27th December 2023

Assess the diplopia axis

Purely vertical diplopia is caused by 3ʳᵈ or 4ᵗʰ nerve palsy, myasthenia gravis, skew deviation (acquired vertical misalignment not due to any single muscle or ocular motor nerve) or processes related to the orbital tissues such as thyroid eye disease. Purely horizontal diplopia is usually related to abduction (6ᵗʰ cranial nerve) and adduction (third cranial nerve) deficits. Identifying problems with extraocular motility can be difficult and requires a detailed examination, observing each eye separately. Whilst assessing the axis, it is important to observe any signs of thyroid eye disease, such as proptosis or lid lag and for signs of myasthenia, such as fatigable ptosis.