Trochlear nerve palsy, also known as 4th nerve palsy, causes a condition called diplopia. Diplopia is more commonly known as double vision, and is caused by the eye’s inability to mesh muscle movement to create a single, unified image. It might not be diagnosed until it becomes more severe because patients tend to compensate for the diplopia by tilting their head.
For instance, if the problem is affecting the right eye, the patient may tilt their head to the left. If it is affecting the left eye, they may tilt their head to the right. This is one of the clues that can help eye doctors diagnose the problem even if double vision isn’t severe. Read below to learn more about this less well-known-eye condition, what causes it and how to treat it.
What Is 4th Nerve Palsy ?
4th nerve palsy is also called superior oblique palsy because it makes it hard for the superior oblique muscle to maintain a vertical gaze. The superior oblique muscle is controlled by the 4th nerve, otherwise known as the trochlear nerve. There are ten known cranial nerves, with the trochlear nerve being the longest one. That makes this nerve the most susceptible to damage from trauma or illness.
When this nerve is damaged and unable to function properly, the eyes become unable to align their separate images. The result is double vision that can either occur with two images next to each other, or one image on top of the other. Rarely, the double vision is torsional, which creates a twisted or rotated image. This most often occurs with acquired cases of 4th nerve palsy, not congenital.
Symptoms of 4th Nerve Palsy
The first and most major symptom of 4th nerve palsy is double vision. It will disappear when one eye is covered.
If you are observing the symptoms of 4th nerve palsy in someone else, you will see:
- Noticeable tilt to the head when gazing at an object or trying to read.
- While looking straight ahead, the eye that is affected may look up.
- Patients may think that objects are tilted, rather than straight up or down.
- Trouble reading or observing things up close.
Sometimes fourth nerve palsy affects both eyes, but more often, the symptoms are bilateral – meaning that they only affect one side.
The Causes of 4th Nerve Palsy
When one develops palsy in the nerve, they are no longer able to efficiently rotate and move the eye downward. Quite often, the doctor cannot figure out what the cause of the 4th nerve palsy is. Other times, the causes for 4th nerve palsy can be either congenital or acquired. If 4th nerve palsy is congenital, meaning present from birth, there will often be other apparent problems with the head, such as a misshapen skull.
Sometimes, it’s caused by a head injury. Many times, this head injury is from a car accident, motorcycle accident or sports injury. Even a mild case of whiplash can cause enough damage to create a problem with this long, sensitive nerve. The most common other causes for injury to this nerve are:
- High blood pressure.
- Circulation malfunction.
- Aneurysm in the skull.
- Tumor in skull.
- Multiple sclerosis.
If limited eye movement in one or both eyes is noticed, the physician will likely recommend a CT scan or MRI to check for these problems or others in hopes that it can be surgically corrected. Surgery is usually not recommended right away, however, as sometimes the palsy can spontaneously correct itself. Usually, the physician will wait for about six months while treating the cause, if possible. During these months of watchful waiting, the doctor will have the patient wear specially prescribed glasses or an eye patch.
4th Nerve Palsy Treatment
If a specific cause for 4th nerve palsy is found, then the physicians will treat the cause and hopefully completely cure the disorder. For instance, if an aneurysm is found, the doctors will go in to do brain surgery, hopefully repairing the damaged blood vessel. If a tumor is found, it will be removed. However, if the cause is multiple sclerosis, then treating the symptoms will be the only option, since there is no cure for multiple sclerosis.
If the cause can’t be found, the treatment will involve more conservative measures, such as:
- Special glasses – glasses with a prism can help to readjust the focus of light coming into the eye, therefore helping to compensate for the double vision.
- Eye exercises – eye exercises help to strengthen the weak muscle over time so that it can focus properly.
- Surgical therapy – sometimes surgeons can correct the muscle weakness with myectomy or recession.
Surgery is not without risks, however. If a doctor over-corrects for the weakness, it could make the vision problem even worse. This is why the doctor will usually wait about half a year before recommending this option, as stated above. Sometimes, the eye surgeon will use an adjustable suture to reduce this risk, so that the correction can be readjusted if accidentally over-corrected or under-corrected.
Surgery is the last resort. Before doing this, the doctor may recommend occlusion, or an eye patch, to eliminate the double vision if prism glasses cannot completely correct the problem. This is not an ideal solution because it prevents depth perception, but it is sometimes necessary. Because 4th nerve palsy often clears up on its own over time, modest treatment options are almost always recommended before surgery. However, in certain congenital cases where it is clear that the problem will not self correct, surgery will likely be the first choice of action.
Most cases of acquired 4th nerve palsy come on suddenly. Therefore, it’s important to seek medical advice from a qualified ophthalmologist if you suddenly notice episodes of double vision or if you have to suddenly start tilting you head in order to see a clear image. The earlier it’s treated, the better, as you may be able to reduce the severity of the attack.