The ocular motility test is able to detect any difference in how the eyes are aligned at different positions of gaze and is an essential part of any binocular vision assessment.

How to perform the test

Usually, the motility test is done without spectacles to prevent obscuration by the frame. Sit the patient comfortably and ask them to try not to move their head, but follow the pen torch with their eyes. The pen torch allows the corneal reflections to be seen and subtle changes in loss of fixation. Also, it allows for end points to be observed when facial features obscure either eye. For children/babies a toy could be used as a compromise, to keep their attention. Start by looking at fixation in the primary position.  Move the pen torch about 50cm from their eyes, at 2 degrees per second. If the target is moved too quickly, it is difficult to pick up deviations and for patients to report diplopia. The light is moved in the eight diagnostic positions of gaze. These are used as the individual EOMs are acting in their primary action and any under action is more easily detectable.

Further investigations

The cover test in each direction of gaze to determine over and under acting muscles can be performed to investigate the magnitude of the under action and to determine if there is incomitancy.  The use of a prism bar can be used to neutralise the movement but is rarely done except before cases of strabismic surgery. In optometric practice, an estimation of the magnitude (minimal, slight, moderate or marked) will be enough to determine if there is incomitancy.

If an under action is found, testing ductions (by occluding one eye) will result in further movement compared to versions in neurological causes but not in mechanical causes.

What to record

When observing the eye movements, both pursuit and gaze is being tested. Look for a smooth movement, noting any nystagmus especially at the extremes of gaze and if it varies. If jerky movements or nystagmus is seen, this may indicate a neurological defect. There are several ways of diagrammatically recording results:

  • Image of what patients see by a diplopia chart
  • Measurement/ estimation of heterotropia/phoria in each position of gaze
  • Use of grading system +1 to 4 to indicate over action and -1 to 4 to indicate under action (used commonly in orthoptic departments)

Interpreting results

In recent onset acquired incomitancies, the underacting action will be the greatest. Then due to Herrings law, there would be over action of the contralateral synergist. There would also be contracture of the ipsilateral antagonist and inhibition of the contralateral antagonist as further sequelae develop over weeks and months. In more longstanding deviations, these are less obvious. Also, in some conditions, such as myasthenia or thyroid eye disease, the deviations can vary dependent on the systemic status and can change between visits. They can be the ones ‘that do not make sense’ and do not seem to follow any rule.

VRICS Exercise

The online exercise is aimed at encouraging practice in viewing and interpreting ocular movements in four patients:

  1. Patient 1 (figure 1) – a young boy whose mother has some concerns that ‘his eyes do not look right’ but who had previously been given the all clear by another clinician
  2. Patient 2 (figure 2) – a patient with a known and previously diagnosed incomitant eye condition who wants further information about her eyes
  3. Patient 3 (figure 3) – an oculocutaneous albino whose binocular stability is influenced very much by her direction of gaze
  4. Patient 4 (figure 4) – a very young and poorly cooperative patient whose mother is convinced that there is a problem 

References

Motility and muscle actions

  • Ennis F. Binocular vision 5- comitant heterotropia and its adaptations. Optician
  • Tang S. Binocular vision walkthrough. Part 3 – Diplopia  Optician, 19 August 2016

Further reading

  • Traboulsi EI, Green WR. Duane's ophthalmology; Ch. 38 An Overview of Albinism and Its Visual System Manifestations. Lippincott Williams & Wilkins. 2006.
  • Myron Yanoff; Jay S. Duker (2009). Duane’s retraction syndrome. Ophthalmology (3rd ed.). Mosby Elsevier. pp. 1333–1334
  • Scott Larson, MD. Pseudostrabismus. American Association for Pediatric Ophthalmology & Strabismus. https://aapos.org/browse/glossary/entry?GlossaryKey=eafce745-dfbf-48e4-aac4-66b9b05868e8