Statements are often made that in combined lateral and vertical strabismus operation for the lateral should always be done first. A counterstatement is frequently made, that the vertical squint should be corrected first. Neither statement is correct, and the decision as to which deviation to correct first rests on fairly definitely determined principles.
These principles include the type of the lateral strabismus, whether a convergent strabismus is simply of the convergence excess type or whether a divergence insufficiency has been added ; or whether a divergent strabismus is a primary convergence insufficiency with a secondary divergence excess or whether, as is frequently seen, there is a definite primary divergence excess with or without a secondary convergence insufficiency. It must also be determined whether the lateral muscles are inherently too strong or too weak. Then it must be determined which vertical muscle or muscles are paretic and whether there are secondary