Delhi Journal of Ophthalmology

A Brief About Hess and Lees Charting

Aakanksha Raghuvanshi, Rishabh Sah, Deepanshu, Subhash Dadeya
Department of ophthalmology, Guru Nanak Eye Centre, Maharana Ranjit Singh Marg, New Delhi, India

Corresponding Author:

Aakanksha Raghuvanshi
Department of ophthalmology, Guru Nanak Eye Centre, 
Maharana Ranjit Singh Marg, New Delhi, India.

Received: 05-JAN-2022

Accepted: 25-FEB-2022

Published Online: 03-APR-2022


Hess chart detects the amount of underaction and overaction of muscles therefore it helps in diagnosis of ocular motility defects. Further by comparing subsequent Hess charts, one can find out improvement or deterioration of a condition. This test can give us repeated and reliable results. Modification of the Hess screen is a Lees screen which uses double sided mirror instead of color dissociation method used by Hess chart. This pg snippet manuscript demonstrates various conditions where Hess or Lees chart can be used and help in diagnosis along with decision making in the management of various ocular motility defects. 

Keywords :Hess Chart, Less Screen, Underaction, Overaction Of Muscles, Ocular Motility Defects

HESS Charting Introduction
Hess chart underwent various modifications in the past. The modern Hess screen is a gray board on which there is a square meter tangent scale and small red lights are mounted which can be illuminated in turn by bulbs behind the screen. Also there is a movable illuminated green indicator.1 The examiner switches on a specific target while the patient bisects the red dot by projecting the green light. For opposite field the goggles are reversed as the red lights are an integral part of the Hess screen. The Hess screen consists of a central dot, an inner square of 8 dots, and an outer square of 16 dots (15 degrees and 30 degrees from the primary position, respectively) (Figure 1). Atleast 33 cm distance is necessary to be maintained to avoid accommodative or convergence factors. 

Principles of HESS screen
Hess screen follows haploscopic principle which involves foveal projection and dissociation of eyes.1 This test can be called as a fovea-to-fovea (maculo-macular) test as each fovea was looking at the point on the screen to which it is deviated (Figure 2)1

Hess chart plotting
Hering’s and Sherrington’s law of innervation are used to plot the chart.2 The patient wears Armstrong goggles during the test. The eye with red glass is the fixing eye. The distance between the screen and patient is kept at 0.5 m with the patient in sitting position keeping his head erect and immobile. The lights of the examination room are dimmed to remove fusional background cues and further dissociate the eyes. The patient is then asked to project green line on the screen to bisect each red dot with the green line. Plotting of each dot is first done for inner field and then the outer one. Once this task is completed, the goggles are reversed while the patient still projects a green line, and the test is repeated.

Patients complaining of diplopia should be tested on Hess chart. Specific indications are
  1. All patients of incomitant strabismus with normal retinal correspondence (NRC).
  2. Patients of esotropia to rule out 6th nerve palsy.
  3. To provide a baseline in conditions likely to develop defective ocular movements e.g. thyroid-related orbitopathy.

Basic Rules for Interpretation of a Hess Plot
  • Small field belongs to eye having defect.
  • Neurogenic paresis will show the muscle sequelae to a greater or lesser extent (dependent on the duration of underlying condition and which eye is used for fixation).
  • The largest underaction is normally in the direction of action of the paretic muscle and the largest overaction is normally of the contralateral synergist.3

Hess Chart in Various Case scenarios 
(Figure 3 - Figure 9) 
Uses of Hess chart
  • In Duane’s or Brown’s syndrome, an obvious restriction in the affected gaze position does not produce the usual overaction of ipsilateral antagonist as it would be in case of palsy. 
  • Useful in planning surgery In conditions like superior oblique muscle palsy, Hess chart is useful in planning surgery by comparing the overaction of the ipsilateral antagonist (inferior oblique) versus the contralateral synergist (opposite inferior rectus) in deciding which muscle should be weakened, or weakened first if two-staged surgery is planned.
  • Other disorders - Most restrictive conditions (e.g. orbital fractures with entrapment of muscles) or thyroid restrictive disease can be differentiated from muscle palsy by the different patterns seen in these diseases on Hess chart.

LEES Screen
Lees designed a new test that would resolve the problems like color separation and difficulty in retaining sufficient color contrast and would be more comfortable to use by the patients, by retaining tangential screen.

It consists of two screens which are independently illuminated and are positioned at 90° to each other. These screens have identical tangent screens printed in back. A double-sided plane mirror is used for dissociation as it bisects the screens such that the virtual image of one screen is superimposed on the other.4 

Plotting of LEES chart
The deviating eye projects a virtual image on the blank screen which is seen by the fixing eye. The patient is handed a pointer with a small ring at the distal end. As the examiner indicates which object has to be fixated on the illuminated screen, the patient is asked to place the pointer on each of the similar positions which are seen by them on another screen and that screen will appear blank to the examiner. 
This blank screen is illuminated by means of a foot pedal pressed long enough for the observer to plot the apparent object position on the chart. The test is repeated using the other eye for fixation. This requires the patient to turn the chin rest and face towards the other screen. The illumination is switched and the opposite screen becomes the one used for fixation. (Figure 10)

Uses of a Lee's Screen
Similar to Hess chart
The Lees test is easier to perform by majority of patients including young (5-6 years) children provided they are able to maintain central fixation.

Interpretation Of Lee's Chart Same as Hess chart.

  1. Roper-Hall G. The hess screen test. American Orthoptic Journal. 2006 Jan 1;56(1):166-74.
  2. Pritchard C, Ellis GS. Approach to visual acuity assessment and strabismus evaluation of the pediatric patient. InPractical Management of Pediatric Ocular Disorders and Strabismus 2016 (pp. 3-23). Springer, New York, NY.
  3. Fung TH, Amoaku WM. Viva and OSCE Exams in Ophthalmology. Springer International Publishing; 2020.
  4. Timms C. The Lees screen test. American Orthoptic Journal. 2006 Jan 1;56(1):180


Raghuvanshi A, Sah R, Deepanshu, Dadeya SA Brief About Hess and Lees Charting.DJO 2022;32:74-78


Raghuvanshi A, Sah R, Deepanshu, Dadeya SA Brief About Hess and Lees Charting.DJO [serial online] 2022[cited 2022 May 19];32:74-78. Available from: