Torsional saccadic oscillations

We present a case of torsional saccadic oscillations and discuss the possible underlying neuroanatomical substrate. Correspondence to: Marcello Cherchi, M.D., Ph.D., Ken and Ruth Davee Department of Neurology, Northwestern Feinberg School of Medicine, Abbott Hall, 11th Floor, 710 N. Lake Shore Drive, Chicago, IL 60611-3078, USA, Tel: 312-908-8266; E-mail: [email protected] Received: April 26, 2015; Accepted: May 29, 2015; Published: June 01, 2015 Case report A 31-year-old man presented with a complaint of imbalance following head trauma. During childhood he had strabismus and “nystagmus” about which no further information was available. At age 29 he sustained head trauma during an assault. After the head trauma he began experiencing headaches, amnesia and episodes of tilting or rocking lightheadedness associated with blurry vision, oscillopsia and nausea (Table 1). The episodes were sometimes triggered by complex visual stimuli (such as following a rapidly moving target) and by changes in position of the head, and could range in duration from hours to days. The symptoms improved over several months, but worsened again after he fell 15 feet from a roof and landed on his back. The symptoms were not alleviated by baclofen, gabapentin, paroxetine or duloxetine. He denied any tinnitus, noise sensitivity, aural fullness, otalgia and hearing loss. Audiometry, otoacoustic emissions and vestibular evoked myogenic potentials were normal. MRI of the cervical spine was normal. MRI of the brain was reported as normal. Video Frenzel oculography was remarkable for modest up beating nystagmus and an approximately 8 Hz pendular torsional oscillation that may be described as torsional flutter or torsional shimmer that was persistent in all directions of gaze and in all positions, unchanged by any maneuvers (Figure 1 and Video). The patient was lost to follow-up. We are not aware of any prior reports of torsional saccadic oscillations. Beyond documenting it, we propose here some ideas regarding the possible underlying neuroanatomy. However, we begin the discussion with the neuroanatomy underlying horizontal saccades, and the possible mechanisms underlying horizontal saccadic oscillations, as these are better understood than their vertical and torsional counterparts. Horizontal saccades and the possible mechanisms of oscillations The brainstem nuclei known to be involved in generating horizontal saccades are the paramedian pontine reticular formation [PPRF] (containing the burst neurons that generate the “pulse” signal needed to initiate a saccade), another area in the pontine reticular formation (containing the neurons that generate the “step” signal needed to maintain the eccentric position) and the pontine nucleus raphe interpositus [RIP] (containing the omnipause neurons that inhibit, or “gate,” the burst neurons) [1-3]. Zee and Robinson [4] proposed a model to account for horizontal saccades and for the quick phase of horizontal nystagmus. This model began with the generally accepted concept that these fast eye movements are created by a “pulse-step” mechanism. The “pulse” component is phasic and encodes eye velocity; it is thought to arise from burst neurons located in the pontine paramedian reticular formation that are normally silent but fire rapidly immediately before and during the eye movement. The “step” component is tonic and encodes eye position; it is thought to arise from other neurons in the pontine reticular formation that integrate (in the mathematical sense) [5,6] the velocity information from the immediately preceding “pulse” component in order to generate a position-coded signal to maintain the eye in the new, eccentric position by overcoming elastic restorative forces of the orbit [7]. Early investigators believed that the pulse was preprogrammed or Figure 1. Videonystagmography. The top tracing reflects horizontal eye movements, the bottom tracing reflects vertical eye movements. Torsional eye movements are not directly measured, but are nevertheless manifest in the subtle horizontal oscillatory eye movement.