**1. Introduction**

Multiple sclerosis (MS) is an immune-mediated neurodegenerative disease of the central nervous system (CNS), which primary affects young adults [1–3]. The large immune response to putative CNS antigens is thought to be driven by an interplay between environmental and genetic factors [4]. There are four different forms of MS that can be distinguished based on the clinical disease pattern, namely: relapsing–remitting MS (RRMS), secondary progressive MS (SPMS), primary progressive MS (PPMS), and progressive relapsing MS (PRMS) [5]. Clinically, RRMS is the most common form of MS, with more than 85% of patients initially present with such form.

Multiple sclerosis is believed to be associated with a wide range of neurological abnormalities, which often interact to cause mobility difficulties, while the impairment in balance is thought to be significant factor in these mobility difficulties [6]. A body line of evidence shows that 85% of MS patients may experience mild ataxia at some point in time, while 32% of MS patients exhibits a severe form that can decline their functional abilities [7–9]. The word ataxia (from the Greek) literally means the

absence of order, disorder, or confusion and is characterized by a loss of coordination of the body's limbs, the trunk, and the gait and it can be brought on by sensory system's dysfunction (sensory ataxia), cerebellum's dysfunction (cerebellar ataxia), or dysfunction of the vestibular system (vestibular ataxia) [10, 11], which may also arise from thalamic and parietal- and frontal-lobe injuries [8].

Clinically, ataxia refers to a collection of abnormal movements, of which tremor is the main symptom. Other clinical manifestations include dysmetria, dysdiadochokinesia, incoordination, and movement delays [8]. It is rarely seen as a single symptom and usually occurs with muscle weakness and spasticity [7]. Nevertheless, severe ataxia may occur alone but it is usually combined with brainstem signs [12]. A range of interventions aimed at enhancing balance in standing and walking are used in clinical practice, including pharmacotherapy, surgical therapies, and the most common is physiotherapy. This chapter provides an overview of different types of ataxia, the current understanding of ataxia in MS, and the currently available therapeutic approaches.
