Spinal muscular atrophy type I mimicking critical illness neuropathy in a paediatric intensive care neonate: electrophysiological features

Sensory Nerve Action Potential Analysis in a Cohort of Patients With Spinal Muscular Atrophy Aged 12 Years and Older

INTRODUCTION/AIMS

Survival Motor Neuron 1 (SMN1)-related spinal muscular atrophy (SMA) is characterized by α-motor neuron degeneration, with sensory function assumed to be clinically preserved. However, recent studies in severely affected patients and animal models have challenged this view. Therefore, we assessed the maximum sensory nerve action potential (SNAP) amplitude of the median nerve in patients with SMA and examined its changes during treatment with SMN-splicing modifying therapies.

METHODS

We longitudinally assessed median nerve maximum SNAPs in 103 genetically confirmed patients with SMA (types 1c-4, aged ≥ 12 years) before and approximately 1 year after treatment with nusinersen or risdiplam. For comparison, we included 53 age- and sex-matched healthy controls, using identical settings. We also compared data with reference values from a previously published cohort.

RESULTS

Maximum SNAPs were abnormal in 6 patients with SMA (6%), which was comparable to controls (8%), even when corrected for age. In patients younger than 50 years, abnormal maximum SNAPs were more prevalent in patients with SMA types 1 and 2. Maximum SNAPs were higher in SMA compared with controls. Maximum SNAPs showed an age-related decline in most cohorts, but the decline was steeper in patients with SMA type 1c. There was no difference in SNAPs after 1 year of treatment.

DISCUSSION

Our findings suggest the preserved sensory integrity of the median nerve in the majority of patients with SMA (94%), even in longstanding disease. The resilience of sensory neurons of the median nerve, and whether this extends to other peripheral nerves, warrants further investigation.

TRIAL REGISTRATION

The study was approved by the local medical ethics committee (no. 20-143) and registered in the Dutch registry for clinical studies and trials (www.toetsingonline.nl-NL72562.041.20, March 26, 2020).

A homozygous missense variant in the YG box domain in an individual with severe spinal muscular atrophy: a case report and variant characterization

The vast majority of severe (Type 0) spinal muscular atrophy (SMA) cases are caused by homozygous deletions of survival motor neuron 1 (SMN1). We report a case in which the patient has two copies of SMN1 but clinically presents as Type 0 SMA. The patient is an African American male carrying a homozygous maternally inherited missense variant (c.796T>C) in a cis-oriented SMN1 duplication on one chromosome and an SMN1 deletion on the other chromosome (genotype: 2*+0). Initial extensive genetic workups including exome sequencing were negative. Deletion analysis used in the initial testing for SMA also failed to detect SMA as the patient has two copies of SMN1. Because of high clinical suspicion, SMA diagnosis was finally confirmed based on full-length SMN1 sequencing. The patient was initially treated with risdiplam and later gene therapy with onasemnogene abeparvovec at 5 months without complications. The patient's muscular weakness has stabilized with mild improvement. The patient is now 28 months old and remains stable and diffusely weak, with stable respiratory ventilatory support. This case highlights challenges in the diagnosis of SMA with a non-deletion genotype and provides a clinical example demonstrating that disruption of functional SMN protein polymerization through an amino acid change in the YG-box domain represents a little known but important pathogenic mechanism for SMA. Clinicians need to be mindful about the limitations of the current diagnostic approach for SMA in detecting non-deletion genotypes.