she had previously been diagnosed with infiltrating ductal carcinoma in her right breast and had known metastases to the liver and bone .
she had undergone a palliative right mastectomy and chemotherapy . when she first complained of horizontal diplopia , headache , and nausea , brain magnetic resonance imaging ( mri ) and cerebrospinal fluid analysis revealed no abnormal findings .
her visual acuity was 20/20 in both eyes , and no abnormal findings were detected on slit lamp examination .
the alternate prism cover test revealed 10 prism diopters of esotropia in the primary position . on
two months later , both eyes were fixed in the midline on attempted right gaze .
in contrast , left gaze was preserved in both eyes and vertical eye movements were normal ( fig .
follow - up brain mri revealed an enhancing round nodular mass at the right facial colliculus of the lower pons , suggesting involvement of either the right abducens nucleus or the paramedian pontine reticular formation ( pprf ) along with the facial nucleus or nerve tract ( fig .
the findings in our patient suggested impairment of the right abducens nucleus , which contains both motoneurons that control right eye abduction and internuclear neurons that control left eye adduction .
involvement of the right pprf could also be inferred given a complete loss of right saccade movements of the right eye . moreover
, the accompanying ipsilateral facial palsy supported the possibility of damage to the right facial nucleus or nerve tract in the vicinity of the abducens nucleus . on mri , a small tumor was found exactly at the facial colliculus of the lower pons ( fig .
the preservation of adduction of the right eye on left gaze implied that the median longitudinal fasciculus ( mlf ) was intact .
pathologic confirmation could not be obtained because we did not think the benefit of biopsy outweighed the risk of pontine injury . however , based on the mri findings and the slow progression of the neurologic deficits , a pontine tumor was favored .
other potential diagnoses , including pontine hemorrhage , infarction , aneurysm , or demyelination , were excluded .
the absence of hypertension and diabetes mellitus as well as an age younger than 55 years also supported the diagnosis .
however , the patient 's history of liver and bone metastases along with a previous report indicating that brain metastasis was detectable in up to 30% of patients with breast cancer strongly supported the possibility of a metastatic lesion . in addition , metastatic brain tumors have been reported to be at least four times as common as primary brain tumors , and breast cancer is known to be the second most common cause of brain metastasis .
yen et al .   reported that the incidence of brainstem involvement in patients with breast cancer metastasis to the brain was as high as 12.4% , higher than that in patients with any other type of cancer .
reported that 28% of cases of brain metastasis in breast cancer showed a single metastatic lesion , supporting the idea that a single brain lesion does not necessarily suggest a primary tumor .
moreover , brainstem gliomas , the most common primary tumor of the brainstem , are usually slow - growing and progressive over several years ; the rapid growth in only three months also supported the diagnosis of metastatic cancer .
neoplasm is known to be one of the causes of abducens nerve palsy and internuclear ophthalmoplegia . however , to the best of our knowledge , this is the first report of unilateral conjugate gaze palsy due to a pontine metastatic mass . while gaze palsy caused by herpes zoster
infection or pontine hemorrhage has been reported , this is the first report of unilateral horizontal gaze palsy due to metastasis to the abducens nucleus . in conclusion ,
a localized metastasis to the abducens nucleus that spares the mlf can cause gaze palsy in a patient with breast cancer .
 a 51-year - old woman with breast cancer presented with progressive diplopia . 
 neuro - ophthalmologic examination revealed right gaze palsy and peripheral facial nerve palsy . 
 brain magnetic resonance imaging ( mri ) was normal . 
 however , two months later a repeat brain mri revealed an enhancing round nodular mass at the right facial colliculus of the lower pons , at the location of the abducens nucleus . 
 localized metastasis to the abducens nucleus can cause gaze palsy in a patient with breast cancer . 