Superior Oblique Myokymia
Key Points at a Glance
Section titled “Key Points at a Glance”1. What is Superior Oblique Myokymia?
Section titled “1. What is Superior Oblique Myokymia?”Superior Oblique Myokymia (SOM) is a paroxysmal monocular abnormal eye movement caused by rhythmic spasm of the superior oblique muscle. It is not pure nystagmus and is sometimes classified as a “nystagmoid movement.”
The term “Superior Oblique Myokymia” was proposed in 1970. Previously, it was called “benign intermittent uniocular microtremor.” It shows no specific age predilection and often occurs in healthy young adults. It is a rare disease with no established treatment protocol.
Strictly speaking, it is not pure nystagmus and is sometimes classified as a nystagmus-like movement. Nystagmus refers to repetitive oscillatory movements of the eyeball, whereas SOM is paroxysmal torsional and downward movement due to spasm of the superior oblique muscle, differing from typical nystagmus in mechanism, findings, and frequency.
2. Main symptoms and clinical findings
Section titled “2. Main symptoms and clinical findings”Subjective symptoms
Section titled “Subjective symptoms”The main symptom is intermittent oscillopsia in one eye (perceived as shaking). It may also be perceived as shimmering or fluttering.
- Nature of oscillopsia: It appears paroxysmally, often lasting a few seconds. In some cases, it may persist for several hours.
- Vertical diplopia: Patients often present with a chief complaint of vertical diplopia when looking downward.
- Symptoms not present: No eye pain or headache associated with the “shaking” sensation.
- How patients describe it: Patients often report that objects appear to “shake up and down.”
Episodes last from seconds to hours and may recur over hours to weeks. In all reported cases, symptoms are unilateral.
Clinical Findings (Findings Confirmed by Physician Examination)
Section titled “Clinical Findings (Findings Confirmed by Physician Examination)”- Intorsional eye movement: Paroxysmal intorsional eye movement is observed under slit-lamp microscopy.
- Induced eye position: Easily induced in eye positions where the superior oblique muscle acts strongly, such as downward and inward gaze.
- Fine movement of bulbar conjunctival vessels: When the patient looks downward and inward with their chin on a chin rest, fine movements of the bulbar conjunctival vessels can be observed.
- Characteristics of nystagmus: The fast phase of rotary nystagmus is in the inward rotation direction, accompanied by a downward deviation. The amplitude is small, less than 4 degrees, and the frequency is high, up to 50 Hz.
- Honda sign: When a stethoscope is placed over the affected eye, a sound resembling a motorcycle engine may be heard (rare).
- Usually normal findings: Visual acuity, intraocular pressure, visual field, pupillary light reflex, fundus findings, and range of eye movements are usually normal.
When a stethoscope is placed over the affected eye, a sound resembling a motorcycle engine may be heard, known as Honda sign. It is thought to originate from frequent spasm of the superior oblique muscle, but it is a rare finding.
3. Causes and Risk Factors
Section titled “3. Causes and Risk Factors”In many cases, no underlying disease is identified (idiopathic).
- Idiopathic: Most common. Usually not associated with other systemic diseases.
- Neurovascular compression: Some reports suggest that neurovascular compression in the dorsal midbrain is the cause. There is a theory that a blood vessel pulsatilely compresses the trochlear nerve in the dorsal brainstem, with possible causes including contact with the superior cerebellar artery, cysts, or brain tumors.
- Secondary: Cases have been reported following trochlear nerve palsy, head trauma, or brainstem infarction.
- Other related diseases: Reports of associations with brainstem tumors, cerebellopontine angle lesions, and multiple sclerosis (MS) 1)
- Triggers: Fatigue, stress, and mood changes can be triggers. Associations with fluorescent lights and flickering lights have also been reported
Statistically, the incidence of SOM in the right eye is higher in women, but MRI shows no anatomical left-right differences in the trochlear nerve or surrounding structures.
Fatigue, stress, and mood changes have been reported as possible triggers for SOM. An association with fluorescent lights and flickering lights has also been noted. Avoiding these triggers may help reduce the frequency of symptoms.
4. Diagnosis and Examination Methods
Section titled “4. Diagnosis and Examination Methods”Examination Methods
Section titled “Examination Methods”The diagnosis of SOM is primarily made by observation with a slit-lamp microscope.
- Basic observation method: Have the patient look downward and inward while resting on the chin rest, and observe the fine movements (paroxysmal inward rotation) of the bulbar conjunctival vessels.
- Provocative maneuver: Moving the patient’s gaze from downward and outward to the center may induce eye movements.
- Observation findings: Intermittent vertical and rotatory fine tremors are observed in one eye.
Imaging Diagnosis
Section titled “Imaging Diagnosis”- Brain MRI: May show no significant findings. Thin-slice MRI (CISS sequence) may reveal neurovascular conflict at the nerve root exit zone.
- MRA: Combined with time-of-flight method improves visualization of the trochlear nerve and enables detection of arterial contact.
