Introduction
Tinnitus is defined as the perception of sound in the absence of corresponding external stimulus, and may be classified as subjective or objective. Subjective tinnitus is characterised by an individual's perception of sound in the absence of any physical source, whereas objective tinnitus refers to a sound that is produced mechanically within the head.
Stapedial myoclonus is a rhythmic movement of the tympanic membrane secondary to repetitive contraction of the tensor tympani and stapedial muscles, and represents a segmental myoclonus. It is a rare cause of objective tinnitus,Reference Zipfel, Kaza and Greene1 with only a few cases reported in the literature.
We describe a previously unreported case of objective tinnitus caused by transitory stapedial myoclonus in a patient diagnosed with benign fasciculation syndrome.
Case report
A 30-year-old man presented with severe, low-pitched tinnitus on the right side which had developed suddenly while he was sitting down watching television. His previous medical history was notable for benign fasciculation syndrome. He did not take any drugs. The tinnitus was described as an unpleasant, buzzing noise resembling that experienced during aircraft landing, but occurring in a rhythmic fashion, dozens of times a minute. There was no pressure in the left ear nor any change in hearing status. The patient denied any precipitating factors, although performing the Valsalva manoeuvre and placing pressure on the tragus were reported to slightly increase the frequency and intensity of tinnitus.
On physical examination, the tympanic cavity was clear. Otoscopic examination revealed rhythmic movement of the tympanic membrane, which was synchronous with the tinnitus. No palatal spasm was noted on nasopharyngeal examination.
Brain magnetic resonance imaging and pure tone audiometry were normal.
Based on these findings, a diagnosis of objective tinnitus due to stapedial myoclonus was made.
Electromyography showed diffuse fasciculation potentials with normal motor unit recruitment, without features indicative of chronic denervation (i.e. neurogenic enlarged motor unit potentials) and/or active denervation (i.e. fibrillation and positive, sharp waves).
The patient's objective tinnitus spontaneously disappeared within 48 hours of its appearance. In the following days, he suffered frequent, brief episodes of objective tinnitus lasting only a few seconds.
Discussion
Stapedial myoclonus is a rhythmic movement of the tympanic membrane caused by repetitive contraction of the tensor tympani and stapedial muscles. It represents a segmental myoclonus, and is a rare cause of objective tinnitus, with only a few cases reported.
A convincing proposal for a pathogenetic mechanism of this condition is still lacking. Furthermore, there are no previously published reports of stapedial myoclonus associated with benign fasciculation syndrome.
Fasciculations are random muscle twitches which can be observed clinically, and are associated with electrical events recorded by a needle electrode as fasciculation potentials. A fasciculation potential therefore represents the spontaneous discharge of a motor unit or part thereof. Fasciculation potentials are seen in many pathological conditions (e.g. peripheral neuropathy, radiculopathy, peripheral nerve hyper-excitability syndromes and, especially, motor neuron diseases).Reference Mills2
Conversely, fasciculation potentials may occur in otherwise healthy subjects.Reference Mitsikostas, Karandreas, Coutsopetras, Piperos, Lygidakis and Papageorgiou3 This phenomenon, termed benign fasciculation syndrome, is a non-progressive, chronic condition characterised by fasciculation, usually in the lower limbs (predominantly the distal leg muscles), which is not associated with any other clinical abnormality and which, unlike motor neuron diseases, has an excellent long-term prognosis.Reference Mitsikostas, Karandreas, Coutsopetras, Piperos, Lygidakis and Papageorgiou3
The origin of benign fasciculation potentials is unknown. It has been suggested that they may arise by ephaptic transmission from irregularly fibrillating fibres,Reference Janko, Trontelj and Gersak4 although a more proximal origin has also been suggested.Reference Sindermann, Conrad, Jacobi and Prochazka5
To the best of our knowledge, the presented patient represents the first report of co-occurrence of stapedial myoclonus and benign fasciculation syndrome. The coexistence of a rare otological disorder such as stapedial myoclonus together with benign fasciculation syndrome could simply be a coincidence. However, the possibility of a non-coincidental association should also be considered, and we present below some considerations regarding a possible pathogenetic relationship.
• Stapedial myoclonus is rhythmic tympanic membrane movement due to repetitive tensor tympani and stapedial contraction
• It represents a segmental myoclonus, and is a rare cause of objective tinnitus
• The presented patient had stapedial myoclonus plus benign fasciculation syndrome
• This suggests the presence of underlying motor unit hyper-excitability
• Some cases of stapedial myoclonus may be due to diffuse motor unit hyper-excitability
It is noteworthy to consider that stapedial myoclonus has been quite frequently described during the course of recovery from peripheral facial nerve paralysis,Reference Watanabe, Kumagami and Tsuda6 a condition in which muscle synkinesis is thought to occur (due to abnormal reinnervation of facial muscles following denervation, or to ephaptic transmission leading to formation of an ‘artificial synapse’, an ephapse, at the site of nerve injury). Furthermore, antiepileptic drugs and botulinum toxin administration have been proposed as treatment for this disorder.Reference Badia, Parikh and Brookes7 Interestingly, both drugs reduce neuromuscular transmission, either by stabilising the muscle membrane and reducing neuronal firing (in the case of anticonvulsants) or by blocking acetylcholine release at the neuromuscular junction (in the case of botulinum toxin). The occurrence of stapedial myoclonus in our patient suggests the presence of an underlying motor unit hyper-excitability. It is worth considering that, in our patient, both the Valsalva manoeuvre and pressure on the tragus were reported to slightly worsen his tinnitus. Since fasciculations may be elicited by muscle percussion, it can be hypothesised that increased tension of the tympanic membrane, leading to stapedius muscle stretching, may mechanically further increase the local hyper-excitability.
Conclusion
The presented case suggests that, at least in some patients, stapedial myoclonus may represent a clinical expression of a functional hyper-excitability of a motor unit or part thereof.
