Sixth cranial nerve palsy secondary to compression by dolichoectatic vertebrobasilar artery

Abstract

Dolichoectasia refers to distinct elongation, dilatation and tortuosity of an artery. We present a rare well-illustrated case of dolichoectatic vertebrobasilar artery compressing the cisternal portion of the sixth cranial nerve resulting in chronic sixth nerve palsy. High spatial resolution, three-dimensional, heavily T2-weighted MRI sequences are uniquely positioned to assess the cranial nerves especially in their cisternal and canalicular portions and need to be performed for all patients with non-resolving cranial nerve palsies. Dolichoectatic vessels can be the cause of neurovascular conflict and cause non-resolving oculomotor palsies.

Background

Dolichoectasia is a term used to describe an elongated, dilatated and tortuous intracranial vessel and can affect the vertebrobasilar and carotid arteries.^1–4 Radiologically vertebrobasilar dolichoectasia (VBD) is defined by basilar artery diameter larger than 4.5 mm and its course lateral to the clivus or basilar bifurcation occurring above the level of suprasellar cistern.⁴ The incidence of intracranial dolichoectasia ranges between 0.06% and 5.8%.¹ It most commonly affects middle-aged men with vascular risk factors (hypertension, hyperlipidaemia and smoking) and usually remains asymptomatic in most cases unless there is compression of adjacent structures by a dolichoectatic vessel.1 3 5 6 As VBD results in widening and elongation of the vertebral or basilar arteries, it can cause compression of nearby anatomical structures which includes the sixth cranial nerve at it exit from the ventral pons. In can also cause other compressive cranial neuropathies as well as compression of the brainstem itself or even acute ischaemia within the vertebrobasilar territory resulting in subsequent transient ischaemic attacks.1 3 5 6 Most commonly affected are trigeminal and facial cranial nerves while dysfunction of the sixth nerve secondary to VBD is very rare with only 10 previous cases described in the literature.⁷ Despite its rarity, VBD is an important entity to consider in a differential diagnosis of a patient with a non-resolving sixth nerve dysfunction and should be looked for using high spatial resolution, three-dimensional, heavily T2-weighted MRI sequences.

Case presentation

A 72-year-old woman was referred because of the history of long-standing intermittent horizontal binocular diplopia present for at least last 5 years but becoming more frequent in the past 6 months. It was mostly noticeable when she looked to the left. The patient was otherwise healthy and did not take any medications. She denied all symptoms of giant cell arteritis and increase dincracranial pressure. She has not noticed ptosis, variability or fatigability of her symptoms. She was examined by an ophthalmologist 1 year ago who reported left abduction deficit and ordered an MRI of the brain which was performed without contrast and interpreted as normal.

On examination, central visual acuity was 6/6 in each eye, there was no relative afferent defect. On motility testing, there was −1 to −2 abduction deficit in the left eye with the corresponding incomitant esotropia of 4 prism diopters (PD) on cover–uncover testing when she was looking to the right, 8 PD of esotropia in primary gaze and 14 PD of esotropia when looking to the left (figure 1). There was no ptosis or proptosis. Anterior and posterior segment examination was otherwise normal in each eye. She had normal orbicularis strength and normal strength in upper and lower extremities. The remainder of her cranial nerve examination was normal as well.

Figure 1.

Extraocular motility in nine cardinal directions of gaze demonstrating left abduction deficit.

Investigations

Acetylcholine antibody titers were previously performed and were normal. Single fibre electromyelogram was also performed 6 months ago and was reported as normal.

As the diplopia was present intermittently for the past 5 years and was reportedly getting worse, a repeated MRI of the brain and orbits was ordered in order to rule out a compressive lesion along the course of the left sixth cranial nerve. Utilisation of constructive interference in steady state (CISS) sequences (high spatial resolution, three-dimensional, heavily T2-weighted MRI protocol) was requested in order to better evaluate the entire pathway of the left sixth cranial nerve and rule out any subtle compressive lesion affecting its course.

MRI revealed the presence of a tortuous, dilated left vertebral artery which was in contact and compressing the left sixth cranial nerve in its cisternal space between its exit from the ventral pons and entrance into the Dorello’s canal (figure 2A). This was deemed to be the cause of chronic non-improving left sixth cranial nerve palsy.

Figure 2.

Three-dimensional T2-weighted space sequence reformatted in a sagittal oblique plane (A) shows the left abducens nerve (long thin arrows) coursing between a dolichoectatic left vertebral artery (long thick arrows) and Dorello’s canal (arrowheads). Axial image from the same sequence (B) shows the left abducens nerve (thin arrows) coursing between the left vertebral artery posteriorly (oblique arrow) and Dorello’s canal anteriorly (oblique arrow). Right sixth nerve does not contact vessels and is shown as it enters Dorello’s canal (arrowhead).

Differential diagnosis

Considering the long-standing and unchanging nature of the abduction deficit, a compressive lesion along the path of the sixth cranial nerve was considered likely and a decision was made to perform a neuroimaging study with contrast and high spatial resolution, three-dimensional, heavily T2-weighted MRI sequences that would allow the visualisation of the entire sixth nerve pathway.

