Timothy C. Hain, MD. • Page last modified: October 9, 2021
Vergence basically means that the eyes are not moving together in the same direction (conjugately). This most commonly occurs when one changes one's point of regard from far to near, or near to far. This is associated with convergence (eyes moving in when looking closer) or divergence (eyes both moving out when looking at something farther away). There is also a minor capability of human eyes to move vertically in different speeds, and presumably some ability for them to also twist (tort) separately.
There are two types of vergence -- "slow" and "fast". Slow vergence movements (less than 2 deg/sec) can be induced by moving a target smoothly towards a patient's nose, as is commonly done during clinical examination.
Fast vergence occurs during saccades (rapid eye movements). One eye moves much more slowly or quickly towards a target than the other.
Vergence is rarely studied in the clinical laboratory, although current equipment certainly has the capability of doing so. Probably the main reason is that vergence disorders are uncommon.
Vergence is classically impaired by midbrain lesions. The pons also controls vergence. Lesions in the NTRP (nucleus reticularis ponti) impair slow vergence. Lesions in the upper pons impairs fast vergence. Certain central disorders such as "PSP" are accompanied by poor vergence.
Excess vergence may be associated with lesions in the upper brainstem. Then it is sometimes called "pseudoabducen's palsy", as the patient may appear as if they have a 6th nerve weakness, but this is intermittent. Associated with pseudoabducens palsy is estropia (eyes turning inward), convergence retraction nystagmus (eyes jumping inward), and spontaneous convergence (Pullicino et al, 2000).
The differential of pseudoabducen's palsy primarily includes voluntary convergence, generally as part of an attempt on the part of the patient to convince the examiner that he/she has diplopia or dizziness. In the author's opinion, voluntary convergence is far more common than convergence difficulties due to midbrain lesions. Voluntary convergence can usually be spotted by it's inconsistency. We have encountered many patients who appear perfectly normal in the light, but begin blinking, closing their eyes or crossing their eyes when they are being recorded.