Fourth Nerve Pathway

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    • Cranial nerve that supplies the superior oblique muscle, which mediates ocular infraduction and intorsion
    • Five anatomic segments: nuclear, fascicular, subarachnoid, cavernous, and orbital
    • Nucleus lies near the midline in the caudal midbrain, ventral to the inferior colliculus
    • Fascicular segment travels dorsally around the Sylvian aqueduct, crossing to the opposite side in the anterior medullary velum caudal to the inferior colliculus
    • Tip: the fourth nerve is unique among cranial nerves in having its fascicles exit dorsally and crossing to innervate a structure on the opposite side of the body
    • Subarachnoid segment travels medial to the tentorium cerebelli, piercing it to enter the cavernous sinus
    • Cavernous segment
      lies in the lateral cavernous sinus wall inferior to the third nerve,
      exiting in the superior orbital fissure
    • Orbital segment
      enters the orbit outside the muscle cone and supplies the superior oblique muscle
    • Tip: intraconal anesthetic injections for ocular surgery often spare intorsion because the fourth nerve lies outside the muscle cone
    • Superior oblique muscle passes through a sleeve of dura on the anterior medial orbital wall called the trochlea, where its tendon bends more than 90 degrees to insert on the top of the globe
    • Nuclear lesion
      • Tip: lesions of the fourth nerve nucleus occur so rarely that you can forget about them!
    • Fascicular lesion
      • Produces a fourth nerve palsy in the eye opposite to side of the damaged fascicle
        • Misalignment pattern obeys the 3 step test
        • Excyclodeviation on the Double Maddox Rod test
      • Tip: in all forms of acquired (non-decompensated) fourth nerve palsy, the hypertropia is always greatest in downgaze
      • May be accompanied by other clinical manifestations of dorsal midbrain dysfunction
      • Common causes: head trauma, thalamic and dorsal midbrain lesions (demyelination, stroke, tumor,
        aqueductal stenosis, ventricular shunt failure)
      • Tip: head trauma often causes bilateral fourth nerve palsies because both nerves are damaged when the dorsal midbrain is battered by the rigid tentorium cerebelli
      • Tip: bilateral fourth nerve palsy of whatever cause can be distinguished from unilateral fourth nerve palsy by these features:
        • Right hypertropia in left gaze, left hypertropia in right gaze
        • V-pattern esotropia
        • Double Maddox Rod testing usually reveals more than 10 degrees of excyclodeviation
    • Subarachnoid lesion
      • Produces an ipsilateral fourth nerve palsy with a misalignment pattern similar to that of a fascicular lesion, but often in isolation
      • Common causes: head trauma, neurosurgery, ischemia
      • Uncommon causes: tentorium cerebelli region masses
        and hemorrhage, meningitis, neuritis, schwannoma
    • Cavernous lesion
      • Produces an ipsilateral fourth nerve palsy with a misalignment pattern similar to that of fascicular lesion, but always in combination with dysfunction of other nerves in the cavernous sinus
      • Tip: the fourth nerve is the most resistant of the ocular motor cranial nerves to cavernous sinus lesions, so do not expect an isolated fourth nerve palsy from a lesion here
      • Common causes: tumors, aneurysms, inflammations
    • Orbital lesion
      • An aging superior oblique muscle often becomes weak,
        displaying a pattern of misalignment that differs from acquired fourth nerve palsy in that the hypertropia is never worse in downgaze
      • Superior oblique trochlea
        may be injured by trauma, including surgery, causing a superior oblique palsy that resembles that produced by other acquired lesions
      • Superior oblique tendon sheath may become inflamed,
        limiting movement of its tendon, and producing ipsilaterally reduced supraduction-in-adduction, a phenomenon known as Brown syndrome
    • Try this flowchart approach

    Ocular Motor Cranial Nerve Pathways

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