Clinical Techniques
Hess Chart
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Get access- Tests incomitant strabismus: squint angle changes depending on the direction of gaze
- Two types of incomitant strabismus
- Neurogenic/myogenic: nerve or muscle palsy
- Mechanical: eg. dysthyroid eye disease or orbital fracture
- In mechanical restrictions, the ductions are the same as the versions
- Assumptions of Hess charts
- Normal retinal correspondence (see above)
- No suppression
- Reasonable visual acuity
- Performed with both eyes open with each eye fixing in turn
- Patient sits 50cm from the screen
- Different coloured stimuli are used to to dissociate the eyes: no fusion
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
Hot Topic
Each Hess chart square represents 5 degrees (or 10 prism dioptres)
- The central field is 15 degrees from the primary position
- The peripheral field is 30 degrees from the primary position
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
- Therefore certain effects can be noted:
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
Hot Topic
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