Fourth nerve palsy (superior oblique palsy): symptoms, causes and treatment

A palsy is a condition in which you lose some or all movement, and usually have weakness, in a part of the body. The fourth cranial nerve is one of three nerves in the head that connect with the muscles that move the eyes from side to side and up or down. In fourth nerve palsy the nerve cannot send its signals as it normally would to the muscle that moves the eye downwards and outwards and rotates it slightly.

University Hospitals Sussex explains that this muscle is called the superior oblique muscle, so the condition is also called superior oblique palsy. Because the muscle is weak, the affected eye does not move downwards as it should and ends up out of line with the other eye.

The main symptom is double vision. According to the University Hospitals Sussex orthoptic leaflet, people with it usually notice one image higher than the other, and it can affect one eye or both. The double vision is more obvious when looking downwards, which can make close work that needs focusing on objects within arm's reach for a long time more difficult, and can make steps and stairs harder to manage.

People often try to tilt and hold their head in a way that reduces the double vision, which can make their posture look unusual. This can be one of the first things family members notice, particularly when the palsy has been present since birth.

The leaflet makes a distinction between the two forms. People with acquired fourth nerve palsy may suddenly get double vision. People with the congenital form may find that it comes on more gradually and comes and goes, and some are not diagnosed until much later in life because the brain can adapt to it, so they may have no symptoms for many years. Double vision may come on when illness, stress or another eye condition, such as the start of cataracts, makes it harder to control the weakness.

The NHS says double vision is not usually serious but is important to get checked, even if it comes and goes. It advises going to an optician or seeing a GP if you think you or your child might have double vision.

Some features make it more urgent. The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you have eye pain and double vision, or double vision that has started suddenly. It says to call 999 or go to A&E if you have a severe headache or an enlarged pupil with blurred or double vision, or double vision after a head injury.

The NHS also says that you must stop driving and tell the DVLA if you are diagnosed with double vision, and that you can drive again if a doctor tells the DVLA it is safe.

The hospital leaflet explains that congenital fourth nerve palsy happens as a baby is developing in the womb. It may be because the nerve is not working as it should or because there is a weakness in the muscle tendon, and the exact cause is not known.

Acquired fourth nerve palsy results from damage to the nerve caused by health conditions or injuries. The conditions listed are diabetes, high blood pressure, stroke, multiple sclerosis and blood circulation (vascular) problems such as blocked or damaged blood vessels, which is more usual in older people. Eye or brain tumours are described as a rare cause. Closed head injuries, such as those from a traffic accident or fall where the skull is not broken, can also damage the nerve.

An orthoptist checks eye movements and how far the eyes are out of position, which allows them to confirm fourth nerve palsy. They ask about your health now and in the past and may check medical records to find the most likely cause. The orthoptist may find signs that suggest you have probably had the palsy for a long time.

Further tests or scans may be arranged if the palsy developed recently with no obvious cause, or if it seems to be changing. The leaflet lists a blood pressure test, blood tests and a CT or MRI scan.

A simple home check described by the NHS is to cover one eye at a time. If the double vision goes away with one eye covered, it is probably affecting both eyes, which is what happens in a nerve palsy. If you still have double vision with the other eye covered, it is probably an eye problem such as dry eyes, astigmatism, cataracts or keratoconus affecting that eye alone.

The leaflet says nearly all fourth nerve palsies get better over time without the need for treatment. Prisms, which are special lenses that bend light rays, can be added to glasses to join the double vision while tests are done or while the palsy is recovering. If prisms are not right, covering one eye with a patch can make symptoms better.

If the palsy does not recover, or only partly recovers, you may need an operation or to wear glasses with prisms long-term. An orthoptist, or sometimes an ophthalmologist, will review it at appointments usually every 2 to 3 months and discuss which treatment may be right.

On the NHS, an optician or GP is usually the first step, with referral to a hospital eye service where an orthoptist and ophthalmologist work together. Scans, if needed, are arranged through the hospital.

Private ophthalmology and orthoptic clinics can offer faster appointments or a second opinion, and the assessments and treatments are the same in principle. The most important practical point is that new, sudden double vision should not wait for a routine appointment.