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Hess Charting or Hess Screen Test
Hess charting is a diagnostic procedure to measure the deviation and the amount of underaction and overaction of extraocular muscles. It helps to rule out the ocular motility defects. Also known as Hess Screen Test, Hess Charting (or Chart) was designed by a well renowned, Nobel Prize (1949) winning neurophysiologist, Walter Rudolf Hess in 1908.
The original Hess screen test used a back screen to mark a square-meter tangent scale. The tangent nature of the coordinate lines converts equidistant points, seen in a virtual sphere like a perimeter, into a two-dimensional chart.
An accurate and simple charting of clinical findings is always essential, and for purposes of record and comparison, this applies especially to the recording of ocular muscle palsies. The usual way (and very often the only way) in the orthoptic or consulting room is to use some form of the Hess screen test.
Hess Chart (Hess Screen Test)
The original Hess screen test used a black screen on which was marked a square-meter tangent scale. The tangent nature of the coordinate lines converts equidistant points, seen in a virtual sphere like a perimeter, into a two-dimensional chart. The electrically operated board has already replaced the original Hess screen.
Each Hess screen consists of a central dot, an inner square of eight dots, and an outer square of 16 dots (15 degrees and 30 degrees from the primary position, respectively). Lees screen test is also a similar test to a Hess chart. But to achieve dissociation, a mirror is used instead of R-G goggles.
The Hess screen is an electrically operated greyboard marked with a grid on which small red lights are individually illuminated and a movable illuminated green indicator. Each of the red lights can be switched on and off by inserting and removing a conductor plug into the switchboard that is controlled by the examiner.
Each eye needs to be tested separately. The test uses red/green complementary filters to create color dissociation which maximizes the ocular deviation. The eye behind the green glass is the testing eye and the other eye with red glass is the fixating eye.
The red goggle only transmits red light and absorbs other wavelengths = “green filter”.
Purpose of Hess Charting
The primary purpose of the Hess chart is to assess the amount of restriction of an extraocular muscle in case of paralytic or restrictive strabismus. It is equally useful in follow-up cases to show improvement or deterioration of the squint. Hess screen test is a reliable and repeatable procedure to keep records of the condition.
Hess chart principle
The procedure employs the haploscopic principle which involves foveal projection and dissociation of eyes. The dissociation of two eyes is achieved by the means of colors. The Hess chart is plotted based on Hering’s law of equal innervation and Sherrington’s law of reciprocal innervation.
Hering’s Law of Equal Innervation
“Every motor innervation arising from cortical centers flows equally to both eyes causing associated muscles called agonists and antagonists to contract and relax respectively”.
Sherrington’s Law of Reciprocal Innervation
“when one of the oculorotatory muscles receives an innervation to contract its direct antagonist will receive a simultaneous innervation to relax”.
Hering’s law applies to the muscles of both eyes, Sherrington’s law applies to the muscles of one eye.
Prerequisites for Hess Charting or Hess Screen Test
The patient needs to fulfill the following given prerequisites to perform the Hess screen test effectively.
- Normal color vision to distinguish between red and green colors
- Normal retinal correspondence and central fixation.
- A reasonable degree of co-operation and level of intelligence who can better understand the procedure. It is the most important criteria as Hess charting is a purely subjective test.
- The ability to wear the red and green close-fitting goggles, which is often difficult, in cases such as post-herpetic neuralgia.
- Good vision in both eyes to see the relatively weak colors of lights.
Clinical Significance of Hess Charting (or Hess Screen Test)
Hess charting is useful in the diagnosis of paralysis of extraocular muscles. Likewise, it also helps to assess the degradation and progress of the deviation overtimes, i.e., to evaluate the results of incomitant squint. So, the Hess chart is the permanent record to compare the results of subsequent examinations.
Similarly, the Hess screen test is a compulsory diagnostic procedure to perform before any type of extraocular muscle surgery. It is also helpful in differentiating long-standing paralysis from the recent onset one.
Another important clinical significance of Hess charting is its use in differentiating paretic strabismus caused by neurological defects from mechanical strabismus caused by thyroid eye disease, blowout fracture of the orbit, etc.
Hess Charting Indications
- Incomitant strabismus (where the squint angle changes depending on the direction of gaze)
- Patients with divergence weakness esotropia to exclude lateral rectus palsy
- To differentiate lateral rectus palsy from divergence palsy
How to perform the Hess Charting or Hess Screen Test
The Hess screen test is performed at a 50 cm distance (distance between the patient’s eye and the Hess screen) with each eye fixating turn by turn (right eye first and then the left eye). The patient is asked to wear R-G goggles in such a way that the red glass is placed on the right eye (fixating eye) first.
