Clinical Reasoning: A 73-Year-Old Woman With Episodic Dysarthria and Horizontal Binocular Diplopia

Section 1

A 73-year-old woman presented with transient episodes of slowing and slurring of her speech over the past year. The episodes were brief, lasting for approximately 20–30 seconds before resolving. Initially, they were occurring 1–2 times a week, but the frequency had increased to 10–15 times a week at the time of first evaluation. Episodes could occur at any time of the day without any specific preceding triggers. She also reported transient episodes of horizontal double vision, which would occur independently from her episodes of dysarthria, resolved with closure of 1 eye, and worsened with rightward gaze. She denied any associated fevers, weight loss, headache, dyspnea on exertion, dysphagia, focal weakness, sensory changes, or word-finding difficulties.

She had a retroclival meningioma and went through suboccipital craniectomy, resection, and stereotactic radiosurgery 18 years ago. She also had a left sphenoid meningioma that is being managed conservatively. On examination (Video 1), she had rightward tongue deviation on protrusion with right-sided tongue fasciculations. There was intermittent right eye abduction impairment without nystagmus or ptosis. The rest of the cranial nerves were intact. She had full strength throughout with normal reflexes, and sensory examination was normal. She did not have dysmetria or intention tremor, and gait was normal.

Section 2

The patient in this vignette presented with recurrent, transient episodes of dysarthria and horizontal diplopia with evidence of right-sided tongue deviation, tongue fasciculations, and impaired abduction of the right eye. The tongue deviation points toward a lesion of the right cranial nerve (CN) XII, and associated fasciculations would favor lower motor neuron involvement. The intermittent impaired abduction of the right eye without any other associated abnormalities of extraocular movements or concomitant facial weakness is suggestive of a focal peripheral lesion of cranial nerve VI. Taken together, these findings suggest multiple cranial neuropathies within the brainstem or subarachnoid space, neuromuscular junction disorder, or myopathy. The intermittent nature of her symptoms raises the possibility of peripheral hyperexcitability as opposed to a fixed structural problem.

When considering the differentials for multiple cranial neuropathies within the brainstem, a single brainstem lesion causing isolated compression of just the right CN VI and XII without additional CN involvement at the same level would be unusual. A treatment effect, such as entrapment of nerve in a surgical scar, or ionizing radiation–induced peripheral hyperexcitability should also be considered.

Multiple cranial neuropathies from extramedullary lesions have wide differential diagnoses, including infectious, inflammatory, and neoplastic etiologies.¹ Relatively chronic presentation, lack of involvement of other cranial nerves, absence of fever, or meningismus made an infectious etiology unlikely. Sarcoidosis and amyloidosis could present with multiple cranial neuropathies, and the slowly progressive presentation could fit with a neuroinvasive process. However, neurologically isolated amyloidosis and sarcoidosis are rare, and there was no suggestion for additional systemic involvement to support these diagnoses. Similarly, a connective tissue disorder or vasculitis appeared to be unlikely given relatively gradual progression, lack of other CN involvement, and absence of associated systemic features. A neoplastic process affecting the skull base or clivus should be a consideration, particularly with her history of retroclival meningioma; however, as mentioned before, the intermittent nature of symptoms suggested against a fixed structural lesion.^1,2

Disorders of the neuromuscular junction were unlikely in the absence of any other symptoms, such as ptosis, proximal weakness, dysphagia, shortness of breath, or autonomic instability. There was no fatigability, and unilateral tongue involvement is also unusual for a neuromuscular junction disorder.

Motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), can present with bulbar predominant findings, but diplopia and episodic symptoms are atypical in ALS. Furthermore, there was no motor weakness or suggestion of upper motor neuron involvement on clinical examination. Although some myopathic processes could present with ophthalmoplegia, isolated dysarthria is unusual in such cases.

Other central causes including transient ischemia, focal seizure, or a migraine with basilar aura were considered but favored to be less likely. Although the patient does have a history of migraine, the frequency of the episodes, brief duration, and lack of associated headache would be less consistent with this diagnosis.

Section 3

The patient underwent MRI of her brain with and without contrast that showed the stability of her left sphenoid meningioma and right retroclival meningioma with mild mass effect on the brainstem (Figure, B–G). There was no evidence of acute intracranial infarction, hemorrhage, or meningeal involvement. Magnetic resonance angiography (MRA) of the head and neck did not demonstrate any hemodynamically significant stenosis, occlusion, dissection, or a structural basis for a vascular compression syndrome. EEG did not show electrographic evidence for seizures or epileptiform discharges.

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Figure EMG and MRI Findings

(A) EMG showing involuntary grouped repetitive discharges of a single motor unit from the right hemitongue, consistent with myokymia. (B–E) Axial and sagittal view of the retroclival meningioma (white arrow) and (F–G): the left sphenoid meningioma (blank white arrow) showing stability over 10 years [all MR images are T1-weighted postcontrast images. B, D, and F from 2010; C, E, and G from 2020].

EMG showed spontaneous depolarizations in bursts at rest with a characteristic sound of “marching soldiers” in the right hemitongue, consistent with myokymia (Video 1, Figure, A). The left side of the tongue was normal. Myokymia is a clinical phenomenon characterized by brief bursts of single motor unit potentials firing at rates of 5–150 Hz that may appear as doublets, triplets, or multiplets.³ This burst activity can occur at regular or irregular intervals with a variable interburst frequency on EMG. The audio description of myokymia has classically been described as “soldier's marching on snow”. Clinically, myokymia will appear as a wave-like rippling of the muscles on examination.

The causes of focal myokymia include Guillain-Barre syndrome, multiple sclerosis, radiation neuropathy, pontine tumors, neoplastic/inflammatory meningoradiculitis, anoxic/ischemic rhomboencephalopathy, syringobulbia, basilar invaginations, and subarachnoid hemorrhage.³ The pathophysiology of peripheral nerve hyperexcitability is thought to be due to ectopic discharges arising from motor neuron axons with compromised myelin sheath integrity.⁴ Given these findings, our patient's presentation of episodic dysarthria was felt to be most consistent with delayed radiation-induced myokymia. The episodes of diplopia were possibly related to underlying ocular myokymia or neuromyotonia.

Section 4

For delayed radiation-induced myokymia, there are limited data available on therapy given the rarity of the syndrome. However, some case reports have noted therapeutic benefit from anticonvulsant medications. Specifically, sodium channel inhibitors (e.g., carbamazepine, oxcarbazepine, and lacosamide) have been used with success largely because of their ability to inhibit the underlying peripheral nerve hyperexcitability.^5,6 Sodium channel blockers can also be effective in treating ocular myokymia/neuromyotonia.^7,8

Our patient was started on low-dose lacosamide, 50 mg twice a day, with notable improvement in the frequency of her symptoms, now occurring only briefly once or twice a month.