1. Is Hess chart useful in all cases of strabismus?

Answer
No. It is useful only for incomitant strabismus.

 
2. What is the difference between Hess screen and Lees Screen?

Answer
In Hess screen, the two eyes are dissociated using lenses of different colours. The fixating eye looks through the red lens ( or filter) and the non-fixing eyes through the green lens (or filter).
In Lees screen, the eyes are dissociated using two opalescent glass screens at right angles to each other bisected by a two -sided plane mirror

 
3. What are the values of Hess chart?

Answer
a. Assessing progress, 
b. Planning treatment and 
c. Evaluating results of incomitant strabismus.

 
4. Is Hess chart alone useful for assessing incomitant strabismus?

Answer
No, it should not be used in isolation but combined with field of binocular single vision and prism cover test.

 
5. What criteria are required for using the Hess charts?

Answer
The patient must have normal retinal correspondence and central fixation.

 
6. What features of the Hess chart are used for interpretation?

Answer
The position, size, shape are compared between the two eyes.
The deviation between the two eyes can also be calculated. 

 
7. What does the position of the fields in Hess chart indicate?

Answer
The deviation of the eye. 
The central dot in each field indicates the deviation in the primary position.

 
8. What information can be obtained from the size of the Hess chart?

Answer
The eye with the small field is the affected eye.
Presence of a compressed field indicates a mechanical cause.

 
9. What can be obtained from the shape of the Hess chart?

Answer
Charts with sloping fields indicates the presence of A and V patterns.

 
10. How can one use Hess chart for measuring the ocular deviation?

Answer
Each small square subtends an angle of 5 degrees at the 50 cm working distance
The displacement of the pointer from the dot is a measure of the size of the deviation
Torsional measurements can be made using a specially adapted linear pointer

 
11. The Hess chart contains a central and outer field. What 
      information may be obtained by analysing the outer field?

Answer
In mild incomitant strabismus, the central field may appear normal. By analysing the outer field, small underaction or overaction may become apparent.

 
12. What are the features of mechanical defects on Hess chart?

Answer
Compressed field either vertically or horizontally
Limited muscle sequelae.

 
13. What are the features of neurogenic defects on Hess chart?

Answer
The smaller field has a proportional spacing between the inner and outer fields.
Muscle sequelae is common  and the Hess chart between the two eyes tend to become more similar in size with time.

 
14. In neurogenic strabismus, the muscle sequelae make the Hess 
      charts between the two eyes become more similar in size. What 
      laws govern the muscle sequelae?

Answer
Hering's law of equal innervation:. Innervation to the extraocular muscles is equal to both eye. Thus, when a nervous impulse is sent to an ocular muscle to contract, an equal impulse is sent to its contralateral synergist to contract.
Sherrington's law of reciprocal innervation. The contraction of a muscle is accompanied by simultaneous and proportional relaxation of its antagonist. For example, if the superior oblique muscle contracts, its antagonist, the inferior oblique muscle, relaxes.

 
15. What are the three stages in the development of muscle 
      sequelae?

Answer
1. Overaction of the contralateral synergist according to Hering's law
2. Overaction of the ipsilateral antagonist as its action is unopposed by the paralysed muscle
3. Secondary underaction of the contralateral antagonist. This occurs for two reasons: a} as the ipsilateral antagonist action is unopposed, less impulse is needed to move it into its desired position and consequently according to Hering's law, its contralateral synergist receives less impulse and therefore underact. b} overaction of the contralateral synergist means that its ipsilateral antagonist will receive equal impulse to relax according to the Sherrington's law.

 
16. Using the information in Q14, can you predict the muscle 
      sequelae in a left superior oblique?

Answer
a. Overaction of the contralateral synergist: right inferior rectus
b. Overaction of the ipsilateral antagonist: left inferior oblique
c. secondary underaction of the contralateral antagonist: right superior rectus.