Anatomy
Summary
The oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves are pure motor nerves that innervate the extra-ocular muscles. These three cranial nerves enter the orbit by passing through the cavernous sinus and exiting through the superior orbital fissure. Palsies of these nerves result in distinct conditions affecting eye movement and are typically characterized by specific symptoms. Damage to the oculomotor nerve can paralyze the 4 extra-ocular muscles innervated by this nerve, resulting in the affected eye deviating downward and outward. CN III palsy can also present with opthalmoplegia and ptosis due to paralysis of the levator palpebrae superioris muscle of the upper eyelid. Additionally, damage to parasympathetic fibers in CN III leads to pupillary dilation that doesn't respond to light or accomodation. Trochlear nerve palsy is marked by vertical diplopia, where the affected eye deviates upward and rotates outward, due to paralysis of the superior oblique muscle. Here, vision disturbances are most apparent when the patient is looking down and inwardÑsuch as when reading a book or walking down stairs. Abducens nerve palsy manifests as a medially deviating eye, a condition known as esotropia,
On the other hand, cavernous sinus syndrome is a complex presentation that can cause multiple deficits in cranial nerves that travel through the cavernous sinus. Symptoms include ophthalmoplegia, involving cranial nerves III, IV, and VI, which results in various eye movement abnormalities. It also often includes Horner's syndrome due to the disruption of postganglionic sympathetic pupillary fibers. Additionally, patients experience loss of corneal sensation (V1) and midface sensation (V2). The syndrome thus involves not only motor deficits but also sensory deficits, making it a multifaceted condition that requires comprehensive evaluation and management.
Lesson Outline
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FAQs
The oculomotor nerve (III) controls most of the eye's movements along with accommodation, pupillary constriction, and eyelid opening. The trochlear nerve (IV) innervates a single muscle, the superior oblique muscle of the eye, which helps in movement of the eye downward and outward. The abducens nerve (VI) innervates the lateral rectus muscle, which is responsible for abducting the eye.
Trochlear nerve palsy can lead to a condition known as vertical diplopia where the person sees two images stacked on top of each other. It also often leads to difficulty in eye movement, particularly moving the eye inward and downward. People affected with trochlear nerve palsy might adopt a head tilt away from the affected side to compensate for the condition.
Damage to cranial nerve VI, also known as the abducens nerve, typically results in difficulty with outward eye movement of the affected eye. Patients may present with diplopiadue to paralysis of the lateral rectus muscle, which is responsible for abducting the eye. When this nerve is impaired, the eye may deviate medially, or inward, due to unopposed action of the medial rectus muscle.
The cavernous sinus, a large vein located at the base of the brain, houses cranial nerves III, IV, and VI, along with the V1 and V2 branches of the trigeminal nerve. Any pathology affecting the cavernous sinus, such as a tumor or thrombosis, could impact these cranial nerves and result in associated symptoms like eye movement disorders, double vision, loss of sensation, and pain around the eye and face.
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