Abducens nerve palsy may occur due to a myriad of infectious, inflammatory, genetic, or malignant diseases. Inability to perform eye abduction, resulting in binocular horizontal diplopia, is the main symptom, whereas additional neurological deficits can be encountered depending on the site of the lesion. The initial diagnosis can be made with a proper physical examination, but imaging studies of the brain and other tests are necessary to determine the cause.
The abducens nerve (cranial nerve VI) is known for its very long travel from the brainstem to the ipsilateral lateral rectus muscle, where it provides an important function by stimulating abduction of the eye  . But because the nerve is quite long, it can be damaged at various points along its pathway. In fact, abducens nerve palsy is one of the most frequent nerve palsies encountered in clinical practice, with an established incidence rate of 11.3 per 100 000 individuals . Certain authors have divided the pathologies that cause abducens nerve palsy according to their anatomical sites (the brain stem, subarachnoid space, the petroclival region, the cavernous sinus, and the orbit), and numerous conditions, both localized and systemic, may cause this condition  . The principal symptom of abducens nerve palsy is the presence of binocular horizontal diplopia, as the eye is not able to perform abduction, resulting in lateral displacement  . In addition, many symptoms accompany sixth nerve palsy depending on the location of the lesion    . For example, contralateral hemiparesis, ipsilateral palsy of the facial, but also trigeminal and vestibulocochlear nerves is seen in conditions affecting the brainstem, such as Raymond’s syndrome, Millard-Gubler syndrome and Foville’s syndrome . Then, papilledema, visual deficits, and symptoms suggestive of a central nervous system (CNS) infection of any etiology (bacterial, viral, fungal) may be seen if the nerve is compromised in the subarachnoid space, as various tumors, sarcoidosis, pseudotumor cerebri and meningitis have been described as potential etiologies . On the other hand, epistaxis, rhinorrhea, and serous otitis media, typically encountered in nasopharyngeal carcinoma, can also be seen together in abducens nerve palsy . Other notable causes of this lesion include hypertension, diabetes mellitus, trauma, and several other tumors (meningiomas, acoustic neuromas, cerebellopontine angle tumors, and metastatic deposits), implying that a broad symptomatology could present together with abducens nerve palsy    .
The diagnosis of abducens nerve palsy is rather easy to make by conducting a proper physical examination that will include a complete assessment of eye muscles. It is necessary, however, to perform a thorough neurological examination, which might reveal other symptoms that could aid in determining the site of the lesion  . After the examination, and a detailed patient history that will assess the course and progression of symptoms, imaging studies should be employed. Computed tomography (CT) is often performed as an initial method, but magnetic resonance imaging (MRI) of the endocranium is recommended for evaluation of the underlying cause of abducens nerve palsy, as it provides a more detailed view of the cranial structures   . Isolated reports have questioned its cost-effectiveness, however, since many patients in whom vascular diseases are responsible for the lesion improve spontaneously within a short period of time  . Nevertheless, MRI is an effective method, while laboratory studies, including a complete blood count (CBC), a full lipid profile, glucose levels, erythrocyte sedimentation rate (ESR), antinuclear antibodies (ANA), and rheumatoid factor, are important in raising clinical suspicion toward vasculitis, diabetes mellitus and atherosclerosis . Because infections and increased intracranial pressure (ICP) are also potential causes of abducens nerve palsy, a lumbar puncture with subsequent examination of the cerebrospinal fluid is recommended as well  .