Isolated bilateral abducens nerve palsy secondary to clivus metastasis of prostate adenocarcinoma undetected by magnetic resonance imaging

Dear Editor:

Bilateral abducens nerve palsy has numerous causes, including cerebrovascular diseases, intracranial hypertension, carotid-cavernous fistulas, infection, trauma, Guillain–Barré syndrome, Wernicke–Korsakoff syndrome, and tumours. The condition rarely presents in isolation; when it does, presence of a tumour in the clivus should be ruled out. We present a case of bilateral abducens nerve palsy secondary to clival metastasis of prostate cancer.

Our patient was a 72-year-old man with 6-year history of prostate adenocarcinoma and bone metastases who visited our emergency department in March 2018 due to diplopia of 2 months’ progression; he was receiving sixth-line treatment with radium-223 and had already completed 4 cycles, showing good tolerance. He initially reported difficulty with left eye abduction, developing difficulty with right eye abduction 2 weeks later.

The most recent imaging studies available, performed a month previously, were bone scintigraphy, which revealed bone lesions in multiple sites, including the skull and the left superior maxillary bone (see online supplementary material), and a chest and abdominal axial CT scan, which revealed no visceral anomalies. A week previously, the patient had undergone a brain MRI study, which was initially interpreted as normal; however, a later evaluation of MR images detected contrast uptake in the left abducens nerve (Fig. 1). An electromyography study conducted the previous day had yielded normal results.

Figure 1.

A and B) Brain MRI. Contrast T1-weighted sequences: A) axial plane; B) sagittal plane. Red arrow: the left abducens nerve, emerging from the prepontine cistern, shows increased contrast uptake, probably due to involvement of the Dorello canal. Green arrow: trigeminal nerve, with no contrast uptake. C and D) Head CT scan (bone window), axial plane. C) Diffuse hyperdensities in the clivus and sphenoid bone (red asterisk), suggesting bone metastases. D) Image from a control, showing no hyperdensities at the base of the skull.

The neurological examination detected isolated bilateral abducens nerve palsy (Fig. 2). A blood analysis detected no abnormalities; acute-phase reactants were within normal ranges. A head CT bone window study revealed diffuse hyperdensities in the clivus and sphenoid bones, suggestive of bone metastases (Fig. 1).

Figure 2.

A) Upward gaze with no alterations. B) Rightward gaze, with partial abduction of the right eye. C) Primary position, with bilateral esotropia. D) Leftward gaze, with mildly impaired abduction of the left eye. E) Downward gaze, with mild right eye esotropia. Preserved convergence (not shown). Examination performed after 14 days of treatment, with partial improvement, mainly of the left abducens nerve (increased contrast uptake on MRI). Diplopia persisted.

We started outpatient treatment with dexamethasone dosed at 4 mg/24 h, which was later down-titrated, and skull base radiation therapy (total dose of 30 Gy); given the progression of the cancer, treatment was switched to a new line of chemotherapy with cabazitaxel. Diplopia resolved within 4 weeks, and has not reappeared after 6 months of follow-up. No follow-up neuroimages are available.

The abducens nerve innervates the lateral rectus muscle, responsible for eye abduction. Its trajectory is subdivided into 5 segments^1,2:

• 1

  Intra-axial: the abducens nucleus is located in the posterior, caudal portion of the pons. It projects axons anteriorly through the medial lemniscus, which is medial to the fascicles of the facial nerve.

• 2

  Cisternal: the abducens nerve emerges at the pontomedullary sulcus, lateral to the bundles of the corticospinal tract, and courses upwards along the prepontine cistern until reaching the posterior, dural surface of the clivus.

• 3

  Dorello canal: after perforating the clival dura mater, the abducens nerve enters the Dorello canal to reach the cavernous sinus.

• 4

  Cavernous sinus: the abducens nerve runs immediately lateral to the internal carotid artery.

• 5

  Extracranial: the abducens nerve enters the orbit through the superior orbital fissure and reaches the lateral rectus muscle.

A clival lesion may therefore damage the abducens nerve bilaterally at the level of the Dorello canal.

The literature includes only 12 cases of isolated bilateral abducens nerve palsy secondary to tumours of varying aetiology. In 7 patients (3 with primary tumours and 4 with metastases), the clivus was the main structure involved: clivus chordoma (2),^3,4 multiple myeloma of the clivus (1),⁵ clivus diffuse large B cell lymphoma (1),⁶ clivus metastasis of Ewing’s sarcoma (1),⁷ clivus metastasis of small-cell lung carcinoma (1),⁸ and clivus metastasis of lung adenocarcinoma (1).⁹ The remaining 5 patients had pituitary adenoma (3),^10-12 primary non-Hodgkin’s lymphoma of the sphenoid sinus (1),¹³ and nasopharyngeal carcinoma (1).¹⁴ The primary tumour was previously unknown in only one of the 3 patients presenting metastasis.⁹ In some patients, abducens nerve palsy was bilateral from diagnosis, whereas other patients initially presented unilateral symptoms, with bilateral palsy developing over the course of several days or weeks.

Our case shows that bone metastases may go undetected in contrast brain MRI scans; head CT scans, particularly bone window images, may be extremely helpful for diagnosis.

To our knowledge, this is the first reported case of isolated bilateral abducens nerve palsy secondary to clival metastasis of prostate adenocarcinoma. Clivus tumours should be included in the differential diagnosis of bilateral abducens nerve palsy.

Funding

The authors have received no funding for this study.