MRI Can Uncover Inferior Rectus Underaction Cause

Person undergoing a CAT scan in hospital. PET scan equipment. Medical CT scan of patient.
Researchers identify distinguishing features of both acquired and congenital inferior rectus hypofunction.

Among patients with inferior rectus (IR) underaction, orbital imaging via magnetic resonance imaging (MRI) can identify and help differentiate between abnormalities, according to research published in the Journal of the American Association for Pediatric Ophthalmology and Strabismus. These technologies are allowing physicians to clarify the mechanisms and guide management, the report shows. 

To compare clinical, imaging, and intraoperative presentation of acquired and congenital IR hypofunction and identify the distinguishing features, researchers conducted a prospective data analysis of “adequately imaged cases categorized as representing IR absence and IR palsy.” 

Participants underwent a complete ophthalmic evaluation, MRI, and evaluation of muscle contractility. The cohort included 15 participants (9 women), among whom 3 mechanisms of inferior rectus underaction were identified. These mechanisms included congenital underdevelopment, acquired denervation, and direct mechanical damage. 

Congenital underdevelopment was noted in the first 3 cases: a 52-year-old woman with progressive congenital strabismus, a 38-year-old mother who had childhood strabismus surgery, and an 8-year-old girl, the child of the mother in case 2, who had congenital left exotropia and hypertropia. 

Acquired denervation was identified in cases 4 through 7, including a 35-year-old woman with diplopia and chin-down torticollis for a period of 18 months, a 40-year-old man with progressive left hypertropia for a period of 20 years, a 62-year-old woman with recurrent diplopia despite repeated strabismus surgeries for presumed thyroid ophthalmopathy, and a 7-year-old girl injured in a traffic accident 3 years prior. 

Direct mechanical damage was identified in cases 8 through 15. These 8 cases included a 46-year-old woman who had experienced left orbital trauma necessitating globe rupture repair and an orbital floor implant; a 37-year-old woman who underwent endoscopic sinus surgery that was complicated by right orbit entry; a 35-year-old man with diplopia following endoscopic sinus surgery with left orbit entry; a 50-year-old woman with diplopia after bilateral laser transconjunctival blepharoplasty; 2 teenagers (1 with diplopia within 1 month of right orbital fracture repair and another who had diplopia 6 months after left orbital floor fracture repair); a 49-year-old man who presented 1 year after trauma with a right orbital floor implant; and a 43-year-old woman with multiple surgeries for severe trauma to the head, face, and left orbit.  

According to the researchers, patients with IR congenital absence or hypoplasia exhibited A-pattern exotropia. Two patients — cases 1 and 3 — had abnormal orbital bands, while the patient in case 1 demonstrated incyclotorsion of the pulley array. Acquired IR denervation was associated with either exotropia or slowed saccades, or hypertropia resulting in an increased IR field of action, excepting the patient in case 7. Seven of 8 patients experienced IR direct damage associated with orbital fracture, and several patients with chronic IR trauma had normal infraducting saccades. 

Results of orbital imaging led to observation, rather than surgical treatment. In cases 8, 11, and 13, lacerated, ruptured, or detached but otherwise functional IR muscles were recovered and reanastomosed. In case 12, IR contusion recovered spontaneously. In cases 9 and 15, where the patients had small-angle strabismus despite “extensive” damage to the IR, prism management was utilized. In case 4, the researchers found that IR reinforcement with SR weakening improved alignment. 

“Orbital imaging demonstrates a variety of abnormalities in patients with congenital or acquired IR hypofunction, helping to clarify the underlying mechanism and guide management,” the research explains. 


Solanes F, Demer JL. Clinical and imaging features of congenital and acquired isolated inferior rectus muscle hypofunction. J AAPOS. Published online February 15, 2021. doi:10.1016/j.jaapos.2020.09.007