Abstract
Superior oblique myokymia (SOM) is characterized by intermittent, small-amplitude, monocular, paroxysmal, high-frequency oscillations. These oscillations are mainly torsional in the primary gaze position and in abduction and evoke oscillopsia during paroxysms (1). The term SOM was first used by Hoyt and Keane in 1970 (2), but the first case recognizable as SOM was described by Duane in 1906 (3). The pathophysiology of this condition is not clear. Often the patients are otherwise healthy, but sometimes vascular compression of cranial nerve IV may be responsible (4, 5), and rarely it is associated with other neurological diseases, such as adrenoleukodystrophy (6), retinitis pigmentosa, astrocytoma, multiple sclerosis and cerebellar tumour (7). The disorder may have spontaneous remissions, which can last for days or up to years, and relapses (1). There have been several reports that anticonvulsants, especially carbamazepine, have a therapeutic effect (8, 9). In recent years, gabapentin has also been reported to be effective (10, 11). When medication is unsuccessful, surgery, involving tenotomy or a tenectomy of the superior oblique muscle, combined with inferior oblique myectomy may be performed (1). Recently, surgical decompression of cranial nerve IV has been found to be beneficial when vascular compression is confirmed on magnetic resonance imaging (MRI), but there is the danger of superior oblique palsy (4). The choice of treatment is related to the intensity and duration of the visual symptoms and to the degree of disability the disorder may have caused. Pareja et al. (12), in a recent and exhaustive review of primary trochlear headache and other trochlear painful disorders, have considered the possible causes of headache secondary to involvement of the trochlear region, including superior oblique muscle myofascial pain, but migraine secondary to SOM was not reported.
Case report
A 27-year-old man presented to our Headache Centre with headache associated with SOM. In the family history, his mother and sister were affected by migraine without aura. The patient's past medical history was negative and, in particular, he had no history of migraine or headache. In the last 4 years he reported ‘ocular tremor’. For the first 2 years, this symptom had an occasional presentation and lasted a few minutes. Thereafter, it became almost continuous with exacerbations lasting up to 1 h and related to ophthalmic stress, in particular, every time the patient strained his sight studying or when, during conversation, he made frequent, sudden changes in gaze position to talk to different people. This occurred about two to three times a week. On ophthalmological examination, there was no alteration of ocular motility. Slit lamp examination demonstrated low-amplitude, high-frequency, torsional movement of the right eye, a feature characteristic of SOM. The patient did not report spontaneous pain in the trochlear region, but moderate pain to compression on the right, upper, inner angle of the orbit. Neurological examination was negative and, in particular, trigeminal sensation was normal. His headache attacks had begun 2 years after the onset of SOM, when SOM became persistent. They were strictly unilateral, always beginning in the inner aspect of the orbit and rapidly spreading on the right side of the head and always triggered by the exacerbations of the paroxysmal myokymia attacks. The pain was pulsating, with presence of phono-, photo- and osmophobia, often with nausea but without vomiting. The pain was incapacitating, increased by movement and effort, lasting for 8–12 h without medication and 2 or 3 h with non-steroidal anti-inflammatory drugs. He never used triptans. The frequency of attacks was variable, but with a mean of about two or three times a week. For this disturbance, he had given up his studies at university. The attacks, according to the International Classification of Headache Disorders-II (13), could be classified as migraine without aura. Laboratory investigations, including complete blood counts, coagulation studies, basic metabolic panel and thyroid function tests, EEG, EKG, chest X-ray, computed tomography scan with contrast, MRI and angio-MRI with gadolinium of the brain were negative. In particular, MRI with focus on the midbrain did not demonstrate neurovascular compression of cranial nerve IV. Previous prophylactic treatments with baclofen, carbamazepine, levetiracetam and bromazepam had been unsuccessful. Therapy was commenced with gabapentin at increasing dosage up to 900 mg/day. This dosage was reached in 3 weeks, resulting in complete remission of SOM and its migraine attacks. Thereafter, the patient was able to return to his studies. After 1 year gabapentin was tapered off. Follow-up visits over the past 2 years have been negative for a reappearance of SOM or migraine attacks.
Discussion
Our patient had no history of migraine and/or headache before the development of SOM; as a predisposing factor his mother and sister suffered from migraine without aura. The presentation of migraine attacks never developed during the night, and was consequent to the worsening of SOM, when after ophthalmic stress he presented with a protracted paroxysmal myokymia attack. The pathophysiology of SOM is unclear. Hoyt et al. (2) have suggested SOM as a primarily nuclear disorder, whereas Ware (14) has hypothesized an aberrant regeneration subsequent to damage of cranial nerve IV. However, thin-slice MRI has shown that the cause of SOM can be attributed to compression of the trunk of cranial nerve IV (trochlear nerve) by the superior cerebellar artery (4). In this sense, SOM should be considered a neurovascular compression condition potentially able to cause migraine, in analogy to trigeminal neuralgia (15) and facial myokymia (16). Another possibility is to consider SOM in the group of paroxysmal dyskynesias; in this case, too, there is some evidence of a correlation with migraine (17). To explain the pathogenesis of migraine related to SOM the relevance of the trochlear region should be considered, as highlighted by Yangüela et al. (18); it is important to explain the so-called ‘trochlear headache’, but is also involved in the worsening of tension-type headache and migraine. In our case, tenderness was found on local examination of the right trochlear region that is not usually found in SOM. We can hypothesize that a continuously repeated muscle contraction during the prolonged attacks of myokymia could cause irritation of local nociceptors, activation of the supratrochlear nerve, a branch of the ophthalmic division of the trigeminal nerve innervating the trochlear region, and, in turn, of the trigeminal nucleus caudalis, thus contributing to central sensitization with a secondary migraine attack (19, 20). Another possibility is related to a central neuronal hyperexcitability underlying both disorders.
In conclusion, for the first time a case of migraine secondary to SOM is presented. We highlight the importance of the trochlear region as a trigger factor for migraine attacks, presenting a cause not previously reported, and confirm the efficacy of gabapentin in the treatment of SOM.