Near vision, which depends on good visual acuity, a normal convergence and accommodative mechanism, a normal voluntary saccadic system, and otherwise stable ocular position, can be affected by many systemic disorders and drugs which modify any of these operative factors. Convergence problems include the more commonly seen primary convergence accommodative synkenesis insufficiency and excess, but as well, result from destabilizing vergence disorders of convergence retraction nystagmus and convergence spasm. Convergence effort alone rarely can induce other forms of nystagmus at near. Saccadic dysfunction occurs in a large variety of muscular, neuromuscular and neurologic disorders. Accommodative insufficiency can result from head trauma, mononeuropathy affecting the third cranial nerve and ciliary ganglion, destructive intraocular disease as well as from systemic diseases such as botulism and diphtheria.
Drugs can affect any of the near vision factors in dose related fashion.
A complete history is paramount in reaching the correct diagnosis.