There are 6 different muscles that move each eye, and there are 3 different nerves involved in moving these muscles. Their interaction is very complex and involves many different areas of the brain.
The particular nerves that are involved are the IIIrd, IVth, and VIth nerve. Determining which nerve(s) may be involved can be complicated and requires a thorough neuro-ophthalmologic exam. More than one nerve may be involved which can complicate the work up. The most common cause for any of these nerves to not function properly is damage to small vessels that supply the nerves. Common causes include diabetes or high blood pressure, but when it occurs in younger individuals, other systemic causes should be considered.