Inferior Oblique Myokymia

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 by Bayan Al Othman, MD on June 30, 2020.


Disease

Inferior Oblique Myokymia (IOM) is a rare disorder, characterized by intermittent episodes of monocular, high-frequency, low-amplitude contractions of the inferior oblique muscle [1]. Patients experience excyclotorsion induced by looking up and out [1]. Transient vertical oscillopsia and excyclotorsion may manifest spontaneously as well [2].

Etiology

The etiology of IOM is idiopathic. IOM may be induced by aberrant, spontaneous discharges in the inferior oblique motor unit [2]. Intact oculomotor nerve functions support a primary muscle problem, possibly excessive neuronal firings of the muscle [1][2]. In the case of alternating superior and inferior oblique myokymia, involvement of 2 different cranial nerves further suggests a local muscle phenomenon rather than vascular compression or brain stem syndrome [3].

Risk Factors

IOM has no known risk factor or association with other systemic disorders. Pyridostigmine was suggested in some studies as a possible trigger [3].

Pathophysiology

The inferior oblique muscle is innervated by the inferior division of the oculomotor nerve and functions to turn the eye superiorly upon adduction [2]. This muscle typically works with other extraocular muscles, but a contraction of inferior oblique muscle alone rotates the eye up and laterally.

The mechanism of IOM is currently unknown, and the presumed pathologic mechanism of Superior Oblique Myokymia (SOM), such as neurovascular compression, seems unlikely [2]. Intact oculomotor nerve functions support a primary muscle problem, possibly excessive neuronal firings of the muscle [1][2]. Aberrant motor neuron firing in the inferior oblique muscle may result in phasic offsetting movements and excyclotorsion with a small amount of supraduction and abduction [2].

Diagnosis

History

Episodes of excyclotorsion with high-frequency, low-amplitude contractions of IOM can affect patients from a few seconds to a minute before resolution (if at all) and range from multiple times a day to once a month [2]. The episodes may present spontaneously and intermittently [2].

Physical examination

Eye movement disorders can be induced by supraduction of patient’s eyes. However, eye movements are random and may not be reproduced on examination [1].

Clinical diagnosis

IOM is a possible differential diagnosis if a patient presents with monocular, involuntary contractions through the inferior oblique muscle. When a patient has a clinical history consistent with an ocular muscle myokymia, such as SOM, and normal findings on examination, asking the patient to look up and out as well as up and in could provoke IOM [1]. IOM can be distinguished from SOM as IOM would cause excyclotorsion of the affected eye instead of incyclotorsion [1].

Imaging

Magnetic resonance imaging (MRI) of the brain and orbits has been unremarkable in all reported cases with IOM [2]. 3D video-oculography of eye movements show monocular phasic movements, excyclotorsion mixed with supraduction and abduction, and slight elevation [2]. Oculographic measurements of IOM indicate a number of phasic and tonic eye movements similar to those of SOM [2].

Management

Medical therapy

There have been 3 reported cases of IOM in the literature, and there is currently no established treatment for this condition. The lack of sufficient cases for treatments and the unpredictability of presentations make the evaluation of medical treatments challenging.

SOM is a well-described disorder, and treatments of IOM have been similar to those of SOM. Treatments of SOM include topical β-blockers, carbamazepine, oxcarbazepine, phenytoin, gabapentin, baclofen, and incisional surgery in severe cases [4]. However, no treatment for SOM has been consistently effective, but some patients may respond to one of the treatments mentioned above [2].

In one case of IOM, there was a markedly reduced severity and frequency of episodes with oxcarbazepine, while another case showed no initial improvement with timolol maleate and oxcarbazepine [2]. Oxcarbazepine, a structural derivative of carbamazepine, inhibits the voltage-dependent sodium channel and decreases the excitability of the cell membrane [5]. Compared to carbamazepine, it has fewer metabolic and hematologic side effects [5]. Thus, medical therapy on a case-by-case basis seems to be a favorable option in IOM patients.

Prognosis

The episodes of IOM are mostly benign. Patients with IOM may have spontaneous improvement or recovery; however, some may not respond to medical treatments initially or at all [2].

Summary

Inferior Oblique Myokymia (IOM) is a rare condition, characterized by episodes of monocular, high-frequency, low-amplitude contractions of the inferior oblique muscle. It may induce excyclotorsion when a patient looks up and out. A case-by-case basis therapy would lead to some relief in patients with IOM.

Additional Resources

A Unique Ocular Motility Disorder: https://edhub.ama-assn.org/jn-learning/video-player/5414740

Alternating Superior and Inferior Oblique Myokymia: https://www.youtube.com/watch?v=Xxa6sS--q88

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Chinskey ND, Cornblath WT. Inferior oblique myokymia: a unique ocular motility disorder. JAMA Ophthalmol. 2013;131(3):404-405.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 Jang J, Jeon H, Choi S, Choi J, Choi, K. Clinical and oculographic analysis of inferior oblique myokymia. Journal of Neuro-Ophthalmol. 2017;37:418-420.    
  3. 3.0 3.1 Smith TA, Cornblath WT. Alternating superior and inferior oblique myokymia. JAMA Ophthalmol. 2014;132:898-899.    
  4. Kattah JC, Fitz Gibbon EJ. Superior oblique myokymia. Curr Neurol Neurosci Rep. 2003;3(5):395-400.    
  5. 5.0 5.1 Wu L, Tsai T, Hu F. Treatment of superior oblique myokymia with oxcarbazepine. Taiwan Journal of Ophthalmol. 2014;4:49-51.