Is Isolated Inferior Rectus Weakness a Strong Indicator for Myasthenia Gravis?

Background: Isolated inferior rectus weakness (IRW) has historically been reported as a possible presenting sign of myasthenia gravis (MG). Orthoptists often identify IRW, prompting further investigations. This service evaluation aims to determine whether patients with IRW or other symptoms were subsequently diagnosed with MG. Methods: A retrospective review of patient case notes was conducted. The cohort included consecutive patients who presented to the adult strabismus service and received acetylcholine receptor antibody (ACR) testing between 2010 and 2023 at a tertiary referral centre in Sheffield. Results: Sixty-two patients were included in the analysis. Patients were grouped based on isolated IRW, suspicion of thyroid eye disease, or the presence of MG characteristics. These characteristics included variable diplopia, ocular motility and ptosis; fatigue on elevation; Cogan's lid twitch; MG-like fatigue; and breathing or swallowing difficulties.Ten patients (16.13%) were diagnosed with MG. Of these, six (60%) had ocular MG (OMG). No patients in the isolated IRW group were found to have MG. All MG patients exhibited weakness of multiple extra-ocular muscles (EOM), ranging from 2 to 12 different muscles affected. Each had one or more characteristic MG symptoms or signs, with variability observed in 80% (n = 8). Conclusion: No cases of MG were identified in patients with isolated IRW. All MG diagnoses were associated with characteristic signs or symptoms. This highlights the importance of detailed clinical history and orthoptic assessment when MG is suspected and poses further questions of whether isolated IRW needs further MG investigative testing.