Is inferior oblique botulinum toxin injection an effective surgical simulator prior to inferior oblique myectomy? A 30-year retrospective review.

Purpose: Botulinum toxin A (BTX) injected into the inferior oblique muscle temporarily simulates the post-operative effect of an inferior oblique myectomy (IOM). This can aid in surgical planning, especially given the irreversible nature of IOM. This study evaluates the efficacy of BTX as a pre-operative simulator and compares its effects to those of IOM. To our knowledge, this is the largest reported series of patients undergoing pre-operative BTX simulations, prior to IOM.

Methods: A retrospective review was conducted on all patients receiving inferior oblique BTX between 1 January 1994 and 1 January 2024, by a single surgeon using an electromyography-guided technique. Data, including diagnosis, procedure, and orthoptic measurements, were obtained from electronic and archived paper records. Successful outcome was defined as subjective improvement in symptoms, or objectively as, reduction of vertical angle deviation in primary position by at least 50%, or post-procedure vertical deviation in primary position of ≤5 Prism Dioptres (PD).

Results: Sixty-eight patients underwent inferior oblique BTX within the study period. Of these, 48/68 patients (71%) had an improvement in subjective symptoms. Subsequently, 36/48 patients (78%) underwent IOM, 4/48 (8%) patients are awaiting IOM, 4/48 (8%) patients requested repeat BTX despite IOM being offered, and 4/48 (8%) patients had complete resolution with no further intervention. Following IOM, 34/36 (94%) achieved the defined successful outcome, with no significant difference between vertical deviation measurements following BTX or IOM (p < .05). No post-operative diplopia or other complications were identified, and 3/36 (8%) patients demonstrated an asymptomatic over-correction. BTX was deemed unsuccessful in 20 patients (29%). Within this cohort, 12 had no improvement (despite repeat treatment with a higher dose), 3 patients were lost to follow-up, 1 patient died and 4 patients developed problematic diplopia in the primary position, which resolved when the BTX wore off.

Conclusions: Inferior oblique BTX is a useful and reliable tool for IOM simulation, prior to irreversible surgery. Its temporary effect on vertical deviation in primary position is comparable to IOM, and can also be used as a tool to identify patients at risk of diplopia, or failure of treatment.