A 26-year-old woman is brought to the emergency department due to acute-onset diplopia. Medical history is significant for episodes of blurry vision involving her right eye that occurred 6 months and 2 years ago. These episodes were associated with deficits in color vision and pain made worse with eye movement. The patient recovered the majority of her visual acuity after a few months in both cases. On neurologic examination, she has a visual acuity of 20/20 OS and 20/40 OD, mild right optic disk atrophy, and a relative afferent pupillary defect in the right eye. There is slowed and impaired adduction of the left eye with right lateral gaze. Convergence testing shows normal adduction in both eyes. The remainder of the neurologic examination is unremarkable. This patient's diplopia is most likely caused by which of the following mechanisms?

Question

A 26-year-old woman is brought to the emergency department due to acute-onset diplopia.  Medical history is significant for episodes of blurry vision involving her right eye that occurred 6 months and 2 years ago.  These episodes were associated with deficits in color vision and pain made worse with eye movement.  The patient recovered the majority of her visual acuity after a few months in both cases.  On neurologic examination, she has a visual acuity of 20/20 OS and 20/40 OD, mild right optic disk atrophy, and a relative afferent pupillary defect in the right eye.  There is slowed and impaired adduction of the left eye with right lateral gaze.  Convergence testing shows normal adduction in both eyes.  The remainder of the neurologic examination is unremarkable.  This patient's diplopia is most likely caused by which of the following mechanisms?

Quizplus · Accepted Answer

11ec160b_39d0_cf80_9249_edf1d98bb4bf_MD0001_00 This young woman has diplopia with features of internuclear ophthalmoplegia (impaired adduction on the left with right lateral gaze). This, along with her prior episodes of optic neuritis (transient blurred vision and eye pain with movement), raises concern for multiple sclerosis (MS), a demyelinating disease of the central nervous system (CNS) caused by an autoimmune response directed at oligodendrocytes. In unmyelinated neurons, signal transmission requires sequential activation of adjacent sodium channels down the entire length of the axon. In myelinated neurons, the depolarization impulse is regenerated at discrete points along the axon (nodes of Ranvier) where sodium channels are concentrated. The myelin sheath serves as an electrical insulator that results in the impulse "jumping" between 2 adjacent nodes (saltatory conduction). This process allows for myelinated neurons to transmit impulses >10x faster than unmyelinated neurons. Due to the acute demyelination seen with MS, propagation of the impulse cannot occur, as the distance between the sodium channels at the nodes of Ranvier precludes sequential activation. This causes a conduction block. Over time, sodium channels eventually redistribute across the naked axon and surviving oligodendrocytes partially remyelinate neurons, allowing for some restoration of signal transmission (explaining why some of the symptoms and signs of MS resolve to a variable extent). (Choice A) Impaired neuromuscular transmission can be seen in myasthenia gravis, which is caused by autoantibodies against acetylcholine receptors at the neuromuscular junction. Affected patients typically present with fluctuating diplopia, ptosis, and/or bulbar muscle weakness. (Choice B) Loss of axons and neurons occurs over time in chronic multiple sclerosis plaques. This patient had prior episodes of optic neuritis that have resolved. However, the internuclear ophthalmoplegia is acute and would demonstrate a relative preservation of axons. (Choice C) Muscle fiber inflammation and necrosis can be seen with various forms of myositis (eg, polymyositis, dermatomyositis, inclusion body myositis). Muscle disorders rarely follow a relapsing-remitting course and are not associated with optic neuritis or internuclear ophthalmoplegia. (Choice D) Optic nerve compression would most likely result in progressive vision loss. This patient's symptoms are more consistent with a relapsing-remitting disease pattern with lesions affecting multiple nervous system locations. Educational objective: Multiple sclerosis is an autoimmune disorder of the central nervous system characterized by recurrent episodes of demyelination, leading to reduced saltatory conduction. Internuclear ophthalmoplegia and optic neuritis are common manifestations.

A 26-Year-Old Woman Is Brought to the Emergency Department Due