Teaching Case

A 31-year-old male presented in the Emergency Department with acute-onset horizontal diplopia. Clinical examination revealed conjugate gaze palsy and lower motor neuron type of left facial nerve palsy. However, pupils were normal in size with normal direct light reflexes. No other neurological deficits were present.

Magnetic resonance imaging showed a punctate area of bright signal on diffusion-weighted imaging  (DWI) with corresponding hypointensity on apparent diffusion coefficient map (Figure 1) and hyperintense signal change on FLAIR (Figure 2)  representing acute infarction in the dorsal aspect of Pons on the left side at the floor of the fourth ventricle at the level of ipsilateral facial colliculus.

No signal alteration was seen on T1 and T2  weighted images.

No perilesional oedema or mass effect was seen. Rest of the cerebral parenchyma was unremarkable.

MR angiography of Extracranial and intracranial segments of ICA and vertebrobasilar system were normal (Figure 3).

Background

Motor fibres of facial nerve loop around the abducens nucleus before exiting the brainstem to produce a prominence at the floor of the fourth ventricle called facial colliculus (Figure 4).Clinical signs and symptoms are determined by the structures involved, especially the abducens nucleus, the 7th nerve that loops around the abducens nucleus, the paramedian pontine reticular formation (PPRF) that is anatomically located near the abducens nucleus, and the medial longitudinal fasciculus (MLF). Facial colliculus pathology causes LMN type of facial palsy, diplopia and horizontal conjugate gaze palsy. The abducens nerve innervates the ipsilateral lateral rectus muscle and directly controls abduction in the ipsilateral eye. It also controls conjugate adduction of the contralateral eye through the MLF connected to the oculomotor nucleus of the contralateral side. Therefore lesion in the abducens nucleus and PPRF cause loss of abduction of the ipsilateral eye as well as loss of conjugate adduction of the contralateral eye. In young age, tumours, demyelination, vascular malformation and infection may be the causative factors while in elderly people, ischemia is the most common cause. [1,2,3]

Clinical Perspective

In this case, the clinical presentations were all the three components - consists of lower motor neuron facial nerve palsy, diplopia and horizontal conjugate gaze palsy. But not all symptoms are invariably present in every patient. The symptoms depend on what component of facial colliculus is involved [3]. 

Imaging Perspective

MRI findings in this young patient revealed a punctate area of diffusion restriction on DWI without perilesional oedema/ mass effect involving the left facial colliculus representing acute infarction. The lesion was conspicuous on DWI and is the most valuable sequence to predict the diagnosis. 3D high-resolution FLAIR sequences with short FOV at the level of pons demonstrated this subtle lesion in this study. The imaging pattern, paucity of oedema and mass effect precluded the diagnosis of neoplasm and demyelination in this case.

Punctate area of diffusion restriction corresponding to the pontine perforator territory and acute onset clinical presentation together helps in the diagnosis of an acute ischemic event in the left facial colliculus.

Outcome

This patient was treated with aspirin and ocular motility, and the seventh nerve function gradually improved. MRI plays a vital role in finding the cause for facial colliculus syndrome. In this case, the cause for the patient's symptoms is acute infarction involving the facial colliculus.

Take-Home Message / Teaching Points

LMN type facial nerve palsy, lateral rectus palsy, and conjugate gaze palsy should raise the possibility of facial colliculus syndrome. Although the clinical picture is characteristic, localizing the organic lesion is challenging. In clinical practice, such syndromes are rarely seen in their pure form, MRI with DWI and 3D High-resolution FLAIR sequences are the best techniques to focus on the brainstem to localise and characterize a subtle lesion.