Advancement of deep brain stimulation (DBS) over the past few decades has progressed the treatment and knowledge of various medical conditions. DBS is a neurosurgical procedure that involves implanting electrodes in specific areas of the brain to intercede misconducting circuits (1). In the field of neurology, DBS has become one of the treatments of choice for severe motor disorders, including Parkinson's Disease, dystonia, and tremor (2). Essential tremor is the most common tremor disorder, characterized by its worsening with action, including postural and/or kinetic exacerbation. Most commonly seen in the upper limbs, it is often unresponsive to first-line medications, including propranolol, and primidone (3). Thalamotomy of the ventral intermediate nucleus (VIM) was previously used to treat disabling, treatment-resistant tremors. With advancements in technology, DBS to the VIM has replaced thalamotomy as the procedure of choice (4). Dysarthria and gait disturbances are the most reported adverse effects of VIM DBS (5, 6). This study presents in detail a case of VIM DBS that resulted in left gaze and facial palsy complications.

The patient's clinical findings and neuroimaging suggest a left dorsal pons localization because of the missed VIM target. The fascicle of the left facial nerve loops around the ipsilateral abducens nucleus, thus an ipsilateral gaze palsy and facial nerve palsy are highly indicative of a lesion in this area.

Deep brain stimulation (DBS) is a procedure that was first approved in the 1990's and has had high success with improving symptomatic outcomes in patients with disabling motor disorders (7). Through neurosurgery, electrodes are placed near certain structures in the brain and then connected to a pulse generator that is implanted into the chest. Though its exact mechanism of action is unknown, different models and hypotheses have been explored to understand its pathophysiology. The “disruption hypothesis” suggests that DBS interrupts inappropriate signaling to nuclei, which in turn disrupts abnormal electrical flow through the corticobasal ganglia loop in pathologic conditions (8). Furthermore, it is believed that this disruption of electrical activity modulates neuroplasticity and plays a role in neurogenesis (9).

Among conditions that utilize DBS for intervention, severe or worsened adverse effects are seen among patients with Parkinson's Disease (PD) (6), with up to 50% of patients potentially experiencing adverse events related to the procedure. In subthalamic nucleus (STN) DBS, mental status/behavioral changes are the most common procedure-related adverse events, with confusion topping the list. For globus pallidus internus (GPi) DBS, mental status/behavioral changes are also the highest reported followed by speech disturbances (10). Complications can be further derived from stimulation, such as electrical flow expanding through association regions and causing non-motor symptoms including laughter and hypomania (11).

In essential tremors, the classic target for DBS placement is the ventral intermediate nucleus of the thalamus (VIM), a structure involved in the dentato-rubro-thalamic tract and important for the abnormal electrical conduction in essential tremors. Other comparable targets in DBS for essential tremors include the posterior thalamic area (PSA) and Zona incerta (Zi) (12). The DBS procedure is efficacious in reducing tremor severity, particularly up to 10 years after the surgery, with efficacy decreasing after 10 years (13). The most common side effects from DBS placement include gait ataxia and dysarthria, reported among all three anatomic targets. In this report, we described a patient with left gaze and facial nerve palsies as the primary complications following a missed VIM target during DBS neurosurgery for refractory essential tremors.

Isolated horizontal gaze palsy is caused by interruption of the abducens nucleus or paramedian pontine reticular formation (PPRF). Inciting events include infarction, inflammation, hemorrhage, or metastasis, with gaze palsy sometimes seen as part of multiple sclerosis or neuromyelitis optica spectrum disorders (14). Clinical ophthalmologic presentations can include impairment of smooth pursuit and saccadic eye movements and can be accompanied by horizontal or vertical nystagmus. In addition to her gaze palsy, this patient also suffered from diplopia due to esotropia. Esotropia causing diplopia has been reported in association with horizontal gaze palsies due to dorsal pontine lesions (15, 16). For isolated facial nerve palsies, almost 75% of causes are attributed to Bell's palsy (17). Other less common causes of facial palsies include cerebellopontine angle tumors and pontine infarcts (18).

For the patient presented in this case, isolated left gaze and facial nerve palsies were attributed to canal insertion into the left dorsal pons during a VIM DBS thalamotomy. The abducens (sixth) and facial (seventh) cranial nerves reside in the pons. The abducens nucleus innervates the ipsilateral lateral rectus muscle and the contralateral medial rectus subnucleus of the oculomotor nuclear complex by way of the MLF. During surgery, the target of interest was the ventral intermediate nucleus of the thalamus, a common landmark for DBS electrode placement in refractory clinical cases. Anatomically, the thalamus is located superomedial to the pons. Overshooting of catheter insertion during initial entry caused an interruption of the pontine pathway. Clinically, this presented as weakness of left horizontal gaze and left facial weakness. Gaze palsies are typically due to interruption of the ipsilateral PPRF, ipsilateral abducens nucleus, or both. PPRF lesions tend to cause primarily saccadic gaze palsies, and as such, it is most likely that in our patient the abducens nucleus was affected (19).

Our case report demonstrates that VIM DBS can present as isolated left horizontal gaze and facial nerve palsies if the thalamic target is overshot during surgery. While DBS can provide acute improvements in refractory movement disorders, neurosurgeons and neuro-ophthalmologists should be wary of the anatomic proximity between the thalamus and the important structures of the brainstem when understanding potential complications of this procedure. Surgery risks should further consider the holistic effects of visual impairment, taking into account the patient's baseline visual function. All patients who undergo VIM DBS surgery should be assessed for potential cranial nerve deficits post-placement.

It should be noted that for the patient presented in this report, visual impairment as a consequence of surgery led to many significant changes, including not being able to drive, not being able to care for others, and a loss of independence. This took a toll on the patient's mental health, ultimately leading to a dosage increase of her Lexapro. The mental health effects of impaired visual function should be considered when weighing the risks and benefits of surgery. For this patient, there was an original decrease in tremor severity right after surgery, but her tremor has since returned to baseline prior to neurosurgery.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

SG examined the patient, established the diagnosis, obtained the images, and revised the manuscript. VC reviewed the literature and wrote the whole manuscript. All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.