Tests incomitant strabismus: squint angle changes depending on the direction of gaze
Two types of incomitant strabismus

The grid uses red lights and the patient wears red and green goggles so that only the one eye (the fixing eye) sees the lights

Each Hess chart square represents 5 degrees (or 10 prism dioptres)

The fixing eye is directed to various positions and the position of the non-fixing eye is recorded

The patient is asked to superimpose the green light onto the red light
Their fixing eye can see the red targets through the red lens but the other eye cannot

Hering’s law: during conjugate movements, equal and simultaneous innervation is directed to corresponding (yoke) muscles to contract
Sherrington’s law: during contraction of a muscle, a simultaneous signal is sent to relax its antagonist
Primary deviation: the extent of deviation is noted when the normal eye is fixing
Secondary deviation: the extent of deviation is noted in the normal eye when the affected eye is fixing

Hess chart interpretation

The smaller chart indicates the eye with the underacting muscle (primary deviation)

The greatest restriction will be seen in the main direction of the affected muscle

The larger chart indicates the eye with the overacting muscle (secondary deviation)

The greatest expansion is seen in the direction of the yoke muscle (according to Hering’s law)

## Muscle sequelae

Defects become more concomitant with time, serving to reduce the squint angle
Stage 1: concurrent overaction of the contralateral synergist according to Hering’s law
Stage 2: overaction of the ipsilateral antagonist due to its unopposed action leading to contracture (shortening) which manifests as overaction
Stage 3: secondary underaction of the contralateral antagonist. Since less innervation is needed in the ipsilateral antagonist due to its contracture, by Hering’s law the contralateral antagonist receives less innervation, so underacts

Strabismus is further explored in the Part 2 package