What is Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)?
The abducens nerve, also known as the sixth cranial nerve, is what allows your eyes to move to the sides. If it’s not working right, this can happen anywhere from its origin point in the brain to the muscle at the side of the eye, causing something we call sixth nerve palsy. To figure out why someone has this palsy, we need to understand the nerve’s path through the brain.
This nerve starts in an area of the brain called the pons, near the seventh cranial nerve. From there, it leaves the brainstem, enters a space known as the subarachnoid space, and follows a pathway along the skull. It continues its path to a part of the skull near the ear called the petrous apex, where it enters an area known as the cavernous sinus.
Inside this sinus, the abducens nerve is positioned close to the big artery that supplies blood to the brain and is next to the trigeminal nerve. It then enters the eye socket through a gap called the superior orbital fissure. Once it’s reached the eye socket, it connects to the muscle that controls eye movement to the sides.
Sixth cranial nerve palsy is the most frequent one-sided palsy in adults and the second most common in children. If a person shows signs of having this palsy, their age is a crucial factor in figuring out why it’s happened and whether more tests, like brain scans, are needed.
In adults, the things that can cause abducens nerve palsy can be divided into two categories: vasculopathic (related to blood vessels) and non-vasculopathic. Older people are more likely to have vasculopathic causes, like diabetes. Non-vasculopathic causes can occur in both adults and children and can be due to various things like injury, inflammation, or pressure.
For children, the most frequent causes of sixth nerve palsy include high pressure in the brain, blood vessel abnormalities, and cancer-related disorders. If a child has this condition, and we’ve ruled out trauma, unknown causes, and viruses, then brain scans are needed to further investigate the reason.
What Causes Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)?
Understanding the pathway of the abducens nerve, which controls some eye movements, is crucial to pinpoint the reasons for its impairment, known as abducens nerve palsy. Tumors and trauma can interrupt the nerve anywhere along its path and result in palsy.
Nerve palsy can stem from problems within the pons, an area of the brain where the abducens nerve starts. These problems could be a stroke, metabolic diseases like Wernicke disease, or loss of protective nerve covering. These causes might also lead to facial nerve palsy because of the close proximity of the abducens and facial nerves.
Other causes of abducens nerve palsy can arise when the nerve enters a space called the subarachnoid space between the brain and skull. The primary cause here is high pressure within the skull. Other symptoms, like a headache, sickness, and swelling of the optic disc, may be present. Several reasons contribute to abducens nerve palsy in these situations, including blood vessel enlargement, cancerous inflammation of the membranes covering the brain and spinal cord, procedural injuries, inflammatory lesions, and various infections.
Special causes of abducens nerve palsy are associated with its course over a specific bone near the ear called the petrous apex. These include complications from an ear infection, sinus blood clot, and base of the skull fracture.
When the abducens nerve goes through a cavernous sinus, a blood-filled space behind the eyes, the most common cause of palsy in this region is stretching or compression of the nerve. These involve clotting in the cavernous sinus, a condition where arteries and veins in the brain become connected abnormally, and diseases of the internal carotid artery, such as aneurysms or dissections.
In the eye socket, various conditions like tumors, inflammation, infection, or trauma can cause abducens nerve palsy.
Tables provided offer a detailed list of causes for abducens nerve palsy in both kids and adults, for example, most common causes include trauma, tumors, nervous system injuries or pathologies, infections, vascular disorders and other conditions related to specific procedures or injuries.
Several syndromes linked with this nerve palsy are listed as well such as “Brainstem syndromes”, “Elevated intracranial pressure”, “Petrous apex syndrome”, “Cavernous sinus syndrome”, and “Orbital syndrome”. These syndromes include various symptoms and difficulties related to the flow of blood, pressure within the skull and brain, tumours and collections of symptoms resulting from diseases in the corresponding location of the brain or skull.
Lastly, it’s important to mention that while some cases of abducens nerve palsy can be due to general conditions like diabetes or hypertension, or to recent viral illnesses, it can occasionally happen without a clear reason and can also recur. Determining the specific type of palsy can thus be challenging when medical scans show mixed results.
Risk Factors and Frequency for Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
The sixth cranial nerve, involved in eye movement, is frequently affected in adults and is the second most common in children after the fourth cranial nerve, with an occurrence of 2.5 cases per 100,000. Poorly managed diabetes is a major risk factor for developing sixth cranial nerve palsy.
In a Korean study, the overall occurrence rate of sixth cranial nerve palsy was reported as 4.66 cases per 100,000 people per year. The rate increased with age, particularly surging at 60 and peaking between the ages of 70 and 74.
- A variety of causes can lead to sixth cranial nerve palsy, with varying incidence rates:
- Trauma-related cases range from 3% to 30%.
- Up to 6% can be due to aneurysms.
- Demyelination or miscellaneous causes account for 10% to 30%.
- Idiopathic (unknown cause) cases make up 8% to 30%.
- Ischemic causes can be as high as 36%.
