Inferior oblique myokymia

Key Points at a Glance

Highlights of This Disease

• Inferior oblique myokymia (IOM) is a rare disorder characterized by intermittent, involuntary contractions of the inferior oblique muscle.
• Main symptoms include transient excyclotorsion and vertical oscillopsia.
• Only three cases have been reported in the literature, and epidemiological data are not established.
• The etiology is idiopathic, with no known risk factors or systemic associations.
• Differentiation from superior oblique myokymia (SOM) is important; IOM is distinguished by excyclotorsion and SOM by incyclotorsion.
• There is no established treatment; oxcarbazepine and other medications used for SOM may be attempted.

1. What is Inferior Oblique Myokymia?

Inferior Oblique Myokymia (IOM) is a rare disorder characterized by monocular, high-frequency, low-amplitude intermittent contractions of the inferior oblique muscle. Excyclotorsion is induced during upward and outward gaze, causing transient vertical oscillopsia.

Only a very small number of cases have been reported in the literature, currently limited to about 3 cases. Epidemiological data (incidence, sex ratio, age distribution) have not been established.

Q How rare is inferior oblique myokymia?

A

Only about 3 cases have been reported in the literature, making it an extremely rare disease. Epidemiological data such as incidence rate, sex differences, and age distribution have not been established.

2. Main Symptoms and Clinical Findings

Subjective Symptoms

• Episodes of extorsion: Lasting from a few seconds to 1 minute. Frequency varies from several times a day to once a month.
• Vertical oscillopsia: The visual field appears to oscillate vertically.
• Onset pattern: Episodes occur spontaneously and intermittently. They appear suddenly without triggers and resolve spontaneously.

Q How long does an attack last?

A

A single episode lasts from a few seconds to 1 minute. The frequency varies greatly from person to person, ranging from several times a day to once a month. Since episodes occur and resolve spontaneously, they often cannot be reproduced during an examination.

Clinical Findings

• Ocular motility findings: Ocular motility disorders may be confirmed by upward gaze provocation. However, eye movements are random and often cannot be reproduced during an examination.
• 3D video-oculography: A pattern of monocular conjugate movement with small-amplitude components of elevation and abduction accompanying excyclotorsion can be recorded. It shows phasic and tonic eye movement patterns similar to SOM.
• Brain and orbital MRI: No abnormalities in all reported cases.

3. Causes and Risk Factors

The etiology of IOM is considered idiopathic. No known risk factors or associations with systemic diseases have been reported.

• Abnormal spontaneous discharge: It is speculated that abnormal spontaneous discharge in the motor units of the inferior oblique muscle is the cause. Since oculomotor nerve (CN3) function is normal, this suggests a primary problem within the muscle itself (excessive neural firing).
• Difference from SOM: In superior oblique myokymia (SOM), vascular compression of the trochlear nerve in the dorsal brainstem is presumed to be the cause, but such a mechanism is unlikely in IOM. Cases where IOM and SOM have alternated have been reported, suggesting that two different cranial nerves are independently involved. This supports a local muscular phenomenon rather than a vascular compression theory or brainstem syndrome.
• Association with medication: Pyridostigmine has been suggested as a possible trigger.

Q What is the difference from superior oblique myokymia (SOM)?

A

IOM presents with excyclotorsion due to contraction of the inferior oblique muscle, whereas SOM presents with incyclotorsion due to contraction of the superior oblique muscle. The innervation also differs: IOM is innervated by the inferior branch of the oculomotor nerve (CN3), and SOM by the trochlear nerve (CN4). Vascular compression theory has been proposed for SOM, but this mechanism is unlikely for IOM.

4. Diagnosis and Examination Methods

Diagnosis of IOM is primarily clinical. IOM is suspected when monocular involuntary contractions of the inferior oblique muscle are observed.

Diagnostic Procedure

1. Confirm clinical history of SOM-like ocular myokymia.
2. During examination, attempt to provoke episodes by having the patient look upward and outward or upward and inward.
3. Even if the examination findings are normal, a typical history supports the diagnosis of IOM.

Differential Diagnosis

Differentiation from SOM is particularly important. The table below shows the main differentiating points between IOM and SOM.

