Sensorimotor postural rearrangement after unilateral vestibular deafferentation in patients with acoustic neuroma

Abstract

Unilateral vestibular lesion, as acoustic neuroma and its surgical removal, leads to impaired balance control. After initial vertigo and postural instability corresponding to unilateral vestibular deafferentation, improvement in symptoms and global balance functions occurs by a process called vestibular compensation. In this respect, this prospective study aimed to assess the differential contribution of sensory inputs to the regulation of posture during the recovery process after acoustic neuroma removal. Twenty-seven patients with acoustic neuroma underwent vestibular and posturographic testings, shortly before and 8 days, 1 month and 3 months after surgical removal of the tumour. Immediately after vestibular deafferentation, vestibular function was asymmetrical, postural performances were altered, especially in eyes closed conditions and in sensory challenged situations. One month, and more particularly 3 months after surgery, restoration and even improvement of vestibular and balance performances occurred, associated with a lower number of falls, development of more appropriate sensorimotor strategies and better resolution of sensorial conflicts. Postural perturbations are related to erroneous vestibular afferences, leading to incoherence in information about head position and interference with somatosensory and visual pathways. The time-course implementation of central adaptive mechanisms, characterized by substitution by other sensory afferences and new behavioural strategies, leads to an improvement of balance performance.

Introduction

Postural control in human is a multidetermined system based on central integration of visual, somesthetic and vestibular peripheral information. This complex integration permits a context-specific motor response, which leads to stabilization of anti-gravity activity and gaze and allows the adjustments of static and dynamic postures (Nashner, 1976, Keshner et al., 1987). Under normal conditions (stable vision, fixed support), this information is redundant but complementary to a higher contribution of proprioceptive inputs than of visual and vestibular inputs (Fitzpatrick and McCloskey, 1994). On the other hand, if either the environmental or task conditions change, a switch between the sensorial information is scheduled according to the resulting balance difficulties, and the gain of the different afferent-postural loops is modified (Marsden et al., 1981, Bronstein et al., 1990). Damage to any of these balance regulation levels influences the common output of the postural system, resulting in postural instability and an increased risk of fall.

Acoustic neuroma is a benign tumour from Schwann cells surrounding the vestibular nerve, which grows within the internal auditory canal and then extends into the cerebellopontine angle (Wiegand et al., 1996, Matthies and Samii, 1997). Dizziness is the principal symptom noted in 10–19% of cases, but its incidence in patients with acoustic neuroma has been reported as between 49 and 66% (El-Kashlan et al., 1998, Moffat et al., 1998, Lynn et al., 1999, Inoue et al., 2001, Kentala and Pyykkö, 2001, Humphriss et al., 2003, Darrouzet et al., 2004). These differences can be explained by the vestibular compensation which accompanies the gradual vestibular dysfunction caused by the tumoral growth (Curthoys and Halmagyi, 1995). Surgical removal of the tumour, which corresponds to an acute unilateral vestibular deafferentation (uVD), leads to a decompensation of this previously compensated situation, and explains that most patients report severe vertigo after surgery. Gradual disappearance of symptoms and improvement in functional capacities vary considerably at any given time, lasting from a few months to several years for complete recovery of social and professional activities (Andersson et al., 1997, Driscoll et al., 1998, Lynn et al., 1999, Briggs et al., 2000, Inoue et al., 2001) and can be considered as a vestibular compensation.

Most studies aiming at evaluating the vestibular pathology impact on gaze and posture stabilization have focused on patients with acute vestibular neuritis or undergoing unilateral vestibular neurotomy for incapacitating forms of peripheral vertigo, particularly Menière's disease (Cass et al., 1991, Lacour et al., 1997, Borel et al., 2002, Alessandrini et al., 2003). uVD induces a perceptual syndrome with vertigo, dysautonomic disorders and typical clinical syndromes, including body and limb deviation toward the affected side, spontaneous nystagmus and ocular tilt reaction (Curthoys and Halmagyi, 1995). These abnormalities can be explained by a tonic imbalance of the vestibulo-ocular and vestibulo-spinal bilateral pathways, leading to gaze and postural control perturbations. Most studies on the effects of acoustic neuroma and its removal have been based on questionnaires which have evaluated the quality of life (Andersson et al., 1997, El-Kashlan et al., 1998, Lynn et al., 1999, Inoue et al., 2001, Humphriss et al., 2003), few however have quantified their impact on postural control and, in addition, certain are contradictory. In this respect, perturbations in dynamic postural performance before surgery were noted for Johansson et al. (1994), but not for Mruzek et al. (1995). Moreover, after surgery or stereotactic gamma irradiation, although the vestibular symptoms usually decline spontaneously, impairment of postural stability often remained and the neurosensory balance information hierarchy is modified, the contribution of visual afferent becoming predominant (Pyykkö et al., 1995, Levo et al., 2004, Ouaknine et al., 2004). Thus, Driscoll et al. (1998) have shown that disequilibrium could last more than 3 months after acoustic neuroma removal and that age more than 55.5 years, female gender, constant preoperative disequilibrium and central findings on electronystagmography were associated with a worse outcome.

Nevertheless, to our knowledge, no longitudinal study on surgical acoustic neuroma removal has examined the evaluation of three modalities of balance control, namely quiet stance, balance disturbances and movements. In this respect, this longitudinal prospective study aimed to assess the differential contribution of sensory inputs – visual, vestibular and proprioceptive – to the regulation of posture during the recovery process after an acoustic neuroma removal.