Describe A and V patterns and dissociated vertical deviation.

A and V Patterns

• An A or V pattern is a horizontal deviation that changes in size between upgaze and downgaze.
• The letter describes the shape made by the visual axes, and the pattern is named for where the letter is wide — that being where the eyes diverge most.
• The naming is the whole difficulty, and it resolves to one idea: the letter A is wide at the bottom, so an A pattern has more divergence (or less convergence) in downgaze; the letter V is wide at the top, so a V pattern has more divergence in upgaze
• Applied to the four combinations:
• V-pattern esotropia — the esotropia is greater in downgaze (least divergence at the narrow foot of the V)
• V-pattern exotropia — the exotropia is greater in upgaze
• A-pattern esotropia — the esotropia is greater in upgaze
• A-pattern exotropia — the exotropia is greater in downgaze
• The thresholds are different for the two patterns, and this is examinable: a V pattern requires a difference of at least 15 prism dioptres between up- and downgaze, whereas an A pattern requires only 10. The larger threshold for V reflects a mild physiological V tendency present in normal eyes

Dissociated Vertical Deviation

• Dissociated vertical deviation (DVD) is a slow upward drift with extorsion and abduction of one eye, occurring when that eye is covered, or during inattention, fatigue or daydreaming, and returning slowly when fixation is resumed
• IT violates HERING’S law, and that is what defines it. In a true hypertropia, when one eye goes up the other goes down by an equal amount. In DVD the fellow eye does not move down — each eye drifts up under cover, and both eyes are "the higher eye", which is impossible in a genuine vertical squint
• The other defining feature is the return: the eye comes down slowly and without a corrective downward saccade. A true hypertropia corrects with a brisk refixation movement
• Associations — infantile esotropia above all, together with latent nystagmus and inferior oblique overaction; it appears at 2 to 3 years, after the esotropia has been operated
• It is bilateral though frequently asymmetric, and the less obvious side is revealed only by covering the better eye