Extra-ocular muscles

4 rectus muscles (inf, sup, med, lat) and 2 oblique muscles (inf and sup).

The origin of the 4 rectus muscles is the common ring tendon (annulus of Zinn) at the orbital apex, and their insertion is on the sclera anterior to the equator.

The origin of the superior oblique is the sphenoid bone and has a long tendinous insertion that hooks round the trochlea, covered by the superior rectus.  The origin of the inferior oblique is the maxilla and has a muscular insertion temporoinferiorly and covers the inferior rectus.

The Oculomotor Nerve

Third Nerve Palsy

  • The primary symptom is diplopia
  • The primary sign is an infraducted, abducted (down and out) eye, with ptosis (involvement of levator palpebrae) and a dilated pupil (loss of parasympathetic innvervation) causing glare in bright light and blurred accommodation

Fourth Nerve Palsy

  • Again the primary symptoms is diplopia
  • A fourth nerve palsy causes a elevated, extorted and adducted eye (the superior oblique intorts, depresses and abducts the eye).  However, these are usually compensated for by the other eye muscles but the patient may adopt a characteristichead-tilttowards the affect side, to reduce the diplopia.
  • Because the fourth nerve is the thinnest and has the longest course, it is susceptible to damage, particularly from trauma.

Sixth nerve palsy

  • Again, diplopia is the primary symptom (horizontal binocular)
  • The classic sign is an esotropic squint (inward) and an inability of lateral gaze in the affected eye.
  • It is the most common cause of a single unilateral nerve palsy, as it is the first to be compressed in raised intracranial pressure.

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