How much superior rectus underaction is considered normal?

Purpose: It is considered normal to have a small amount of superior rectus weakness in laevo and dextro elevation; however, there is no documented definition for these normal parameters within a healthy young adult population using ocular movement testing and the synoptophore. The aim of this study was to collect normative data on the degree of superior rectus underaction in healthy young adults.

Method: Twenty-nine healthy adults (3 males and 26 females, mean age 20.30 ± 1.70 years) were recruited. Superior recti underactions and inferior oblique overactions were recorded during routine ocular movement testing and mean and median values calculated. Subjective horizontal, vertical and torsional measurements were taken in degrees on the synoptophore in primary position, laevo elevation and dextro elevation.

Results: Most participants (79.31%) had some degree of observable superior rectus underaction in either eye or in both eyes on ocular movement testing (mean superior rectus underaction of –0.69 units in laevo elevation and –0.71 units in dextro elevation, range = –1.5 to –0.5 units; median –1 units, interquartile range (IQR) = –1 to –1 units). Most participants (62.07%) had some degree of superior rectus underaction in either eye or in both eyes on the synoptophore (mean left and right superior recti underactions of –0.48 degrees, range = –3 to –1 degrees; median 0 degrees, IQR = –1 to –1 degrees).

Conclusion: The majority of young healthy adults in this study showed some degree of superior rectus underaction. On ocular movement testing, –0.70 units of underaction, and on the synoptophore, –0.48 degrees of underaction are the mean levels of weakness to be expected. Superior rectus underactions greater than –1 units for ocular movement testing and –1 degrees on the synoptophore in healthy young adults should be carefully evaluated, together with other important clinical signs.