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    By John D. Ferris, FRCOphth
    Squint Clinic
    Strabismus

    Almost all lateral rectus palsies are acquired in later life and are caused by conditions that have damaged the VIth cranial nerve, which supplies the lateral rectus muscle.

    The causes of a lateral rectus palsy are:

    • Poor blood supply to the VIth nerve caused by a combination of factors such as high blood pressure, diabetes, high cholesterol and smoking. This is known as a microvascular palsy.
    • Direct pressure on the VIth nerve caused by tumours, middle ear infections or swelling of neighbouring blood vessels can damage the VIth nerve. Lateral rectus palsies can also be a sign of raised intracranial pressure.
    • Head injuries can cause a lateral rectus palsy, but this is usually due to raised intracranial pressure.
    • Occasionally inflammation in the region of the nerve can cause a lateral rectus palsy.

    Typical features of a lateral rectus palsy include:

    • Sudden onset of horizontal double vision, which is worse when the patient looks to the affected side.
    • Limited outward movement of the affected eye. Patients often compensate for this by turning their head to the affected side.
    • A convergent strabismus that is large when the patient tries to look at an object in the distance.

    All patients will be seen by an Orthoptist and an Ophthalmologist. A detailed history will be taken and specific tests carried out to measure the strabismus and assess the range of eye movements. A chart may be plotted to measure the size of the area of single vision and to measure the movements of the eyes in different directions. Blood tests and an MRI scan are carried out to investigate what could have caused the palsy.
    What are the treatment options?

    Most (80%) microvascular superior oblique palsies will resolve within 3-6 months. However spontaneous recovery is less likely to occur if the lateral rectus palsy has been caused by a head injury or a tumour.

    In adults it may be possible to have temporary plastic prisms fitted to the patient’s glasses that will reduce or in some cases completely correct the strabismus and double vision. If the angle of the strabismus is too large to correct with prisms Botox treatment can be considered.

    Botox injections into the medial rectus muscle will reduce the size of the convergent strabismus. It will also prevent the medial rectus from contracting and shortening which can cause reduced outwards movement of the eye, even if the lateral rectus muscle starts to work normally again.

    Republished, with permission, from www.squintclinic.com