Item	IOM	SOM
Eye movement	Excyclotorsion	Incyclotorsion
Innervating nerve	Oculomotor nerve (CN3) inferior branch	Trochlear nerve (CN4)
Vascular compression	Not involved	Suspected involvement

Imaging and electrophysiological tests

• Brain and orbital MRI: No abnormalities in all reported cases. Used to exclude organic lesions.
• 3D video-oculography: Can record monocular phasic movements mainly involving extorsion with slight elevation and abduction.

Regarding the anatomy of the inferior oblique muscle, the oculomotor nerve (CN3) originates in the midbrain, passes ventrally, and divides into superior and inferior branches within the orbit. The inferior branch innervates the medial rectus, inferior rectus, and inferior oblique muscles. The main function of the inferior oblique is extorsion, with secondary actions of elevation and abduction.

5. Standard Treatment

Only three cases have been reported in the literature, and no established treatment exists. Due to the unpredictability of symptoms and the small number of cases, evaluating the effectiveness of drug therapy is difficult.

Individualized treatment based on SOM therapy is attempted. The main options are listed below.

Drug/Treatment	Category
Oxcarbazepine	Antiepileptic drug (Na⁺ channel blocker)
Carbamazepine	Antiepileptic drug
Phenytoin	Antiepileptic drug
Gabapentin	Antiepileptic drug
Baclofen	Muscle relaxant
Topical beta-blocker	Eye drops
Incision surgery	Severe cases

No consistently effective treatment has been established for SOM either, with only some patients responding. The situation is similar for IOM.

The outcomes of reported cases are as follows.

• Case 1: The severity and frequency of episodes significantly decreased with oxcarbazepine.
• Case 2: No initial improvement was observed with timolol maleate and oxcarbazepine.

The mechanism of action of oxcarbazepine is the reduction of cell membrane excitability through inhibition of voltage-gated sodium channels. Compared to carbamazepine, it has the advantage of fewer metabolic and hematological side effects.

Individualized treatment based on each patient’s condition is recommended.

Precautions in Treatment

Since there is no established treatment protocol, predicting treatment efficacy is difficult. Drug selection, dosage, and duration should be determined in consultation with a specialist, based on individual cases.

Q Is there a treatment?

A

There is no established treatment. Antiepileptic drugs such as oxcarbazepine may be tried following the treatment for SOM, and improvement has been reported in some cases. However, since only three cases have been reported, evaluation of efficacy is difficult. Individualized treatment for each case is recommended.

6. Pathophysiology and Detailed Mechanism

The mechanism of IOM remains unknown.

The inferior oblique muscle is innervated by the inferior branch of the oculomotor nerve (CN3). Its primary function is excyclotorsion, with secondary actions of elevation and abduction. The oculomotor nerve originates in the midbrain, runs ventrally, and divides into a superior branch (superior rectus and levator palpebrae superioris) and an inferior branch (medial rectus, inferior rectus, and inferior oblique) within the orbit.

• Hyperexcitability of the muscle itself: Since oculomotor nerve function is preserved, a primary muscle problem (hyperexcitation of motor neurons) is suggested.
• Difference from neurovascular compression: In SOM, vascular compression of the trochlear nerve is considered a cause, but given the anatomical course of the oculomotor nerve (CN3) and the aforementioned findings of alternating SOM, vascular compression is unlikely in IOM.
• Eye movement pattern: Abnormal motor neuron firing leads to phasic cancellation movements, resulting in eye movements predominantly of excyclotorsion with slight elevation and abduction.

References

1. Chinskey ND, Cornblath WT. Inferior oblique myokymia: a unique ocular motility disorder. JAMA Ophthalmol. 2013;131(3):404-405. doi:10.1001/2013.jamaophthalmol.365. PMID: 23494050
2. Jang JY, Jeon H, Choi SY, Choi JH, Choi KD. Clinical and Oculographic Analysis of Inferior Oblique Myokymia. J Neuroophthalmol. 2017;37(4):393-395. doi:10.1097/WNO.0000000000000535. PMID: 28991103
3. Smith TA, Cornblath WT. Alternating superior and inferior oblique myokymia. JAMA Ophthalmol. 2014;132(7):898-899. doi:10.1001/jamaophthalmol.2014.366. PMID: 25010174
4. Patel BC, Malhotra R. Superior Oblique Myokymia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. PMID: 35593867. Available from: https://www.ncbi.nlm.nih.gov/books/NBK580560/