216 and sensory axons, PSMA by loss of motor axons, and CIAP by length-dependent loss of motor and sensory axons. Cold paresis was reported by two-thirds of patients with distal upper limb muscular atrophy (Hirayama disease), a disorder affecting peripheral motor neurons in the anterior horn of the cervical cord, which results in denervation of hand muscles [23]. Nerve conduction studies in 11 patients with Hirayama disease, and one patient with hypothenar atrophy due to ulnar neuropathy, showed that cold induced excessive conduction delay and waning of the compound muscle action potential during 20-Hz repetitive stimulation [15, 18]. These ﬁndings were attributed to an increased sensitivity of reinnervated muscle ﬁbers to develop depo- larizing conduction block in cold. Cold paresis in MMN was suggested to be related to the existence of inﬂammatory nerve lesions where axons are depolarized but just able to conduct impulses at ambient temperature [11]. At these lesions, thermal reduction of Na/ K-pump activity due to cooling may induce further depo- larization to a point where impulse conduction fails. Some ﬁndings may support this hypothesis. First, animal models of inﬂammatory spinal root lesions indicated that inﬂam- mation may induce nitric oxide-mediated mitochondrial dysfunction, energy depletion of the ATP-ase dependent Na/K-pump, and axonal depolarization [13]. Secondly, excitability studies in MMN indirectly suggested that axons at the site of block may possibly be depolarized [14, 17]. Thirdly, cooling of normal human motor axons results in their depolarization, probably due to decreased Na/K-pump activity [7]. Some of the steps in this hypothesis were corroborated by experiments, but it must be emphasized, however, that its entire sequence has not been proven. Moreover, some experiments were carried out in central nervous system neurons so that the results pos- sibly cannot be extrapolated to peripheral motor neurons. Alternatively, cold paresis in MMN may be caused by development of conduction block in reinnervated muscle ﬁbers due to the lower temperature [15, 18]. The possibility that this mechanism, which may be similar to that descri- bed in Hirayama disease, occurs in MMN is supported by several studies. Extensive needle electromyography studies in MMN patients showed signs of reinnervation, indicating collateral sprouting, in non-atrophic as well as in atrophic muscles [26]. Some pathological studies of motor nerves in MMN have shown prominent loss of axons and little demyelination; the loss of axons may lead to collateral sprouting and reinnervated muscle ﬁbers [24]. In conclusion, symptoms of cold paresis occur in several peripheral nervous system disorders, but are consider- ably more common in MMN. 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