Ipsipulsion: A forgotten sign of lateral medullary syndrome.

Ipsipulsion is a clinical sign specifically seen in lateral medullary syndrome. It is characterized by two involuntary phenomenons. One is static eye deviation ipsilateral to the side of lesion especially in the absence of visual fixation. Second is the saccadic lateropulsion whereby voluntary saccades towards the side of lesion are hypermetric and saccades towards opposite side are hypometric. The vertical saccades may also appear oblique. Ipsipulsion is produced due to damage to the contralateral olivocerebellar pathways that crosses midline in medulla and pass through the ipsilateral inferior cerebellar peduncle to supply ipsilateral cerebellar hemisphere.

Introduction

Lateral medullary syndrome is a well-defined posterior inferior cerebellar artery infarct characterized by ipsilateral palatal palsy and contralateral sensory loss. A variety of eye movement abnormalities are associated with lateral medullary syndrome that includes skew deviation, primary position horizontal/torsional/upbeat nystagmus, gaze-evoked nystagmus, and ipsipulsion.[1] Unlike other eye movement abnormalities, ipsipulsion is more specific to lateral medullary syndrome and is characterized by two involuntary phenomenon. One is static eye deviation ipsilateral to the side of lesion especially in the absence of visual fixation. Second is the saccadic lateropulsion whereby voluntary saccades towards the side of lesion are hypermetric and saccades towards opposite side are hypometric.[12] Despite being a clinical sign with a good localizing value, this clinical sign is very sparingly discussed in neurology textbooks and therefore seldom taught to medical students.

Case Report

A 52-year-old gentleman presented with sudden onset of difficulty in swallowing for 1 day. Difficulty in swallowing was for both solids and liquids with associated nasal regurgitation of liquids. Patient also complained of nasal quality of his sound. There was no limb weakness or sensory complaints. There was no past history of diabetes mellitus, hypertension, or coronary artery disease. On examination, patient was conscious and oriented. He had rightward deviation of eyes. He was able to move his eyes in all direction. However, while looking in the primary gaze, if patient was asked to close his eyes, his eyes moved involuntarily to right side. Horizontal saccades to left to fixate on a target required multiple small saccades; whereas, saccades towards right were fast and effortless [Video 1]. Primary position left-beating nystagmus was seen in primary position that changed direction to right on right gaze. Vertical saccades produced slight oblique movements and pursuit movements were broken by multiple small saccades (not documented in video). He had right facial hypoalgesia, palatal palsy with diminished gag-reflex on right side, right-sided cerebellar signs, and hypoalgesia on left half of body. Motor examination was normal and there was no posterior column sensory loss. His magnetic resonance imaging (MRI) of brain revealed a small infarct in the rostral lateral medulla and part of right cerebellar hemisphere. MR angiography of brain and cervical vessels was normal. His electrocardiography and 2D-echocardiography was also normal. Over 4 weeks, the ipsipulsion improved spontaneously; however, he continued to have rebound nystagmus [Video 1], oblique vertical saccades, and palatal palsy.

Discussion

Acute conjugate eye deviation gives impression of either a cerebral stroke with eyes deviation towards the cerebral lesion or a pontine stroke with eye deviation opposite to the side of pontine lesion. However, it is very unique that patients with lateral medullary syndrome show conjugate deviation of eyes towards the side of lateral medullary infarct. Often, this sign remains unnoticed because in majority of patients with lateral medullary infarct, visual fixation needs to be removed to elicit ipsilateral gaze deviation. In one study, conjugate eye deviation was seen in 4 of 14 patients with eyes open and 12 of 14 patients with eyes closed.[3] As compared to involuntary ipsilateral conjugate eye deviation, ipsilateral hypermetric saccades and contralateral hypometric saccades (saccadic lateropulsion) is more common among patients with lateral medullary syndrome.[24] This can be tested by asking the patient to move the eyes in either direction in order to fixate on an object. In the direction ipsilateral to the side of lesion, movement of eyes will be brisk and overshoot the target while in the direction opposite to the side of lesion, saccades will be slow and multiple saccades will be generated to compensate for undershooting of target. In addition to saccadic lateropulsion in horizontal direction, vertical saccades also show ipsilateral oblique trajectories.

The exact mechanism of ipsilateral gaze deviation is not known. It is believed that the contralateral olivocerebellar fibers crosses midline in the rostral medulla and passes through the ipsilateral inferior cerebellar peduncle to contribute as inhibitory climbing fibers to the cerebellar hemisphere. These fibers are damaged in the lateral rostral medullary infarcts. Due to loss of inhibitory climbing fibers, the Purkinje cells currents increases transiently to several folds. These Purkinje cell projections inhibit the ipsilateral vestibular nuclei. As a result, the contralateral vestibular activity increases which act by generating a slow-phase eye velocity towards the side of lesion.[5]

Deep cerebellar nuclei are implicated in producing saccadic lateropulsion.[5] The ipsilateral fastigial nucleus of cerebellum when stimulated produces contralateral saccades by its connections to burst neurons of the contralateral parapontine reticular formation (PPRF). Due to lesions in the rostral medualla and loss of olivocerebellar tracts, the transiently increased Purkinje cell activity inhibits the ipsilateral fastigial nucleus that in turn inhibits the burst neurons in the contralateral PPRF. This results in a bias towards ipsilateral saccades. Since, a part of fastigial nucleus also contributes to vertical saccades; its inhibition also produces oblique vertical saccades. The Purkinje cell activity returns to normal after few weeks and therefore the ipsilateral gaze deviation and saccadic lateropulsion also improves.

The ipsilateral gaze deviation is specific for lateral medullary syndrome, but very rarely may also be seen in isolated infarcts of inferior cerebellar peduncle. Similarly, the saccadic lateropulsion is rarely seen in infarcts involving the superior cerebellar peduncle. It is because of the fact that the fastigial projections passes through the contralateral superior cerebellar peduncle to supply the PPRF.[5] It is also worth noticing that the saccadic lateropulsion should not be confused with axial or body lateropulsion which is characterized by a tilt of body in the vertical axis in patients with midbrain infarcts.[6]