Conjugacy refers to alignment of the eyes such that the central light ray simultaneously falls on the fovea of each eye, with the entire retinal image falling on corresponding retinal points and leading to a single image. The eyes can become misaligned in any of the three principal axes of rotation, leading to dysconjugate gaze which the patient experiences as diplopia. Dysconjugate gaze can occur due to paresis of the extraocular muscles, dysfunction of the nuclear or supranuclear pathways innervating those muscles, or intra-orbital masses that can push the globes out of alignment.

One method of assessing conjugacy is to observe the corneal light reflection of each eye. Normally, a point of light will be seen reflecting back from a region centered over the pupil of each eye; if dysconjugacy is present, the light will reflect from over the iris, away from the pupil as shown above.

Another method, in addition to direct observation, is the use of Maddox rod testing, which is discussed in further detail elsewhere.

Etiologies of Diplopia include:

* Mechanical impairment such as extra-ocular muscle entrapment from orbital fracture
* Extraocular muscle hypertrophy due to hyperthyroidism
* Myositis of orbital muscles
* Myasthenia gravis and other disorders of the neuro-muscular junction
* Lesions of brainstem nuclei or nerve fascicles affecting cranial nerves III, IV, or VI
* Dysfunction of supranuclear oculo-motor pathways
* Effects of toxins such as alcohol
* Side effects of medications such as anti-convulsants