A 15-year US study involving 137 cases, showed an adjusted occurrence rate for the sixth nerve palsy to be 11.3 per 100,000, with the highest occurrence in people in their seventies. Among these cases, there were 4 instances of bilateral sixth nerve palsy and 16 cases where multiple cranial nerves were affected.
Signs and Symptoms of Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
Abducens nerve palsy causes a specific type of double vision where objects appear side by side, often due to a weakness in the muscle responsible for moving the eye outwards. This can cause some people to perform constant head-turning movements to align their vision and reduce the double sight. The double vision becomes more noticeable when looking into the distance or when their eyes move to the side.
Other symptoms might include vision loss, headaches, vomiting, trauma, hearing loss, recent lumbar puncture, or a recent viral illness. Esotropia (crossed eyes), turning of the head, facial pain, and numbness can also occur. Some people might have other underlying conditions like giant cell arteritis, shooting pains in the temple, and facial pain. An eye examination is essential for all those presenting these symptoms.
Patients suspected with abducens nerve palsy will go through a comprehensive evaluation that includes:
- Evaluating visual acuity
- Examining binocular single vision
- Checking depth (stereopsis)
- Assessing ocular movement
- Evaluating squint
- Taking near and distance measurements
- Checking all positions of gaze
- Assessing fusional amplitude
- Manifest and cycloplegic refraction
- Examination of the front and rear parts of the eyes
Tests during side gaze and assessing movement and versions of the eye may be particularly useful in diagnosing the degree of nerve palsy. For children, the most common causes of sixth nerve palsy are tumors and trauma, so a thorough examination is necessary to identify these conditions. It is crucial to evaluate each eye separately to avoid any misleading results.
In patients with abducens nerve palsy, double vision is the most common symptom. This double vision typically affects distant vision more than near vision, and is more pronounced when looking in the direction of the affected muscle. The specific pattern of this double vision helps in diagnosing the condition.
The other symptoms can vary based on what is causing the sixth nerve palsy. Symptoms of increased pressure in the brain can include headaches, ocular pain, nausea, vomiting, and ringing in the ears. If the nerve palsy is due to a leakage of cerebrospinal fluid, it can lead to symptoms like headache, which can resemble the symptoms of increased intracranial pressure. If the cause is neurological, it can present with other neurological signs and symptoms. Additionally, if the cause affects the brainstem or multiple nerves, it can lead to symptoms like weakness on one side of the face, sensory issues, and weakness on the opposite side of the body.
Testing for Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
When trying to identify the cause of the sixth cranial nerve palsy, it’s crucial to take into account the patient’s specific circumstances, and a variety of tests may be used.
If children are affected, doctors will need to conduct detailed examinations because there’s a greater risk of the problem being caused by cancerous growths. If there’s been a physical injury leading to nerve damage, neuroimaging, such as brain scans, should be done promptly.
If it’s thought that the cause might be increased pressure within the skull, a lumbar puncture or spinal tap, can be performed. When doctors suspect that a lack of blood supply is behind the nerve palsy, an MRI scan is recommended because it’s particularly good at providing detailed images of the base of the brain.
MRI scans are also particularly beneficial for patients under 50 who don’t have pre-existing conditions affecting their blood vessels, especially if they have other symptoms like pain or neurological changes. Patients with a history of cancer, double vision or signs of increased brain pressure can also benefit from an MRI scan. If there’s no improvement after 3 to 4 months or if other nerves are affected, comprehensive medical, neurological and imaging checks are needed.
Various laboratory tests can also contribute to the diagnosis. A complete blood count can help identify any abnormalities. Meanwhile, a series of diabetes-related tests can help rule out this condition. Inflammation can be determined by an erythrocyte sedimentation test or a C-reactive protein test. For elderly patients, a platelet count can help evaluate if there’s a lack of blood platelets. If there’s a chance of having a condition that weakens the immune system, an acetylcholine receptor antibody test is done. If syphilis is suspected, rapid plasma reagin and fluorescent treponemal antibody-absorption tests can be used. If doctors think the patient may have Lyme disease, they can order a Lyme titer test. Various thyroid function tests can be done to check the levels of thyroid hormones.
Autoantibodies associated with numerous autoimmune conditions can be detected with an antinuclear antibody test, while the presence of underlying rheumatoid arthritis can be determined with a rheumatoid factor test.
Finally, it’s also necessary to check a patient’s blood pressure and carry out a comprehensive medical history, paying particular attention to a history of trauma, eye infections, transient vision loss, and fluctuating symptoms. You’ll also need to examine the other cranial nerves thoroughly to find out if any other neurological abnormalities could be causing the symptoms. Children should also have a detailed ear examination to rule out otitis media, a type of ear infection.
Treatment Options for Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
The best way to manage sixth nerve palsy, also known as abducens nerve palsy, largely depends on what’s causing it. At first, much of the focus should be on getting a patient’s other medical conditions under control. While for most people, this condition goes away on its own without any specialized treatment, children may need interventions.