Differential Diagnosis
Section titled “Differential Diagnosis”It is important to differentiate from diseases with similar amplitude, frequency, and direction as SOM. Refer to the comparison table below.
| Disease | Direction | Amplitude | Frequency |
|---|---|---|---|
| Superior oblique myokymia | Torsional and downward | <4 degrees | Up to 50 Hz |
| Hyman-Bielschowsky phenomenon | Vertical | Up to 30 degrees | <5Hz |
| Square-wave jerks | Horizontal | Small | High frequency |
Other differential diagnoses include the following.
- Monocular pendular nystagmus (multiple sclerosis): Differentiated from SOM by its intermittent occurrence and faster micro-movements.
- Spasmus nutans: Accompanied by head tremor and abnormal head posture, with a clearly different clinical picture.
- Eyelid myokymia: Oscillopsia due to eyelid twitching is easily confused with SOM.
5. Standard Treatment
Section titled “5. Standard Treatment”There is no established effective treatment. Treatment is selected based on symptom severity and patient preference.
Medication Therapy
Section titled “Medication Therapy”As standard treatment in Japan, the following medications are tried to alleviate symptoms. However, a cure is difficult.
- Carbamazepine: May help reduce symptoms
- Baclofen: May help reduce symptoms
Overseas, varying degrees of success have been reported with the following medications (supplementary information).
- Gabapentin: Considered to have relatively few side effects and is sometimes used as a first-line treatment
- Others: Reports include phenytoin, clonazepam, mirtazapine, memantine, etc.
- Beta-blockers (propranolol, timolol, betaxolol): May reduce blood pressure amplitude and alleviate symptoms.
- Botulinum toxin injection: Success rate is inconsistent and may only provide temporary relief.
Surgical Treatment
Section titled “Surgical Treatment”Considered when conservative treatment is ineffective.
- Combined superior oblique tenotomy and inferior oblique recession: Reported to be effective.
- Anterior transposition of the superior oblique muscle : one of the surgical options
- Microvascular decompression (MVD) : considered when MRI identifies vascular compression as the cause. Immediate and complete symptom resolution has been reported with MVD via the lateral suboccipital approach, with no recurrence at 24-month and 17-month follow-ups1)
If conservative treatments such as carbamazepine or baclofen are ineffective, surgical treatment may be considered. Options include combined superior oblique tenotomy and inferior oblique recession, or microvascular decompression (MVD) if MRI confirms vascular compression. For details, see the “Standard treatment” section.
6. Pathophysiology and Detailed Mechanism
Section titled “6. Pathophysiology and Detailed Mechanism”The superior oblique muscle is innervated by the trochlear nerve and rotates the eye downward during adduction (depression). When the superior oblique muscle contracts alone, the eye deviates downward and outward.
The fast phase of the rotary nystagmus observed in SOM is in the direction of the superior oblique muscle’s action (intorsion), accompanied by a downward deviation. Electromyography shows abnormally long action potentials (7–8 ms) in the superior oblique muscle, but not in the inferior oblique muscle. This abnormal electrical activity is thought to cause spasm of the superior oblique muscle.
The following mechanisms are hypothesized for the pathology:
- Ephaptic transmission: Damage to the trochlear nerve due to vascular compression, etc. → Segmental demyelination → Electrical crosstalk between adjacent nerve fibers may cause SOM.
- Hyperexcitability of the trochlear nucleus: Pathological hyperactivity of the trochlear nucleus is thought to underlie the condition1).
- Disorder of the nerve root exit zone: The trochlear nerve may be affected at the nerve root exit zone.
Statistically, it is more common in the right eye of women, but MRI shows no anatomical left-right difference in the trochlear nerve or surrounding structures, and the cause of this sex and laterality difference is unknown.
7. Latest Research and Future Prospects (Reports under Investigation)
Section titled “7. Latest Research and Future Prospects (Reports under Investigation)”Effectiveness of Microvascular Decompression (MVD)
Section titled “Effectiveness of Microvascular Decompression (MVD)”Immediate and complete symptom resolution has been reported after MVD using the lateral superior cerebellar approach. No recurrence was observed during follow-up periods of 24 months and 17 months 1). MVD can be a viable treatment option in cases where neurovascular compression is confirmed on MRI.
Potential of Beta-Blockers
Section titled “Potential of Beta-Blockers”Beta-blockers (propranolol, timolol, betaxolol) have recently been reported to potentially reduce symptoms of SOM by decreasing blood pressure amplitude. Comparative studies with standard pharmacotherapy are needed in the future.
Research Challenges
Section titled “Research Challenges”Because SOM is a rare disease, conducting randomized controlled trials (RCTs) is challenging. In addition to difficulty in securing a sufficient number of patients, the unpredictable and fluctuating course of symptoms makes it difficult to objectively evaluate the efficacy of existing drugs. Establishing treatment protocols requires multicenter collaborative studies and the development of patient registries.
8. References
Section titled “8. References”- Gurnani B, et al. Nystagmus and Abnormal Eye Movements: A Comprehensive Review of Types, Causes, and Diagnostic Approaches. Clinical Ophthalmology. 2025;19:1617-1642.