Delayed Recovery From Radiation-Induced Oculomotor Nerve Palsy

Dear Editor,Therapeutic radiation is widely utilized in the management of various intracranial and skull base tumors. 1 However, this treatment modality has potential long-term adverse effects such as radiation-induced cranial neuropathy that can emerge anywhere from 6 months to 10 years after treatment, which may present significant clinical challenges.Radiation-induced neuropathy typically has a progressive course and is largely irreversible even with immunomodulatory interventions.2,3 Here we present a patient with oculomotor nerve palsy that developed 6 years after therapeutic radiation, but which showed substantial clinical and radiologic improvements over 1 year.We discuss the possible mechanisms underlying the development and recovery of delayed neuropathy in order to broaden the insight into this rare condition.A 44-year-old female was referred to our department due to painless binocular diplopia and left-sided ptosis with a 2-month history.Six years earlier she had undergone endoscopic subtotal resection of pituitary macroadenoma followed by Gamma Knife surgery (GKS) that delivered 11 Gy in three fractions to the 50% isodose line.Brain MRI performed 1 week after symptom onset showed a stable remnant tumor without any significant surrounding abnormality.A neurologic examination at presentation revealed left mydriasis with partial light and near reflexes (Supplementary Video 1 in the online-only Data Supplement), left ptosis, and limitations in supraduction, infraduction, and adduction of the left eye (Fig. 1A).No neurologic signs or symptoms other than the ocular findings were observed, and so she was diagnosed with left oculomotor nerve palsy.To exclude potential microstructural abnormality, she underwent orbital MRI with MR angiography 2 months after symptom onset, which disclosed an atrophic and linearly enhanced left oculomotor nerve within the cavernous sinus (Fig. 1B).Serologic evaluations including vascular risk factors and autoimmune disorders were unremarkable.Serum acetylcholine receptor antibody and repetitive nerve stimulation test results were normal.Given the history of GKS and MR findings, she was diagnosed with radiation-induced delayed oculomotor nerve palsy.Empirical high-dose intravenous methylprednisolone (1 g/day) was administered for three consecutive days and followed by oral prednisone tapered over 2 weeks, but there was no positive response over the following 4 months.The lack of corticosteroid response was consistent with the typical irreversible profile of radiation-induced cranial neuropathy, further supporting the diagnosis.However, her symptoms began to improve approximately 7 months after symptom onset, resulting in the limitations in left ocular motility resolving with only mild ptosis and mydriasis remaining at 1 year after symptom onset (Fig. 1C).Follow-up orbital MRI showed an atrophic left oculomotor nerve with markedly decreased enhancement (Fig. 1D).The high density of nerve tissues in the head and neck make these regions particularly vulnerable to radiation.The pathophysiology of radiation-induced neuropathy is not yet fully understood, but it is considered multifactorial involving direct neural damage, inflamma-