Incomitant Strabismus/Module 1: Clinical Investigations of Incomitant Strabismus/Part 5: Mapping Ocular Deviations

Interpretation of Hess/Lees Charts
General points
 * Hess (or Lees) chart shows the direction of the deviation; it does not differentiate between a latent and manifest deviation.
 * The subjective angle is recorded, not the objective angle, therefore the patient must have NRC.

Position
 * Foveal projection is used in this test therefore the position of the field of movement reflects the position of the eyes, e.g. a lower field (or recording point) belongs to the lower eye.
 * The position of the central dot indicates the deviation in the primary position, fixing with the RE on the left chart and with the LE on the right chart.
 * The left chart indicates the deviation of the LE, the right chart indicates the deviation of the RE.
 * The graph is read as if the examiner is the patient.

Size
 * Based on Hering's Law of equal innervation the smaller field, plotted with the normal eye fixing, belongs to the eye with the primary limitation of movement. This is the primary deviation.
 * Displacements of the recordings interior of the normal template are underactions; displacements exterior of the normal template are overactions.
 * After selecting the eye with the primary limitation of movement, the primarily affected muscle is found by selecting the muscle which has the greatest underaction.
 * The contralateral synergist will show the greatest overaction in the non-palsied eye.
 * The ipilateral antagonist of the palsied muscle will show the next greatest overaction.
 * The secondary inhibition of the contralateral antagonist will be seen as an underaction (usually but not always smaller than the primary palsy).
 * The outer fields should be examined for small under- and overactions which may not be evident on the inner field.
 * The consequent over- and underactions of the muscle sequelae will decrease in size with time as the deviation becomes more concomitant therefore fields of equal size may indicate a long standing palsy or a non paralytic strabismus.
 * A narrow field which is restricted in opposing directions of movement shows a mechanical restriction ie. The normal muscle sequelae of a neurogenic palsy are not evident.

Shape
 * Sloping fields denote 'A' & 'V' patterns
 * Sloping fields should not be interpreted as indicating torsion.

Summary
 * The smallest field belongs to the primary affected eye.
 * Look for the greatest underactions and corresponding over action of contralateral synergist and other muscle sequelae (NB torsion is noted by the tilt of the torch light NOT by a sloping field).
 * Mechanical restriction show fields of restriction in opposing directions and do not follow the normal pattern of muscle sequelae of a neurogenic palsy.
 * Fields of differing sizes indicate a recent palsy; fields of the same size indicate either a long standing palsy or a non paraletic squint.