Background Nusinersen, an antisense oligonucleotide (ASO) capable of increasing SMN protein production, has been used in clinical trials in infants and children with SMA type 1 or type 2 and has been associated to a significantly better motor development, motor function, and survival compared with placebo groups. Because of the lacking ability of ASOs to cross the blood-brain barrier, nusinersen is administered intrathecally. While most infants with SMA type 1 die in easy childhood without a muscular and nutritional support, SMA type 2 patients often reach adulthood, and patients with SMA type 3 generally have a normal lifespan. A further feature of SMA patients is a progressive neuromyopathic scoliosis, so spinal nusinersen administration can be challenging because of poorly palpable surface landmarks, as in elderly and in obese patients. In patients with anticipated difficult anatomy, the failure rate of neuraxial anesthesia may be as high as 17%. Furthermore, multiple needle punctures are associated with hematoma, post-dural-puncture headache, back pain, and patient dissatisfaction. Current data suggest that landmark identification using a pre-procedure ultrasound (US) is a useful adjunct to neuraxial anesthesia that facilitates technical performance in obstetric and pediatric patients. In adult patients with difficult spinal anatomy a pre-procedure US reduces the number of attempts and the number of needle passes necessary for successful spinal anesthesia and can predict technical difficulty; notably, compared to fluoroscopy, sonography allows for the elimination of radiation exposure for patient and physician and for a cost reduction for institution. Despite these potential advantages, reports of lumbar central neuraxial blocks US guidance in patients with predicted placement difficulties are limited. Most reports to date involve a small number of patients with normal anatomy.

Methods The study will be conducted in Fondazione Policlinico "A.Gemelli" operatory rooms, Rome, Italy. Written informed consent will be obtained from all patients enrolled in the study. SMA adult patients referring to NEMO center of this institution will be enrolled. Exclusion criteria will be: coagulation defects, patients medicated with anticoagulation, any finding of infection in puncture site, high intracranial pressure. A visit will be performed before the procedure.

Patients will be randomly assigned in ratio 1:1 to one of two groups using a sealed envelope: Group 1 (US-assisted nusinersen administration) and Group 2 (landmark based nusinersen administration). Nusinersen administration will be performed with a single-operator technique by one of two anesthesiologists trained on US-guided or -assisted central neuraxial blocks. With patient in sitting position, or in lateral position if forced posture will be needed, after skin disinfection with 2% chlorhexidine in 70% alcohol, the spinal administration will be performed as follows: Group 1 (US- guided nusinersen administration). The paramedian sagittal oblique view will be used to identify specific lumbar interspaces and perform the procedure. Starting at the sacrum and moving cephalad, the L2-3 or L3-4 intervertebral interspace will be identified. After local anesthesia with lidocaine 2% (3-5 mL), a 25-gauge Withacre spinal needle will be used to identify subarachnoid space. After confirmation of the flow of cerebrospinal fluid and after removing 5 ml of CSF, nusinersen will be administered intrathecally over 1-3 min. Group 2 (landmark based nusinersen administration).

The desired intervertebral space (L2-3 or L3-4) was first identified by manual palpation of surface landmarks and marked on the skin. After local anesthesia with lidocaine 2% (3-5 mL) a 25-gauge Withacre spinal needle will be used. After confirmation of the flow of cerebrospinal fluid and after removing 5 ml of CSF, nusinersen will be administered as previously described. After successful administration, patients will be positioned in the supine position for 2-3 h. Within 72 hours of administration a phone survey will be conducted to evaluate the development of postdural puncture headache. Another outpatient visit will be performed at 30 days after the procedure.