Dear Doctors: I had an episode where it felt like my left eye went out of alignment and I couldn’t focus. A brain scan was clear, but the doctor said it’s cranial nerve palsy. He says it’s from a concussion I had last spring. I’m fine again but really worried. Will it happen again? Is there a treatment?
Dear Reader: You have been diagnosed with a condition known as cranial nerve IV palsy. It causes weakness or paralysis of the nerve that animates the muscle responsible for moving the eye downward and inward toward the nose. The Roman numeral in the name refers to the fourth cranial nerve, also known as the trochlear nerve. The word "palsy" indicates limited or impaired movement. The medical term for this condition is trochlear nerve palsy.
To understand what is going on, we should begin with a bit of anatomy. The human body has 12 cranial nerves, each with a distinct function. The third, fourth and sixth cranial nerves control the six muscles involved in eye movement. For the fourth cranial nerve, that’s the superior oblique muscle. In addition to moving the eye downward and inward toward the nose, the superior oblique muscle plays a role in moving the eye outward. Damage to the fourth cranial nerve can interfere with the impulses it carries to the superior oblique muscle. This can lead to significant vision impairment, including the symptoms you have described.
Trochlear nerve palsy can be congenital, which means it is present at birth. It can also be acquired, which means it occurs due to injury. Although the trochlear nerve is the shortest of the 12 cranial nerves, it takes the longest path through the skull. This leaves it uniquely vulnerable to injury, particularly when there is head trauma. That is why your doctor has drawn a connection between a concussion you suffered recently and the onset of this condition.
Symptoms include the sensation of misalignment and the inability to focus as you described. Many people experience double vision, which can be worse at night. Additional symptoms include dizziness when looking down, a need to tilt the head to keep things in focus and crossed eyes. Diagnosis is based on symptoms and a vision test, and treatment depends on the cause of the condition.
It is possible that you will have another episode. However, the good news is that when trochlear nerve palsy results from an injury, it often resolves on its own, typically in six to eight months. During the period of watchful waiting, you might be prescribed specially crafted eyewear, known as prism glasses. They have wedge-shaped lenses that bend the light before it reaches the eye. This improves vision by adjusting the misalignment caused by the malfunctioning muscles. Prism glasses are calibrated to the specific needs of each person through a vision test. If they don’t help, an eye patch may be needed. After eight months, if the condition has not improved, surgery to realign the eyes becomes an option.
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