6th nerve palsy is characterised by double vision (diplopia) caused by damage to the nerve controlling the muscle (the lateral rectus muscle) responsible for lateral (side-to-side) eye-movements.
The left lateral rectus muscle pulls the left eye outwards and the right pulls the right eye outwards. Thus if the left sixth cranial nerve is damaged, it causes double vision when looking left and if the right is affected, it causes double vision when looking right.
This nerve is called the Cranial Nerve VI or Abducens Nerve which arises in the Pons area of the Brainstem. This condition is associated with several conditions including multiple sclerosis, mysathenia gravis, mechanical aggravation of the nerve, diabetes, circulation problems, hypertensions, tumors, Moebius Syndrome (a genetic condition), childhood virii and other known and unknown causes. In 25% of adults, the cause of the palsy is unknown and the condition is self-limiting.
There is a specific test that can be performed called a Tensilon test which can help rule in-or rule out myesthenia gravis. Most people with MS and sixth nerve palsy have lesions in the Pons that show up on MRI scans.
Treatment often involves wearing a patch or press-on prisms for spectacles which can help eliminate the double vision.
6th nerve palsy links:
Handbook of Ocular Disease Management - CRANIAL NERVE VI PALSY
Yale- Cranial Nerve 6, pg. 1
Acute VIth cranial nerve dysfunction in multiple sclerosis