromuscular disease causing muscle weakness
QWhen is the Hess test performed?
A
It is performed when there is paralytic eye movement disorder or diplopia, and the degree and changes over time need to be objectively recorded. It is also used for recording movement disorders in orbital floor fractures, abducens nerve palsy, and thyroid eye disease, as well as for pre- and post-operative evaluation of paralytic strabismus.
The Hess chart graphically represents extraocular muscle function and deviation in different gaze directions. The first step in interpretation is to check the deviation at the center point (primary position) of the chart, which allows initial assessment of the type and magnitude of strabismus.
Small chart
Eye with paretic muscle: The eye wearing the green lens has the palsy.
Maximum restriction: The chart is most reduced in the direction of action of the paretic muscle. The muscle that acts farthest from the grid is the paretic muscle.
Inner chart reduction: An inner chart smaller than normal indicates muscle dysfunction.
Large chart
Overaction of the contralateral yoke muscle: Indicates the eye where the yoke muscle (yoke muscle) contralateral to the paralyzed muscle is overacting.
Maximum expansion: The chart expands most in the direction of action of the overacting yoke muscle.
Enlargement of the outer field: An outer field larger than normal suggests muscle overaction.
Check primary position: Read the ocular deviation in primary position (first eye position). Evaluate the direction and amount of exotropia, esotropia, and vertical deviation.
Determine comitance: If the left and right charts are symmetrical, there is no ocular muscle palsy or it is comitant strabismus. In comitant strabismus or skew deviation, the pattern size is equal in both eyes, and only horizontal and vertical shifts are recorded.
Estimate the causative muscle: Check for restriction of eye movement on the chart of the eye with poor movement, and infer the causative muscle from the gaze direction where the misalignment is greatest.
Monocular paralytic strabismus: When fixing with the healthy eye, the chart of the paralyzed eye (primary deviation) is compressed; when fixing with the paralyzed eye (secondary deviation), the chart is enlarged.
V and A patterns: Appear as inward or outward tilting of the visual field on the chart.
Divergence paralysis: Often shows a mild V-pattern exotropia.
Convergence paralysis: Often shows an esotropia pattern.
Overaction of the ipsilateral antagonist: In the eye with a paralyzed muscle, the antagonist of the paralyzed muscle may show overaction. This is because the normal function of the paralyzed muscle is lacking, so the action of the antagonist is not inhibited.
Underaction of the contralateral antagonist: Occurs as a result of the overaction of the contralateral yoke muscle suppressing its contralateral antagonist. The presence of this interaction suggests a chronic palsy.
QWhat can be determined from the size of the chart?
A
The smaller chart indicates the eye with the paretic muscle (affected eye), and the most constricted direction within it corresponds to the action direction of the paretic muscle. The larger chart indicates the eye where the contralateral yoke muscle is overacting.
The Hess test relies on foveal projection and is based on the principle of confusion. This principle is a different concept from diplopia.
Diplopia
Double vision: A condition in which a single object is perceived as two images due to misalignment of the eyes.
This occurs when different images from a single object are projected onto each eye.
Confusion
Image superimposition: A phenomenon in which corresponding points (usually the fovea) of both eyes are stimulated simultaneously, causing two different objects to appear superimposed.
The Hess test uses this confusion to evaluate ocular deviation.
In the Hess test, dissociative tools such as red-green goggles are used to artificially create confusion. A red lens is placed in front of the fixing eye and a green lens in front of the test eye (non-fixing eye), and the position of the non-fixing eye required to superimpose the image seen by the fixing eye and the image seen by the non-fixing eye is recorded.
QHow are diplopia and confusion different?
A
Diplopia is a condition in which a single object appears double due to misalignment of the eyes. In contrast, confusion is a phenomenon in which two different objects appear superimposed because corresponding points (fovea) of both eyes are stimulated. The Hess test uses this principle of confusion to measure ocular deviation.
The Hess screen is a tangent screen with a checkerboard pattern on a dark gray background. Twenty-five red lights are individually controlled and represent the diagnostic gaze directions. The grid consists of an inner circle (15° from center) and an outer circle (30° from center), with each square corresponding to 5° of eye rotation.
Mild palsy: If the deviation is small, it may be difficult to detect with the central 15° inner circle; therefore, it is necessary to examine up to the 30° outer frame.
Blowout fracture: Even if the ocular deviation is not clear, examine up to the peripheral 30° outer circle.
Anisometropia: Examination may be difficult. If strong correction is needed, pay attention to the prismatic effect of the corrective lenses.
Children: Examination is difficult if the head position in the sitting position is unstable.
Muscle sequelae are patterns of muscle overaction and dysfunction that appear sequentially after the onset of incomitant strabismus. They are based on Sherrington’s law of reciprocal innervation and Hering’s law of equal innervation.