Compressive lesions can affect any portion of the sixth nerve, starting from its nucleus and fascicle in the dorsal pons continuing to the prepontine cistern where the sixth nerve is vulnerable to compression by large cerebellopontine angle tumours or by a surrounding dolichoectatic vessel. The nerve then travels through the Dorello’s canal where it can also be compressed by neoplasms arising from the surrounding structures, continuing to the cavernous sinus where it can be affected by a variety of lesions: aneurysms of the intracavernous carotid arteries, increased venous pressure from carotid-cavernous fistula, infectious lesions most commonly originating from the surrounding sinuses spreading to the cavernous sinus or producing cavernous sinus thrombosis, neoplasms arising from the nasopharynx or metastasising to the cavernous sinus, laterally expanding pituitary gland tissue after pituitary apoplexy, by inflammatory entities that have predisposition to affect cavernous sinuses (IgG-4 disease, sarcoidosis and idiopathic orbital cavernous inflammatory disease) and by perineural spread of malignancies, most commonly squamous cell carcinomas usually arising from the skin. From the cavernous sinus, the sixth nerve travels to superior orbital fissure where it can again be compressed by neoplastic or inflammatory lesions arising from the surrounding structures and also by enlarged extraocular muscles in thyroid orbitopathy, or a variety of lesions arising from the surrounding sinuses (malignancies, infections, particularly fungal ones, expanding mucoceles, etc).

Other entities to consider in the diagnosis for patients with non-resolving sixth cranial nerve palsy are myasthenia gravis (although it typically produces variable deviation along with ptosis and tends to more commonly mimic third rather than sixth cranial nerve palsy), a restrictive process such as thyroid ophthalmopathy causing medial rectus enlargement and restriction (it is usually accompanied by other stigmata of thyroid eye disease such as proptosis, lid lag and retraction), a recently described sagging eye syndrome that causes progressive inferomedial displacement of lateral rectus muscles, and type 1 Duane’s syndrome that is a congenital syndrome caused by the absence of sixth cranial nerve nucleus and usually producing large abduction deficits accompanied by globe retraction on adduction.

Our patient was found to have a large dolichoectatic vertebrobasilal artery that was compressing left sixth cranial nerve in its cisternal portion. The diagnosis was made possibly only by high-resolution MRI study employing CISS imaging technique, which allowed direct visualisation of the entire pathway of the sixth cranial nerve that is often not possible with conventional imaging techniques.

Treatment

After worrisome, aetiologies of the sixth nerve palsy were ruled out, the patient was managed with prisms in her spectacles which allowed her to be diplopia-free most of the time.

Outcome and follow-up

Patient was followed for a year with unchanged left abduction defect and stable incomitant esodeviation. She is not experiencing diplopia except when looking to the extreme left side with the prisms in her spectacles.

Discussion

Cranial nerve sixth palsy secondary to VBD is rare and is an important diagnosis to consider in patients with non-resolving abduction deficits who have no other obvious explanation for their findings. The diagnosis requires a high-quality MRI study preferably employing CISS imaging that allows one to visualise the entire course of the sixth cranial nerve, starting from its nucleus in the dorsal pons, the fascicle in the prepontine cistern, in the Dorello’s canal, along its course over the clivus, and finally in its intracavernous and orbital portions.

There were only 10 previously described cases of VBD causing sixth nerve paresis and previous reports have described potential risk factors for developing dolichoectasia of the cranial vasculature which include male gender, hypertension and connective tissue diseases affecting large vessels.1 3 5–7 Our patient was a woman and did not have any of the previously described risk factors. While VBD vessels can produce life-threatening complications such as compression of the third ventricle, acute ischaemia of the brainstem and intracranial haemorrhage, in many patients, it is a benign entity and does not cause morbidity other than variable dysfunction of the cranial nerves it is in contact with, and in some it is an incidental imaging finding without any clinical manifestations.1 3 5–7

MRI and MR angiography play an important role in investigating any patient with non-resolving cranial nerve paresis and in addition to using high-definition sequences (such as balanced steady-state echo gradient sequences), the study needs to be interpreted by a neuroradiologist experienced in skull-based imaging as the pathological findings such as VBD can be subtle and missed.

Prognosis for most cases of sixth cranial nerve palsies caused by VBD is usually favourable with most patients remaining stable.^{3 7} Placement of prisms in the spectacles can be used for small deviations and strabismus surgery can be offered to patients with larger ocular misalignment for the symptomatic relief of diplopia.^{3 7}

Learning points.

• Non-resolving cranial nerve sixth palsy can be caused by dolichoectasia of vertebrobasilar arteries.

• Presumed mechanism of cranial nerve palsies in dolichoectasia is direct compression and/or pulsating pressure on the nerve by the artery.

• In order to visualise the entire course of the involved cranial nerve, high-resolution MRI sequences preferably employing balanced steady-state gradient echo sequences should be used.

• Careful review of the imaging study with a neuroradiologist is warranted and dolichoestasia of the vertebrobasilar arteries should be looked for as a potential explanation of the sixth nerve paresis.

• There is no established treatment for the dolichoectasia but most patients do well with prisms in their spectacles or strabismus surgery in order to produce orthophoria in primary position of gaze.