The patient’s head should be erect and centered on the fixation spot with eyes in primary position throughout the test. If he is unable to maintain this position, it is necessary to hold the head in the correct position by another person.
A green light projection pointer is given to the patient. Red fixation light is projected on the screen and the patient is asked to superimpose the green light over the red. In normal conditions, both the lights will superimpose.
The goggles are then reversed so that red glass is in front of the left eye and the procedure is repeated. The responses are analyzed by connecting the response marks with lines.
Hess Chart Recording
The recording of the left eye (with green goggles) is done at first and the red-green goggles are reversed with the red filter now in front of the left eye. And, the recording of the right eye is done.
It is advisable to start from point A then go to above point B then proceed clockwise from C to I. The examiner marks the positions indicated by the patient on the small card with a reduced copy of the screen. The points found by the patient are connected by straight lines and permit the examiner to determine which, if any, muscles react abnormally.
Hess Chart Interpretation
The Hess chart position, shape, size, and measurement explain the type and amount of deviation in a particular eye. So, the interpretation of the Hess chart is based on these primary characteristics.
Position of Hess Chart
The position, shape, and size of the Hess chart give a clear clinical picture in the early stages of ocular deviation (when there is no spread of comitance).
Unlike diplopia charting (where the higher image is of the lower eye and vice versa), the lower field belongs to the lower eye and the higher field belongs to the higher eye in Hess charting. This is guided by the basic principle on which the Hess chart is performed, i.e. foveal projection.
The central dot in each field indicates the deviation in the primary position. Therefore, it is important to mark and notice the central dot fixing the right eye and fixing the left eye.
Size and Shape Measurement of Hess Chart
The difference in the size of the Hess chart is governed by Hering’s law of equal innervation. A smaller chart indicates the eye with the underacting muscle (primary deviation). Likewise, the larger chart indicates the eye with the overacting muscle (secondary deviation).
The smaller chart shows the greatest restriction in the main direction of action of underacting muscle whereas the larger chart shows the greatest expansion in the main direction of action of the yoke muscle.
Inward displacement of the dots (and the curve) depicts the underaction of the muscle. The eye cannot move far enough to plot in a normal position. Similarly, the outward displacement of dots indicates overaction (or contracture). Due to excessive movement of the eye, the dots are displaced beyond the normal position.
When there is the development of muscle sequelae, the size of both fields will be equal. It also indicates the non-paretic strabismus. In muscle sequelae there are:
- Underaction of the affected muscle
- Overaction of the contralateral synergist
- Contracture of an ipsilateral antagonist of the paretic muscle
- Secondary underaction of the contralateral synergist of contracted muscle.
It is equally necessary to examine the outer fields for small underaction and overactions which may not be seen in the inner field.
To calculate the amount of deviation, the position of the central dot can be measured. The small square on the chart subtends a 5 degrees angle at the testing distance of 50 cm. in the primary position, the amount is calculated by fixing either eye by the displacement of the pointer from the center dots.
In a similar way, the amount of overaction and underaction can be calculated. To measure the degree of cyclotropia, the chart is placed in a torsion position.
Features of Neurogenic Defects
- The smaller field has more or less equal or proportional spacing between the inner and outer fields.
- With the tendency of becoming comitant the Hess chart between the two eyes tends to be more similar in size with time.
- Follow muscle sequelae (common).
Features of Mechanical Defects
- Disproportional spacing between the inner and outer fields, i.e., compressed either horizontally or vertically.
- Marked overaction of the contralateral synergist.
- The underaction of the contralateral antagonist and overaction of the direct antagonist is not obvious.
Hess chart vs diplopia charting
The diplopia charting is based on the diploscopic principle. It means the patient is looking at one object and seeing two different images that can be superimposed or not depending on whether the patient is having diplopia or not.
But the Hess charting is based on the haploscopic principle. Here, the patient looks at two different objects and sees two different images.
Diplopia charting gives us a picture of the patient’s double vision in nine diagnostic positions of gaze. However, we get an actual picture of each muscle action in each of the six cardinal positions of gaze.
In diplopia charting, R-G goggles are used to form two different images of the same object. But, R-G goggles are used in the Hess screen test to dissociate the two eyes.
Both diplopia charting and Hess charting are subjective tests and can’t be used in abnormal retinal correspondence, suppression, and color vision deficiency as diplopia can’t be elicited.
Some Examples of Hess Charting or Hess Screen Test
Hess charting is an important tool for eye doctors to plan the treatment and to evaluate the result. However, the result of Hess charting shouldn’t be used in isolation to measure ocular deviation. Regular clinical examination, cover test, diplopia charting, and binocular visual field testing along with Hess screen test help in the better management of the patient with ocular deviation and double vision.
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