Objective: To investigate spinal excitability and plasticity in patients with pure hereditary spastic paraparesis (HSP).

Background: Pure HSP (SPG4) is a neurodegenerative disorder characterized by slowly progressive lower limb weakness and spasticity due to the impairment of corticospinal descending fibres [1]. The pathophysiology underlying motor disorders in HSP is still largely elusive [2]. Also, the possible pathophysiological role of abnormal spinal excitability and plasticity in HSP is unknown.

Method: Fifteen patients with pure HSP (SPG4) and ten age-matched healthy subjects (HS) participated in this study. Patients were clinically assessed by using standardized scales including the Spastic Paraparesis Rating Scale, modified Ashworth scale and Barthel scale. To examine spinal excitability, we evaluated the H reflex (HR) and calculated the reciprocal inhibition (RI) curve, at seven different interstimulus intervals (ISIs), in the soleus (SO) muscle of the dominant limb. To investigate spinal plasticity, we assessed long-term changes in the HR amplitude and RI curve, before and after applying a specific spinal cord plasticity-inducing protocol based on focal muscle vibration (fMV) (i.e., 30-minutes of fMV delivered over the musculotendinous complex of the SO muscle) [3].

Results: When considering excitability, HSP patients showed higher HR amplitude and lower HR inhibition at 2, 3 and 20 ISIs of the RI curve than HS. Concerning plasticity, HS showed significant long-term depression of the HR amplitude at 5 and 30 minutes after fMV delivered over the SO muscle. Conversely, in HSP patients, fMV left the HR amplitude and the RI curve unchanged.

Conclusion: HSP patients are characterized by abnormal spinal cord excitability (e.g. increased motoneuron excitability associated with a decreased excitability of the RI pathways). Also, the reduced modulation of the HR and RI curve following fMV points to abnormal spinal plasticity in HSP. Overall, these findings support the hypothesis that abnormal spinal excitability and plasticity play a role in the pathophysiological mechanisms underlying motor disorders in HSP.

References: [1] de Souza PVS, de Rezende Pinto WBV, de Rezende Batistella GN, Bortholin T, Oliveira ASB. Hereditary Spastic Paraplegia: Clinical and Genetic Hallmarks. Cerebellum. (2017), 16, 525-551. doi: 10.1007/s12311-016-0803-z [2] Crone C, Petersen NT, Nielsen JE, Hansen NL, Nielsen JB. Reciprocal inhibition and corticospinal transmission in the arm and leg in patients with autosomal dominant pure spastic paraparesis (ADPSP). Brain (2004), 127, 2693–2702. doi: 10.1093/brain/awh319 [3] Rocchi L, Suppa A, Leodori G, Celletti C, Camerota F, Rothwell J, Berardelli A. Plasticity Induced in the Human Spinal Cord by Focal Muscle Vibration. Front Neurol. (2018), 9, 935. doi: 10.3389/fneur.2018.00935

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MDS Abstracts - https://www.mdsabstracts.org/abstract/spinal-excitability-and-plasticity-in-hereditary-spastic-paraparesis-a-neurophysiological-study/