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Zorluk: OrtaPhysical Examination Findings

A 62-year-old man presents to the clinic with sudden-onset double vision that began this morning. He reports that the double vision resolves when he covers his right eye. Medical history is significant for type 2 diabetes mellitus for 15 years, hypertension, and hyperlipidemia. His medications include metformin, lisinopril, and atorvastatin. Vital signs are within normal limits. On physical examination, the right eye is deviated downward and outward at rest, and there is significant right-sided ptosis. The left eye has normal extraocular movements. Pupillary examination reveals that the right pupil is 3 mm and the left pupil is 3 mm; both react briskly to direct and consensual light. Visual acuity is 20/25 bilaterally. Based on the physical examination findings, which of the following is the most likely underlying mechanism for this patient's presentation?

  1. Ischemia of the central somatic motor nerve fibersCevap
  2. B
    Aneurysmal compression of the superficial parasympathetic nerve fibers
  3. C
    Demyelination of the medial longitudinal fasciculus
  4. D
    Impaired acetylcholine transmission at the neuromuscular junction
  5. E
    Microvascular infarction of the abducens nerve

Cevap

The most likely mechanism is ischemia of the central somatic motor nerve fibers.
The correct answer accurately identifies that a 'pupil-sparing' CN III palsy is caused by central ischemia of the somatic motor fibers. The somatic motor fibers are located centrally within the nerve and are supplied by the vasa nervorum, making them highly susceptible to microvascular ischemia associated with diabetes mellitus and hypertension. The parasympathetic fibers, which control pupillary constriction, run superficially and are spared in this ischemic process.

Adım Adım Çözüm

1
Analyze the physical examination findings of the extraocular movements and eyelids.
The right eye is 'down and out' with significant ptosis, indicating a deficit of the oculomotor nerve (CN III).
CN III controls the levator palpebrae superioris (preventing ptosis) and all extraocular muscles except the superior oblique (CN IV) and lateral rectus (CN VI). Unopposed action of CN IV and CN VI pulls the eye downward and outward.
2
Analyze the pupillary physical examination findings.
Both pupils are equal (3 mm) and reactive to direct and consensual light.
This establishes that the parasympathetic pupillomotor fibers of CN III are functionally intact, making this a 'pupil-sparing' CN III palsy.
3
Correlate the physical examination with nerve anatomy and pathophysiology.
The somatic motor fibers are located centrally within the nerve (supplied by vasa nervorum), while parasympathetic fibers run on the superficial periphery.
Microvascular ischemia (due to diabetes) affects the central core, sparing the peripheral parasympathetic fibers. Conversely, external compression (e.g., aneurysm) affects the superficial parasympathetic fibers first, leading to pupillary dilation.

Anahtar Kavram

In a patient presenting with an oculomotor nerve (CN III) palsy, the physical examination finding of an intact, reactive pupil strongly points to a central microvascular ischemic etiology (e.g., due to diabetes), whereas a dilated, non-reactive pupil suggests external compression (e.g., posterior communicating artery aneurysm).
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Physical Examination Findings

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