Introduction

Hemiballism or hemichorea is a rare neurological disorder, but the crucial involvement of the subthalamic nucleus (STN) in its pathophysiology has been appreciated for decades (Jakob 1923; Martin 1927; Glees and Wall 1946; Whittier and Mettler 1949; Carpenter and Carpenter 1951; Crossman 1987). Only recently have serious doubts come forward. Postuma and Lang (2003) have described the STN as being involved in only a minority of cases, and indicated unrecognized causes such as non-ketotic hyperosmolar hyperglycaemia and complications of human immunodeficiency virus (HIV) infections. Moreover, the crucial involvement of a lesion of the STN is in doubt (Guridi and Obeso 2001; Postuma and Lang 2003). On the other hand, idiopathic Parkinson’s disease (Battistin et al. 1996; Usunoff et al. 2002) is a common neurodegenerative disorder, but the key role of the STN in the pathophysiological origin of the parkinsonian state has become evident only recently (Miller and DeLong 1987; Mitchell et al. 1989; Bergman et al. 1990, 1994; Hollerman and Grace 1992; Guridi et al. 1993; Parent and Hazrati 1995b; Hassani et al. 1996; Levy et al. 1997, 2002; Blandini et al. 2000; Hirsch et al. 2000; Ni et al. 2000; Alvarez et al. 2001; Guridi and Obeso 2001; Magill et al. 2001; Marsden et al. 2001; Rodriguez-Oroz et al. 2001; Bevan et al. 2002; Houeto et al. 2002; Salin et al. 2002; Tintner and Jankovic 2002; Hamani et al. 2004). Surgery, primarily in the form of the bilateral, high-frequency stimulation of the STN (Benabid et al. 2000), is highly effective in parkinsonian patients who are responsive to levodopa but who experience marked motor fluctuation or other complications (Hamani et al. 2004; Tintner and Jankovic 2002; Perlmutter and Mink 2006; Kleiner-Fisman et al. 2006 and references therein). Houeto et al. (2002) point out that following STN stimulation, the parkinsonian motor disability improved by more than 60% (see also Sect. 2.3 of Part II of The Subthalamic Nucleus) and the levodopa equivalent daily dose was reduced by 60.5%. However, according to Houeto et al. (2002), the improvement in parkinsonian motor disability induced by STN stimulation is not necessarily accompanied by improvement of psychic function.