Features of paralytic squint

The clinical picture of paralytic squint will depend upon the precise muscle(s) weakened. However, the following comments may be made.

Secondary deviation - i.e. the position of the normal eye when the affected eye is in fixation - is greater than that of primary deviation - the position of the affected eye when the normal eye is in fixation. This is a consequence of Hering's law of equal innervation. The affected eye requires relatively more innervational effort to maintain fixation, and this excess effort is distributed to the muscles of the normal eye.

Diplopia is present if previously there was binocular single vision (BSV), and is most marked when the eyes are rotated into the field of action of the affected muscle. Diplopia cannot occur if the individual has never developed BSV.

Changes in head position are common in an attempt to neutralise the diplopia:

• the face is turned towards the field of action of the paralysed muscle in paralysis of any of the recti muscles
• the chin is elevated in paralysis of the elevators of the eye - superior recti and inferior obliques. It is depressed in paralysis of the depressors of the eye - infectior recti and superior oblique.
• the head is tilted towards the normal side in paralysis of the superior oblique. It is tilted towards the side of the paralysed muscle in paralysis of the superior and inferior recti, and the inferior oblique muscle.