CN4 anatomy
- The fourth nerve originates in the caudal midbrain.
- 2 unique anatomic features of the fourth nerve are
- Fibres exit dorsally
- Fibres that they decussate, via the superior medullary velum, to the opposite side.
- The fibers course ventrally along the edge of the tentorium, to enter the cavernous sinus
- Within the cavernous sinus, the fourth nerve courses ventral to the third nerve and dorsal to the ophthalmic division of the fifth nerve.
- After exiting the cavernous sinus, the nerve passes through the superior orbital fissure to enter the orbit, where it innervates the superior oblique muscle
- The frontal, trochlear, and lacrimal nerves can be seen through the periorbita.
- The trochlear nerve crosses above the orbital apex to reach the superior oblique muscle.
Function
- The superior oblique muscle depresses and intorts the eye.
- Superior oblique depresses the eye when the eye is adducted
Assessment
- Anatomy textbooks deal with the function of the muscle.
- Acting alone, superior oblique depresses and abducts the eye (ie. moves it down and out).
- Clinical textbooks are concerned with dysfunction.
- A neurological examination includes assessment of the trochlear nerve (CN IV) which innervates the superior oblique muscle. The test for CN IV function is for the patient to depress and adduct the eye (ie. to look down and in).
- This movement is impaired in the presence of a trochlear nerve palsy.
- If the muscle moves the eye down and out, why do we test for a CN IV palsy by instructing the patient to look down and in?
- The superior oblique muscle is inserted in front of the coronal equator of the eye, and the line of pull passes medial to the axis of rotation of the eye. As a result of this line of insertion, the inferior rectus muscle is primarily responsible for depression when the eye’s gaze is directed forward but superior oblique is the main depressor of the eye that is held in adduction. Therefore we test the CN IV by moving the eye ‘down and in’. However, the superior oblique muscle does turn the eye ‘down and out’ when acting independently. In clinical practice this is seldom seen as it is unusual to have ocular nerve palsies that spare the trochlear nerve.
CN4 lesions
Presentation
- Vertical Diplopia if recently acquired or decompensating
- Head Tilt to unaffected side
- Face turn to unaffected side
- Chin depression
- + BIELSCHOWSKY HEAD TILT TEST
- Performed at 3 metres
- Head is tilted 30 Degrees to the affected side and if the hypertropia increases = Superior Oblique palsy
- Head tilt to the unaffected side should show very little difference in the deviation = Contralateral SR under activity
- Positive result should be minimum 5deg difference from tilting right to left
Causes
- Congenital
- Bilateral
- Acquired (Unilateral)
- The majority of these cases are caused by microvascular problems
- Rule of 10-20-30-40 is used:
- 10% neoplasms/aneurysms
- > 20% ischemic
- > 30% idiopathic
- > traumatic
Location
- Nuclear and fascicular
- Isolated nuclear or fascicular lesions of the fourth nerve are rare.
- Subarachnoid space
- The most common fourth nerve lesion is a contusion of the nerve in its subarachnoid course against the edge of the tentorium.
- Bilateral lesions are not rare.
- In a unilateral palsy
- The patient complains of vertical diplopia that is made worse when the gaze is directed down and to the opposite side (such as is required to walk down stairs).
- These patients tilt their heads to the side opposite the paretic muscle to decrease the degree of diplopia.
- Cavernous sinus and superior orbital fissure
- Isolated lesions of the fourth nerve in the in the cavernous sinus or the superior orbital fissure are uncommon.
- Orbital vault
- Vertical diplopia due to a lesion in the orbit is more often caused by direct damage to the superior oblique muscle or trochlea.
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