We have quite a few treatment options for children with sixth nerve palsy. These involve alternate eye patching, prism therapy, eye muscle surgery, and botulism toxin injections. Alternate patching involves covering each eye every few hours. This prevents the weaker eye from getting too lazy. With prism therapy, a temporary prism is placed on the affected eye’s lens to align the eyes better. If there’s no progress with prism therapy, eye muscle surgery may be considered to realign the eyes. Botulinum toxin injections into an eye muscle can be used to prevent the muscle from stiffening and the eye from deviating towards the nose.
Using filters or patches can help to avoid seeing double. They also prevent the weaker eye from getting lazy in children. Furthermore, these filters or patches can reduce the risk of a muscle in the eye stiffening. A type of prism called a Fresnel prism can help patients maintain single vision when placed in an outward position.
In most minor cases of sixth nerve palsy, no action is required as the condition goes away on its own with time. Patients just need to be patient and wait for symptoms to disappear, which usually takes about 3 to 6 months. However, the selected treatment highly depends on what’s causing sixth nerve palsy.
For example, steroid medications may be prescribed if temporal arteritis is the cause. If intracranial pressure due to a condition like pseudotumor cerebri or cancer is causing the palsy, surgery or a lumbar puncture might be needed to reduce the pressure.
If sixth nerve palsy doesn’t go away on its own, the same approaches used in children may be considered. However, alternate eye patching doesn’t seem to work in adults.
Surgical treatment for this condition is kept as last resort, and only considered if the patient’s orthoptic status has been stable for at least 6 months. Before any surgery, a patient has to go through a forced duction test to help in planning the surgery. Surgical options, to fix the imbalance of eye muscle function, depend on the underlying cause of the condition and patient’s condition.
An operation called the “Resect and Recess” may be done if there is still some lateral rectus function left. This option involves shortening the eye muscle on one side and lengthening the muscle on the other side to fix the alignment. If the function is totally lost, however, procedures that help adjust the position of the tendons in the eye may be done.
What else can Abducens Nerve Palsy (Sixth Cranial Nerve Palsy) be?
When a medical professional is trying to diagnose a problem with the sixth cranial nerve, also known as the abducens nerve, they need to consider a range of other possible conditions that may cause similar symptoms. These include:
- Myasthenia gravis (a condition causing muscle weakness)
- Duane retraction syndrome (a type of eye movement disorder) in children
- Thyroid eye disease (a condition related to overactive or underactive thyroid)
- Syphilis (a sexually transmitted infection)
- Pseudotumor cerebri (a condition that mimics a brain tumor)
- Spasm of the near reflex (an eye disorder related to focusing)
- Fractures in the medial wall of the eye socket
- Lyme disease (a bacterial infection transmitted by ticks)
- Damage due to an injury or trauma
- Cancerous growths (neoplasms)
- Old blowout fracture
- Conditions like diabetes or high blood pressure
- Blood vessel conditions (like aneurysms or vasculopathy)
- Conditions like congenital esotropia, sphenoiditis, lateral rectus myositis, chronic suppurative otitis media, Miller-Fisher syndrome, neuromyotonia, etc.
In Duane retraction syndrome, the opening between the eyelids narrows as the affected eye moves inward. However, this doesn’t happen in a sixth nerve palsy which helps distinguish the two conditions.
People with thyroid eye disease may experience symptoms in one or both eyes, such as bulging eyes and inflamed conjunctiva.
Myasthenia gravis patients may experience visual problems, like fluctuating double vision, along with fatigue, shortness of breath, and hoarseness.
What to expect with Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
The outlook for those with abducens nerve palsy depends on what caused it. If a viral illness is behind it, people typically recover completely. However, if physical trauma caused the palsy, some leftover symptoms could still exist. The most notable improvements usually occur within the first six months. While most patients dealing with idiopathic sixth cranial nerve palsy – or the type of palsy appearing without a clear cause – recover fully, a small number may face permanent changes to their vision.
Possible Complications When Diagnosed with Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
Abducens nerve palsy is a medical condition that can have various complications, depending on the root cause. One of the key complications comes after surgical correction. This can result in too much (overcorrection) or too little (undercorrection) adjustment being made during the surgery. These issues can be managed after surgery by using special glasses called prisms to align the eyes properly.
The following are possible complications after surgical correction:
- Overcorrection
- Under correction
- Muscle slip
- Muscle loss
- Bleeding in the muscle
- Accidental muscle injury
- Anterior segment ischemia (decreased blood flow to the front part of the eye)
- Double vision
- Glaucoma caused by steroids
Recovery from Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
It’s very important for all patients who have had strabismus surgery to keep up with their regular check-ups. They should continue taking their post-surgery medicine in decreasing doses as directed. Patients need to understand the significance of using topical steroids and lubricants regularly and on time. They should also be aware of the possibility of experiencing double vision after surgery and know that this can be managed using special lenses called prisms.
Preventing Abducens Nerve Palsy (Sixth Cranial Nerve Palsy)
Patients should be made aware that their condition might improve on its own. Therefore, they might be recommended to wait and see if there’s any improvement for about 6 months. If no improvement can be noted during this time, then they may want to consider surgery. However, if a patient has another medical condition that is causing this problem, that needs to be dealt with first. So, the primary focus of any discussions with a patient is usually about addressing that underlying issue.
