Ocular neuromyotonia (ONM) is a rare eye movement disorder characterized by intermittent and sustained tonic spasms in one or more extraocular muscles. It causes recurrent episodes of paroxysmal strabismus and diplopia. The mechanism is thought to involve involuntary neural discharges from damaged nerves and delayed muscle relaxation.
Most cases are unilateral and occur primarily in adults. The oculomotor nerve (CN III) or abducens nerve (CN VI) is most commonly affected. Typical cases develop after radiation therapy for tumors in the sellar or parasellar region, with onset ranging from 2 months to 18 years after irradiation 1).
Non-radiation-induced ONM is rare but can occur in patients without a history of radiation exposure. Cases triggered by peribulbar block have also been reported 1).
It most commonly occurs in adults with a history of radiation therapy, mainly after treatment for sellar or parasellar tumors (e.g., pituitary tumors). It is usually unilateral and can appear from months to 18 years after irradiation 1).
In a case after retrobulbar block (Zhang 2025), a man in his 60s developed intermittent vertical diplopia several weeks after left eye pterygium surgery. Symptoms were more noticeable in the morning and evening, occurred sporadically during the day (especially while driving), and had been stable since onset1).
When the patient gazes in the direction of action of the affected muscle for several seconds to 10 seconds, the eye becomes fixed, and attempting to return to the primary position induces spasm and diplopia. This “gaze-evoked ocular fixation” is key to clinical diagnosis.
Radiation-induced ONM
Intracranial radiation therapy: Most common cause. Often occurs after treatment of sellar and parasellar tumors.
Chemotherapy during radiotherapy: Concurrent use of cisplatin or fluorouracil may be involved.
Time to onset: Ranges from 2 months to 18 years after irradiation.
Non-radiation-induced ONM
Vascular or meningeal nerve compression: Direct compression of cranial nerves is the mechanism.
Autoimmune diseases: Examples include myasthenia gravis and thyroid disease.
Retrobulbar block: Nerve damage due to local anesthesia. Zhang et al. (2025) reported this for the first time in the literature1).
Other rare causes include vitamin B12 or D deficiency, brainstem demyelination, botulinum toxin injection, and cataract surgery.
In vertical strabismus after peribulbar block, the risk of inferior rectus muscle injury is involved. Needle injury from local anesthesia and/or local anesthetic affecting nerve fibers innervating the extraocular muscles may cause ephaptic transmission.
ONM can be diagnosed clinically. The basis is a careful history and a thorough ocular motor examination observing the eyes in primary position and after a few seconds of lateral gaze.
The main differential diagnoses are listed below.
| Disease | Key differentiating features |
|---|---|
| Superior oblique myokymia | Rhythmic contraction of the superior oblique muscle. Common in healthy young adults. |
| Convergence spasm | Paroxysmal bilateral adduction. Accompanied by accommodative spasm and miosis |
| Ocular myasthenia gravis | Fatigability, diurnal variation. Improves with Tensilon test |
| Periodic oculomotor palsy | Periodic changes in ptosis and miosis |
| Graves’ disease (early stage) | Proptosis, orbital inflammatory signs |
Membrane-stabilizing drugs are the standard treatment.
Check liver function, kidney function, and complete blood count at treatment initiation and at 6-week follow-up.
Surgery is considered in cases resistant to drug therapy or when long-term medication is difficult.
In the case by Zhang et al. (2025), a 3.5 mm recession of the left inferior rectus (adjustable suture) was performed, and diplopia markedly improved 4 months postoperatively. Stereopsis of 140 seconds of arc and Worth 4-dot fusion at distance and near were achieved, and no worsening of deviation was observed even in downgaze extension1).
It is known that discontinuing membrane-stabilizing drugs often leads to recurrence1). If side effects are a concern, switching to lacosamide or considering strabismus surgery are options.
The basic mechanism of ONM is ephaptic transmission caused by segmental demyelination of cranial nerves due to radiation injury, compressive diseases, microvascular disorders, etc.
Ephaptic transmission is a phenomenon in which signal crosstalk occurs between adjacent demyelinated nerve fibers, and nerve impulses are transmitted along lateral contacts without synapses. This explains the persistence and repetition of tonic contractions 1).
Other proposed mechanisms are as follows:
In thyroid-related orbital disease, inflammatory changes and crowding of the oculomotor nerve and muscle cells cause ONM. Inflammatory cell infiltration of the endomysium and mucopolysaccharide deposition increase nerve excitability, and anti-thyroid peroxidase antibodies may directly induce demyelination.
In ONM after retrobulbar block, needle injury and/or local anesthetics may damage nerve fibers innervating the extraocular muscles, leading to ephaptic transmission. The therapeutic response to carbamazepine supports this hypothesis 1).
Ephaptic transmission between demyelinated nerve fibers causes signals from gaze activating a specific muscle to also spread to adjacent damaged nerves, resulting in sustained tonic contraction 1). This is why the eye becomes “fixed.”
Zhang et al.（2025）は、球後局所麻酔後に発症したONMを文献上初めて報告した1)。球後ブロックが新たなONM発症原因として提唱されており、球後ブロック後の垂直斜視例の一部にONMメカニズムが関与している可能性がある。測定値のばらつきとして見逃されている症例が存在すると著者らは指摘している。
Bodi et al. (2024) reported a study on the clinical features, diagnosis, and outcomes of ONM 2). It was shown that spontaneous remission of ONM is rare and long-term management is required.
Prospective studies are difficult to conduct due to the rarity and heterogeneity of vertical strabismus after retrobulbar block, but further investigation is needed to accurately assess the involvement of ONM mechanisms 1).