The order of appearance of muscle sequelae is as follows:
Order
Change
Law
Stage 1
Overaction of the contralateral synergist
Hering’s law
Stage 2
Overaction of ipsilateral antagonist
Sherrington’s law
Stage 3
Dysfunction of contralateral antagonist
Combination of both laws
If muscle sequelae are absent, it suggests a recent-onset palsy. If muscle sequelae are present, it indicates a long-standing, chronic palsy, and the more extensive the sequelae, the more chronic the condition is likely to be.
A case of a 66-year-old man who developed right fourth nerve palsy (trochlear nerve palsy) after neurosurgery for pleomorphic pineocytoma has been reported.
Chief complaint: Diplopia on left gaze, right hypertropia (right eye higher than left eye)
Hess chart findings: Dysfunction of the right superior oblique muscle (impaired intorsion and depression), especially prominent on left gaze
Muscle sequelae: Overaction of the left inferior rectus (contralateral synergist), overaction of the right inferior oblique (ipsilateral antagonist), dysfunction of the left superior rectus (contralateral antagonist)
All muscle sequelae were present, suggesting a chronic palsy.
Patient cooperation: The accuracy of the test depends heavily on the patient’s understanding and concentration.
Time: The test takes time to perform and requires patience from both the examiner and the patient.
Need for binocular vision: Normal binocular vision is a prerequisite. It cannot be performed if there is suppression (a condition where the brain ignores the image from one eye) or abnormal retinal correspondence.
Poor vision: The test cannot be performed if vision in one eye is poor.
Color vision deficiency: Accurate testing is not possible in cases of red-green color blindness.
Large-angle deviation: If the deviation is very large, the perceived point may fall outside the chart range, making accurate assessment difficult.
Cyclodeviation: The grid scale primarily measures horizontal and vertical deviations, making evaluation of cyclodeviation difficult.
Children: Testing is difficult in patients who cannot maintain a stable head position while seated.
Bilateral palsy: If there are movement restrictions in both eyes, the deviation may be offset, and a comprehensive judgment must be made by also referring to the results of the duction test. If both eyes are paralyzed, a determination is impossible.
Old palsy: Secondary motor adaptations may develop, making the initial diagnosis unclear.
Manifest vs. latent deviation: The Hess test cannot distinguish between manifest strabismus (tropia) and latent strabismus (phoria).
In recent years, technological advances such as eye tracker devices have provided alternative methods that are less influenced by patient or examiner factors, enabling more objective and accurate assessments. Computerized Hess charts (Ocular Motility Analyzer) can be measured in a comparable time to conventional methods and are reported to be slightly easier for patients to perform [2]. Furthermore, examination devices that enable automated data analysis and electronic recording, such as digital KM screens and digital Hess screens for orbital fractures, have been developed, and are expected to shorten examination time and standardize procedures while maintaining accuracy equivalent to conventional methods [3][4].
QCan the Hess test be performed in patients with color vision deficiency?
A
In cases of red-green color vision deficiency, the conventional Hess test using red-green glasses cannot be performed accurately. As an alternative, the use of the Lees chart with a double-sided mirror should be considered.
Grenga PL, Reale G, Cofone C, et al. Hess area ratio and diplopia: evaluation of 30 patients undergoing surgical repair for orbital blow-out fracture. Ophthalmic Plast Reconstr Surg. 2009;25(2):123-125. PMID: 19300155. doi:10.1097/IOP.0b013e31819a41d5
Watts P, Nayak H, Lim MK, et al. Validity and ease of use of a computerized Hess chart. J AAPOS. 2011;15(5):451-454. PMID: 21958903. doi:10.1016/j.jaapos.2010.12.021
Thorisdottir RL, Sundgren J, Sheikh R, et al. Comparison of a new digital KM screen test with conventional Hess and Lees screen tests in the mapping of ocular deviations. J AAPOS. 2018;22(4):277-280.e6. PMID: 29852255. doi:10.1016/j.jaapos.2018.02.007
Akkina SR, Shabbir A, Lahti A, et al. Quantifying Eye Alignment in Orbital Fracture Patients: The Digital Hess Screen. Facial Plast Surg Aesthet Med. 2020;22(6):427-432. PMID: 32456473. PMCID: PMC7703130. doi:10.1089/fpsam.2020.0080
Keene KR, de Nie JM, Brink MJ, et al. Diagnosing myasthenia gravis using orthoptic measurements: assessing extraocular muscle fatiguability. J Neurol Neurosurg Psychiatry. 2023;94(2):151. PMID: 36261286. doi:10.1136/jnnp-2022-329859
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