Fourth nerve palsy refers to a condition that results from a weakness of the superior oblique muscle leading to eye misalignment. The disorder is caused by a combination of horizontal, vertical, and torsional (twisted or rotated) eye misalignment, with the most noticeable being vertical. Palsy means the muscle is fragile.
The disorder can affect one or both eyes. It can also cause diplopia (double vision) where the brain perceives two images from two separate directions. Double vision may occur horizontally or vertically and rarely torsional. Acquired cases of superior oblique palsy are frequently torsional. There is no double vision in congenital superior oblique palsy. A child with diplopia will often tilt the head to compensate for the double vision or better eye alignment.
The fourth cranial nerve, also called the trochlear nerve, is one of three nerves (the others being third and sixth) that helps in eye movement. It controls the eyelids' position and the pupils' size. The fourth nerve also controls the activities of the superior oblique muscle. The muscle extends from the eye socket's back to the top of the eye. This muscle passes through the trochlea, a tissue loop close to the nose. Its function is to turn the eyes inward and downward. The fourth nerve is the only one that originates from the back of the brain. Damage to this nerve through injury or disease can cause paralysis to the superior oblique muscle.
Also Known As
- Trochlear nerve palsy
- Superior oblique palsy
Causes & Risk Factors
Fourth nerve palsy can be present at birth (congenital) or acquired, with congenital being the most common. The child may have other health problems that accompany the congenital type. Acquired superior oblique palsy is commonly caused by trauma to the head, even if the trauma is relatively minor. For instance, if a child is injured in a motor vehicle accident and gets a concussion, that is sufficient to cause the condition. Rarely do aneurysm, tumor, or stroke cause superior oblique palsy.
In some cases, the cause is unknown (idiopathic). Idiopathic fourth nerve palsy goes away on its own, unlike the one caused by trauma. Other causes of fourth cranial nerve palsy include infection, increased pressure in the skull, an aneurysm (bulging artery), and diabetes which causes poor blood flow.
Signs & Symptoms
Signs and symptoms may include:
- An iris that is elevated above the other one
- Tilting the head as a compensatory mechanism to help with the problem of vision
- Pain above the eyebrow if the disorder has been caused by diabetes or is of unknown cause
- Lack of double vision for those who have had the disorder for a while. This is because the brain has learned to ignore images from the affected eye
- Double vision which occurs with both eyes open where one image seems to appear above the other. Or the child may see two images, but one appears a little bit more turned than the other. A little separation between the images will cause the child to see an overall blurry image instead of double. If the child shuts one eye, they will see one image
A diagnosis can be made through the following:
- Medical history, especially of recent symptoms. The doctor also wants to know about a child's past health
- Medical examination where the doctor checks the cranial nerves. This is done through observing the eyes’ position when at rest and let the patient follow an object with their eyes. The pupils are also examined when the doctor checks for their reaction to light. The doctor will check eye pressure and examine the back of the eye. Old photos of the patient can assist the doctor in detecting when the problem began
- Tests to differentiate fourth cranial nerve palsy from other diseases that present similar symptoms, such as Grave disease, myasthenia gravis, migraines, Wernicke syndrome, etc. These tests include blood tests to check the thyroid hormone level and autoimmune conditions
- An MRI or CT scan can be used to examine the cranial nerves and brain. Ultrasound helps assess the eye's muscles
- A lumbar puncture (spinal tap) helps find the cause of increased pressure in the skull
Treatment aims to improve vision by getting rid of double vision. It also depends on the cause. The idiopathic type tends to resolve on its own. Fourth nerve palsy caused by injury may go away with time, or the underlying cause may be treated first before it can disappear. The ophthalmologist waits for at least six months for the condition to disappear after treating the cause. Meanwhile, prisms help manage double vision during this waiting period.
Prisms help with vision by merging both images into one, but without strengthening the muscles. One eye is patched to eliminate double vision where prisms have failed. Over-the-counter medications can help to manage the pain.
If prisms and patching do not help in the six-month waiting period, the child can undergo surgery. Surgery is also the treatment of choice for congenital fourth cranial nerve palsy. It can be performed on one eye or both, depending on the severity of eye misalignment and other factors. With surgery, double vision is minimized, the compensatory head tilt is corrected, and the eye's upward drift is reduced.
Prognosis & Long-Term Outlook
If the fourth nerve palsy does not go away on its own, it can affect how the eyes work together, i.e., when the affected eye moves, the unaffected one moves as well. The condition can also affect the child's facial appearance (asymmetry), particularly when the head keeps tilting. Because the muscles on either side of the face do not develop together, the child’s two sides begin to look different with time.
Prevention & Follow Up
Follow-up will depend on the type of treatment. For instance, those undergoing surgery will be provided with a schedule.