A 28-year-old woman comes to the physician because of a 4-day history of double vision when looking to the left. She also describes occasional shooting sensations down her spine when she bends her head forward. On physical examination, her visual acuity is 20/20 in both eyes. When the patient is asked to look to the left, her right eye remains at the midline and fails to adduct, while the left eye abducts but demonstrates prominent horizontal nystagmus. On rightward gaze, both eyes move normally. Convergence is preserved. The pupils are equal and reactive to light.
Which of the following is the most likely location of this patient’s lesion?
The correct answer is:
C) Medial longitudinal fasciulus
This patient is presenting with internuclear ophthalmoplegia (INO) affecting the right eye, characterized by the inability to adduct the right eye on leftward gaze and dissociated nystagmus of the left abducting eye. This condition results from a lesion in the medial longitudinal fasciculus (MLF), a highly myelinated white matter tract that coordinates horizontal gaze by connecting the abducens nucleus (CN VI) in the pons to the contralateral oculomotor nucleus (CN III) in the midbrain. In a young patient, bilateral or even unilateral INO is highly suggestive of multiple sclerosis (MS), a suspicion further supported by this patient's report of electric-shock sensations with neck flexion, which is called Lhermitte sign. Convergence remains intact in most cases of INO because the pathway for convergence typically bypasses the MLF to reach the medial rectus subnuclei directly.
Answer choice A: Edinger-Westphal nucleus, is incorrect. The Edinger-Westphal nucleus provides preganglionic parasympathetic fibers for pupillary constriction and lens accommodation. A lesion here would cause a fixed, dilated pupil and loss of the near response, rather than a gaze-induced motility defect.
Answer choice B: Lateral rectus muscle, is incorrect. A lesion of the lateral rectus muscle or its nerve (CN VI) would result in an inability to abduct the ipsilateral eye, not an inability to adduct. On examination, the affected eye would be medially deviated at rest (esotropia).
Answer choice D: Medial rectus muscle, is incorrect. While a primary lesion of the medial rectus muscle would impair adduction, it would do so during both horizontal gaze and convergence. Furthermore, a muscle lesion would not explain the rhythmic nystagmus seen in the contralateral abducting eye.
Answer choice E: Paramedian pontine reticular formation, is incorrect. The paramedian pontine reticular formation (PPRF) is the horizontal gaze center. A lesion here results in an ipsilateral horizontal gaze palsy, where neither eye can look toward the side of the lesion.
Key Learning Point
Internuclear ophthalmoplegia (INO) is caused by a lesion in the medial longitudinal fasciculus (MLF). It presents as impaired adduction of the eye ipsilateral to the lesion and horizontal nystagmus of the contralateral abducting eye during horizontal gaze. In young adults, it is a classic finding associated with multiple sclerosis.
