Sixth nerve palsy, also known as abducens nerve palsy, affects the sixth cranial nerve, which controls the eye’s ability to move outward. Damage to this nerve, which originates in the brainstem, causes weakness or paralysis of the lateral rectus muscle. This impairment results in difficulty moving the affected eye laterally, away from the nose. The lack of coordinated movement often leads to the immediate onset of double vision.
How the Sixth Nerve Controls Eye Movement
The sixth cranial nerve, or abducens nerve, is a purely motor nerve dedicated solely to the lateral rectus muscle. This muscle is positioned on the outer side of the eyeball and pulls the eye directly outward. When activated, the nerve causes the eye to abduct, or turn away from the midline.
The nerve begins deep within the brainstem, in the pons, and follows a long pathway before reaching the orbit. This extended course through the skull base makes it vulnerable to injury or compression. Its sole purpose is to ensure the eye can properly look to the side, working in coordination with other eye muscles to maintain aligned vision.
Recognizing the Signs of Palsy
The most noticeable symptom of sixth nerve palsy is the sudden appearance of horizontal double vision, known as diplopia. This occurs because the affected eye cannot fully turn outward, causing the visual axes to become misaligned. The patient sees two distinct images side-by-side, which often increases in severity when looking at distant objects.
When attempting to look toward the damaged side, the affected eye lags behind and often appears turned inward toward the nose, a misalignment called esotropia. The double vision disappears if the patient covers one eye, confirming the problem is binocular.
To avoid seeing double, many individuals adopt a compensatory head posture, instinctively turning their face toward the affected side. This head turn minimizes the required action of the weak nerve, helping to fuse the two images.
The palsy can affect one eye (unilateral) or both eyes (bilateral). While adults experience diplopia, younger children may unconsciously suppress the image from the affected eye to avoid double vision. This suppression can lead to amblyopia, a long-term reduction in visual acuity.
What Leads to Nerve Damage
The causes of sixth nerve palsy are diverse and often indicate a serious underlying medical issue. The most frequent cause in older adults is microvascular ischemia, which is damage to the small blood vessels supplying the nerve. This damage is strongly associated with systemic conditions like diabetes and uncontrolled high blood pressure. The lack of adequate blood flow causes the nerve fibers to malfunction, but these cases often resolve spontaneously as microcirculation recovers.
Physical trauma, such as a severe blow to the head or a skull fracture, is another common cause of nerve injury. Because the abducens nerve has the second-longest path of all the cranial nerves, it is susceptible to stretching or tearing as the brain shifts within the skull during an impact. Head injuries can cause direct damage or lead to swelling and pressure that compress the nerve along its trajectory.
Increased intracranial pressure (ICP) is a concerning cause, resulting from brain tumors, hydrocephalus, or bleeding within the skull. The nerve’s long course makes it especially vulnerable to being stretched or compressed against the skull base when pressure rises, leading to a “false-localizing” palsy that can occur even if the mass is far from the nerve itself. Tumors located at the base of the skull or in the brainstem can also directly compress the nerve, causing a slowly progressive palsy.
Inflammatory and infectious conditions also cause nerve damage through swelling and demyelination. Multiple sclerosis, an autoimmune disease, is a known cause, especially in younger adults. Infections like meningitis, Lyme disease, or viral illnesses can trigger an inflammatory response affecting nerve function. Finally, in a significant number of cases, no specific cause is identified, and the condition is classified as idiopathic.
Confirming Diagnosis and Treatment Options
Diagnosing sixth nerve palsy begins with a comprehensive clinical examination assessing eye movements and alignment. The physician observes the eye’s inability to turn outward and measures the degree of double vision using specialized tools like prisms. This initial assessment confirms the nerve malfunction and differentiates it from mimicking conditions.
Determining the root cause requires further investigation, as the palsy often signals an underlying systemic issue. Neuroimaging is a standard diagnostic step, typically involving a magnetic resonance imaging (MRI) or computed tomography (CT) scan of the brain and orbit. These scans are used to rule out serious causes like tumors, aneurysms, or signs of increased intracranial pressure.
Blood tests are also routinely performed to check for common vascular risk factors, such as high blood sugar levels (diabetes), or to identify markers of inflammation and infection.
Treatment is tailored to the specific cause. If the palsy is due to microvascular ischemia, initial management often involves observation, as these cases frequently improve spontaneously within three to six months.
During this waiting period, temporary measures alleviate double vision and improve comfort. These include placing a prism on the glasses to help fuse the two images or simply patching one eye to eliminate the second image entirely.
For underlying causes, specific medical intervention is required, such as managing high blood pressure or administering antibiotics for an infection. If the palsy persists without resolution for six months to a year, or if the underlying cause is permanent, surgical options may be considered. Surgery, such as a muscle transposition procedure, aims to correct eye alignment and restore binocular vision.