Cranial Nerve 4

Aka: Cranial Nerve 4, Cranial Nerve IV, Trochlear Nerve, CN 4, Trochlear Nucleus, Cranial Nerve 4 Palsy, Trochlear Nerve Palsy, CN 4 Paralysis

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II. Physiology

  1. Innervates extraocular Muscle: Superior Oblique Muscle
  2. Superior Oblique Muscle passes via pulley (trochlea)
  3. Long thin nerve that is susceptible to compression (Cranial Nerve 4 and 6 are similar in this way)
    1. Paralysis (unilateral or bilateral) may occur even with generalized Increased Intracranial Pressure
    2. Contrast with the Cranial Nerve 3
      1. Thick cable-like nerve requiring significant compression for paralysis
      2. CN 3 parasympathetic fibers are susceptible to injury as they lie on the outside of CN 3

III. Anatomy

  1. Nucleus
    1. Trochlear Nucleus is at the floor of Cerebral Aqueduct in the Midbrain
    2. Unlike all other Cranial Nerves:
      1. CN 4 crosses the midline, innervating the contralateral side (all other Cranial Nerves are ipsilateral)
      2. CN 4 exits posteriorly (all other Cranial Nerves exit anterior to the Brain Stem)
  2. Course
    1. neuroTrochlearCN4.png
    2. Tracks around superior cerebellar, cerebral peduncles
    3. Enters tentorium cerebelli at posterior clinoid process
    4. Proceeds through lateral wall of Cavernous Sinus
    5. Enters orbit via superior orbital fissure

IV. Exam

  1. Extraocular Movement
    1. See Extraocular Movement
    2. eye_eom.png
  2. Normal function of the Superior Oblique Muscle
    1. Eye movement inferior laterally
  3. Paralysis of the Superior Oblique Muscle
    1. eye_eom.png
    2. Head Tilt toward the contralateral Shoulder
      1. Diplopia improves when patient laterally bends their neck to contralateral side (contralateral ear to Shoulder)
      2. Diplopia worsens when patient laterally bends their neck to ipsilateral side (ipsilateral ear to Shoulder)
    3. Affected eye slightly elevated in primary position (looking straight ahead)
      1. Can only depress affected eye slightly in the primary position
      2. When patient looks laterally or medially, eye returns to normal position and Diplopia improves

V. Causes: Cranial Nerve 4 Palsy

  1. Congenital Palsy
    1. Head position changes may occur to compensate for Strabismus and Diplopia
  2. Trauma
    1. Longest nerve course with higher risk of injury
    2. Increased Intracranial Pressure may also cause palsy (as nerve is relatively thin)
  3. MIcrovascular (small vessel disease)
    1. More common in older patients with Cardiovascular Risk Factors (e.g. Diabetes Mellitus)

VI. References

  1. Gilman (1989) Manter and Gatz Essentials of Neuroanatomy and Neurophysiology, Davis, p. 87-113
  2. Goldberg (2014) Clinical Neuroanatomy, p. 24-39
  3. Netter (1997) Atlas Human Anatomy, ICON Learning, p. 110-129

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