Orthoptics I - Convergence

What is convergence?

the inward rotation of both eyes to maintain binocular alignment when viewing a near object

What is convergence normally accompanied by?

pupil miosis and accommodation

What is convergence measured in?

metre angles (MA) - relates to the amount of convergence required for each eye to fixate an object at 1m in the medium plane, independent of IPD

How do we calculate convergence?

1/object distance (m)
- same as accommodative demand

What is individual convergence measured in and what is it dependent on?

measured in prism dioptres and dependent on IPD (cm)
e.g. 65mm IPD = 6.5PD at 1m, 19.5PD for 0.33m

What is tonic convergence?

position of rest when awake (natural anatomical position is divergent)

What is accommodative convergence?

convergence occurring when accommodation is induced

What is proximal convergence?

convergence stimulated by the awareness of a close object

What is fusional convergence?

convergence occurring to maintain single vision, overcoming binocular disparity

What is the stimulus for vergence movements?

retinal disparity

How is the vergence system unique?

only visual response system resulting in disjugate movement

Describe how the nervous system is involved in convergence

higher centre for vergence control is poorly understood:
- oculomotor nerve innervates the medial rectus
- medical rectus fibres carries signals
- cerebellum regulates the vergence movements

Can convergence be controlled?

yes can be voluntary

How does convergence develop?

demonstrated in full term infants but is uncoordinated
- accurate convergence is developed by approx. 3 months (alongside accommodation responses)

What is a convergence insufficiency?

Inability to obtain or maintain adequate convergence for comfortable binocular vision at near
- may be associated with a convergence excess exophoria