Examkin

Question

Difficulty: MediumPhysical Examination Findings

A 31-year-old woman is evaluated in the neurology clinic for horizontal double vision that occurs primarily when she attempts to focus on objects to her far right. She recalls a distinct episode six months prior when she experienced mild weakness in her right leg, which resolved over a few weeks without any medical intervention. Her temperature is 37.0°C (98.6°F), blood pressure is 118/76 mmHg, and pulse is 72/min.

During a detailed cranial nerve evaluation, the physician instructs her to follow a moving target to her right. The patient's left eye fails to move medially beyond the midline, while her right eye completely abducts but develops prominent horizontal nystagmus. When a target is brought progressively closer to the bridge of her nose, both eyes converge symmetrically. Pupillary light reflexes are intact bilaterally.

Based on these specific physical examination findings, which of the following is the most likely structural abnormality?

  1. A
    Compression of the right abducens nerve
  2. B
    Demyelination of the right medial longitudinal fasciculus
  3. Demyelination of the left medial longitudinal fasciculusAnswer
  4. D
    Infarction of the left paramedian pontine reticular formation
  5. E
    Inflammatory lesion of the left oculomotor nerve

Answer

The most likely structural abnormality is demyelination of the left medial longitudinal fasciculus (MLF), leading to an internuclear ophthalmoplegia.
The patient's physical examination reveals an internuclear ophthalmoplegia (INO), characterized by impaired adduction of the left eye and nystagmus of the abducting right eye during rightward gaze. The medial longitudinal fasciculus (MLF) is a highly myelinated tract that carries signals from the contralateral abducens nucleus (CN VI) to the ipsilateral oculomotor nucleus (CN III) to coordinate horizontal conjugate gaze. An adduction deficit on rightward gaze localizes the lesion to the left MLF. The preservation of normal convergence is a critical finding because it confirms that the left oculomotor nucleus and left medial rectus muscle are functional, solidifying the MLF as the site of pathology. In a young woman with a history of separated neurological deficits in time and space, this presentation is highly suspicious for multiple sclerosis.

Step-by-Step Solution

1
Analyze the specific eye movement deficit during rightward gaze.
The left eye fails to adduct (move medially), and the right eye exhibits horizontal nystagmus upon abduction.
Recognizing this specific dissociation of eye movements is essential for identifying an internuclear ophthalmoplegia (INO).
2
Evaluate the convergence response.
Both eyes converge symmetrically upon viewing a near target.
Preserved convergence confirms that the medial rectus muscles and the oculomotor nuclei (CN III) are fully functional, localizing the lesion specifically to the pathway connecting the abducens and oculomotor nuclei.
3
Localize the side of the lesion based on the adduction deficit.
The adduction deficit occurs in the left eye.
In an INO, the lesion is located in the medial longitudinal fasciculus (MLF) ipsilateral to the eye that fails to adduct. Therefore, the left MLF is the site of the abnormality.

Key Concept

An internuclear ophthalmoplegia (INO) is characterized by an ipsilateral adduction deficit and contralateral abducting nystagmus with preserved convergence, indicating a lesion in the medial longitudinal fasciculus (MLF).
Rate this question

Topics

Physical Examination Findings

More questions from this topic