Thursday, 17 May 2012

Following a tutorial Prof. Giovannoni gave in his clinic last week I would like to share an excellent link about eye movements. Tutis Vilis of the Department of Physiology, University of Western Ontario has developed a flash tutorial explaining many normal and abnormal eye movements, which can be found here:

There are a few points to develop around the link, once you've worked through it:

1) The accommodation reflex or near response comprises three movements - the first two are mentioned in the eye movement tutorial:
  • both eyes rotate medially towards the target  - vergence
  • the ciliary muscle contracts, causing the lens to be come more spherical/convex - accommodation
  • the pupils constrict
Why do the pupils need to constrict? The quality of the lens results in spherical aberration - light passing through the edge of the lens will be focussed more poorly onto the retina, reducing the quality of the image. Pupillary constriction prevents light from passing through the edges of the lens and ensures the eye can produce a sharp image on the retina. In a given light level, a near object will reflect more light onto the retina than one further away, so the pupillary constriction does not typically impair vision by inadequate illumination.

2) Humans have 6 extraocular muscles which are responsible for positioning the eye and one which elevates the eyelid (levator palpebrae superioris). Cows (for example) manage with just four muscles to rotate the globe. Why does Man need two more?

This image of the right orbit (from above) shows the origins and insertions of the orbital muscles.The orbit is cone shaped and this cone is externally rotated by about 20 degrees. The rectus (straight) muscles all originate from a common point at the back of that cone - the common tendinous ring (also known as the Annulus of Zin). Due to the cone's shape and position, the rectus muscles act not only in the direction they are named, but also cause rotation in other axes.

For example superior rectus (in the middle of the image above) when contracting rotates the globe upwards (elevation) but also has some internal rotation (intorsion) and medial rotation (adduction). The inferior rectus (not seen on the illustration) causes not only depression of the eye but extorsion and adduction. With only four muscles, there would be no control over in- and ex-torsion of the globe. Our bovine friends do not encounter this problem due to the shape of their orbits - presumably the tendinous ring lies deep to the centre of the globe, so their rectus muscles pull straight, as the name "rectus" would normally suggest.

The addition of two oblique (slanted) muscles allows for control over intorsion and extorsion. Superior oblique acts to intort, depress and abduct the eye. Some use the acronym SO-LID as a reminder - Superior Oblique Lateral rotation, Intorsion, Depression. The muscle is of interest as it originates in the common tendinous ring, then passes through a sling or pulley attached to the superio-medial orbit before attaching at the superior posteriolateral segment of the globe (see below):

Inferior oblique acts to extort, elevate and abduct the eye; this muscle does not arise from the common tendinous ring, but instead from the maxillary component of the anterior orbit and attaches to the inferior posteriolateral globe, see below, deep to the inferior rectus:

3) Why do we examine eye movements in an "H" shape?

Although above I have talked about eye movements from the neutral position, in clinical practice it is not possible to ask a patient to contract, for example, only their superior oblique muscle. If we examined simply elevation/depression and adduction/abduction, we would get a mixed picture of which muscle was acting. The picture below is of the right eye movements in a patient, as observed by the examiner, i.e. lateral rectus abducts the eye:

By examining in an "H" pattern, we can isolate the action of individual muscles. This image is also of right eye movements in a patient, as observed by an examiner. For the muscles in the left eye, simply consider a mirror image.

You'll note that even though the action of the superior rectus is elevation, intorsion and adduction, once the eye is already abducted by lateral rectus, superior rectus acts solely to elevate the eye. This is because once the eye is abducted, the other eye elevator - inferior oblique - becomes slightly slack and is unable to contribute to elevation.

Similarly even though the action of inferior oblique is extortion, elevation and abduction, once the eye is adducted by medial rectus, the superior rectus is unable to act as it now is slack, leaving inferior oblique as the only muscle able to elevate the eye.

The previous two paragraphs probably do not make much sense unless you've understood the attachments and direction of rotation caused by each muscle earlier in this post. Please feel free to ask questions or for clarification below!

Next time from me: what happens when these movements go wrong...

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