Diagnostic and prognostic value of Transcranial Magnetic Stimulation in Mucopolysaccharidosis-related cervical myelopathy

Introduction: Mucopolysaccharidosis (MPS) are lysosomal diseases due to defective catabolism and storage of glycosaminoglycans in the skeleton and soft tissues. Cervical myelopathy (CM), the main cause of neurological morbidity and disability in MPS [1], is caused by atlanto-axial subluxation, odontoid hypoplasia, periodontoid soft tissue masses, and spinal canal narrowing, eventually leading to spinal cord compression. Given that early CM detection is associated with best surgical and post-operative outcome, accurate diagnosis and strict monitoring are recommended [2]. We applied Transcranial Magnetic Stimulation (TMS) in MPS-related CM. Materials / Methods: Eight patients (two males), median age 14.5 years (range 13.0-41.0), were included. Four of them, with clinical and MRI signs of CM, had previous surgical decompression, although three still complained symptoms. The other subjects did not have significant clinical or radiological evidence of CM. Motor evoked potentials (MEPs) to single-pulse TMS were recorded from the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles, bilaterally. MEPs latency, shape, and amplitude, as well as central motor conduction time (CMCT) were recorded at rest and during moderate isometric voluntary contraction [3]. Results: Among those underwent surgery, MEPs were absent from FDI of two patients and TA of one patient. Motor latencies were prolonged in all treated patients; in three of them, MEPs had also reduced amplitude and polyphasic shape. CMCT was increased in two of treated subjects from both upper and lower limbs, whereas responses to cervical root stimulation could not be evoked in the other two. Among the four apparent neurologically normal subjects, MEPs were abnormal in terms of reduced amplitude, increased latency, or altered shape in at least one of the examined muscles. Discussion: Abnormal TMS findings from both upper and lower limbs were consistent with diffuse axonal damage and demyelination, suggesting that a spinal disease was clinically present before the occurrence of an overt CM and might persist despite surgery. However, MEPs analysis also revealed functional impairment even in patients without a clear evidence of CM, thus allowing a preclinical diagnosis. Conclusions: TMS was able to detect MPS-related CM, even subclinically, and to provide reliable electrophysiological data after surgical decompression. Objectives: TMS is a safe, painless, and non-invasive neurophysiological technique assessing excitability and conductivity of the cortical-spinal tract. TMS screening for CM should be performed in MPS. Baseline and longitudinal exams are helpful for an early diagnosis and prognosis. References: [1] Galimberti, et al. Ital J Pediatr 2018 [2] Boor, et al. Neuropediatrics 2000 [3] Rossini, et al. Clin Neurophysiol 2015