Superior Oblique Myokymia: Some Novel Observations.

Superior oblique myokymia is a rare condition, characterized by spontaneous rhythmic contractions of the superior oblique muscle and was first described by Duane in 1906. However, the pathophysiology of this condition remains poorly understood even today. A number of medical and surgical treatment modalities have been tried, with variable results. We report a case of superior oblique myokymia in a pregnant female, which could be triggered with flashlight stimulation, and the result of treatment with timolol maleate 0.5% ophthalmic solution.

Introduction

Superior oblique myokymia is a rare ocular disorder characterized by rhythmic involuntary contractions of the superior oblique muscle and most commonly presents with oscillopsia. The pathophysiology of this condition still remains unclear. Numerous medical and surgical modalities have been tried for this condition, with variable results. We describe a case of superior oblique myokymia in a pregnant lady with some novel observations and relief from the symptoms with timolol maleate 0.5% ophthalmic solution.

Case Report

A 25-year-old, junior public health nurse at 27 weeks of her first pregnancy presented to us with oscillopsia of the left eye of 1-month duration. She had intermittent vertical double vision during the episodes of oscillopsia, and it was aggravated by looking toward objects on the right side. She had multiple episodes occurring unpredictably on a daily basis and worsening of the symptoms with reading, using computer and with outdoor activity on sunny days. She had noticed that with slight head tilt to the right and chin up posture, her symptoms seemed to improve. There was no tinnitus, vertigo, vomiting, or headache. There was no history of deviation of eyes, blurred vision or any neurological complaints. She was not on any medications. There was no history of any systemic diseases, surgery or trauma. No alcohol, tobacco, substance, caffeine abuse, or drug allergies. She had an uneventful antenatal course. No history of multiple sclerosis or neurological disorders in the family

On examination, her unaided visual acuity was 20/20 both eyes, near vision was N6. There were occasional jerky repeated movements of intorsion of the left eye on dextrodepression, which increased on exposure to flashlight for 5–10 s. There was no ocular deviation, no evidence of vertical misalignment. Anterior segment, fundus, automated visual fields, color vision, and stereoacuity were normal. Intraocular pressure (Goldmann applanation tonometry) was 18 mmHg in both eyes. Hemogram, erythrocyte sedimentation rate and urinalysis were normal. Antinuclear antibody and rheumatoid factor were negative. Magnetic resonance imaging of the brain was normal, no evidence of vascular compression.

The diagnosis of the left eye superior oblique myokymia was made. Since she was pregnant, we decided to observe her, and she was given tinted spectacles for outdoor use.

She presented 5-month postpartum with worsening of symptoms after resuming her work, especially with outdoor activity. She was prescribed timolol maleate 0.5% ophthalmic solution twice daily for 1 week. At 1 week follow-up, there was subjective improvement in the quality of vision and there was no oscillopsia. Timolol maleate 0.5% ophthalmic solution twice daily was continued for 3 weeks. On holding the drops, oscillopsia episodes were less frequent and less bothersome, and she was symptom-free after 4 weeks. After 18 months of onset of symptoms, she conceived for the second time, and there was no recurrence of the symptoms during pregnancy or lactation.

Discussion

Superior oblique myokymia is a rare ocular disorder characterized by spontaneous rhythmic contractions of the superior oblique muscle. Superior oblique myokymia was first described by Duane in 1906[1] as “unilateral rotary nystagmus.” Stress, fatigue, alcohol, caffeine, nicotine, and fluorescent lighting and trauma have been suggested as trigger factors.[23456] This condition has been treated using oral drugs[45] and surgery[6] with variable results. The greater prevalence in females has resulted in postulating a link with pregnancy hormones. Our patient had onset of symptoms during her pregnancy and worsening postpartum. Although there are reports describing onset during pregnancy or lactation,[7] there was no published study with onset of symptoms during pregnancy and worsening postpartum. Our speculation is that the progesterone receptors on the superior oblique muscle may have abnormal hyperexcitatory signals during pregnancy and lactation in these patients.

Flashlight as a stimulant for superior oblique myokymia has not been previously reported in literature. This was an observation made by the patient herself as she had more difficulty during outdoor activity on sunny days. We could induce abnormal intorting movements on stimulating with torchlight or slit lamp illumination for 5–10 s. Our hypothesis is that photic stimulation can cause hyperexcitation in the superior oblique muscle and this can lead to abnormal intorting movements of the muscle. Hence, the conduction of light through the optic nerve may also have a role in the pathogenesis of the condition. However, this is purely speculative, and our report fails to explain the exact mechanism of the disease.

Mehta and Demer[8] have reported the smaller size of the affected superior oblique muscle as per magnetic resonance imaging studies and suggested that these anatomical changes in the superior oblique muscle of patients with myokymia secondary to an antecedent injury to the trochlear nerve. However, the magnetic resonance imaging of the superior oblique muscle in our patient failed to demonstrate such a difference.

Topical timolol for treatment of superior oblique myokymia was first reported by Bibby et al.[9] and the mechanism of action was thought to be due to the systemic absorption of the drug causing membrane stabilization. But Borgman[10] observed that it was due to the local action on the superior oblique muscle endplates. In our patient, triggering of the symptom with flashlight stimulation suggest a possible link with light reflex pathway as well. We postulate that the membrane stabilizing action of timolol maleate happens at the local level on lysosomal endplates and this may result in abnormal action potentials. Our patient had alleviation of symptoms following topical timolol maleate 0.5% even after the 1st week of starting the therapy, and the symptoms did not recur after a drug holiday or second pregnancy. This could be due to a long-term membrane stabilization effect of topical timolol maleate.

Our case report highlights the use of topical beta blocker for this rare disease entity and this can avoid the undesirable effects of oral drugs or surgery as this cannot be performed in pregnant or lactating women. Our follow-up of 2 years with nonrecurrence of symptoms even in the next pregnancy indicate the possible cure effect. Although a possibility of selflimiting disease can be put forward, the fact that our patient had abrupt relief of symptoms of 9 months duration on starting the drug points towards more of a therapeutic effect.

This case highlights the successful treatment of superior oblique myokymia following the use of topical timolol maleate. The mechanisms by which this effect was achieved could not be elucidated from the present report. We believe that the membrane stabilizing properties of the drug must have contributed to the therapeutic effect. However, this is a single case report, and more investigations have to be made regarding the use of this drug in the treatment of superior oblique myokymia.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.