Sensory nerves — those responsible for transmitting sensory information — are progressively damaged in children with spinal muscular atrophy (SMA) type 1 as these patients age, according to a small study in Italy.
Importantly, major functional deficits in the sensory nerves of both the arms and the legs were detected in children ages 7 and older. These findings provide the first evidence that this type of damage is related to age, but not to nerve length, according to the researchers.
The results add to the increasing number of studies highlighting the body-wide nature of SMA, particularly in its more severe forms, affecting not only motor neurons, but also other cells and organs.
Larger studies are needed to confirm these findings and to assess the clinical value of sensory nerve tests to monitor disease progression in this patient population, the researchers said.
The study, “Age-related sensory neuropathy in patients with Spinal Muscular Atrophy type 1,” was published in the journal Muscle & Nerve.
SMA is caused by low to no levels of SMN, a protein essential for motor neuron and muscle health, which leads to muscle wasting and weakness. However, a growing body of evidence suggests that “there are other neurological cells and other tissue types that are selectively vulnerable to reduced levels of SMN protein,” the researchers wrote.
Particularly, a few studies in both SMA type 1 animal models and patients have reported sensory nerve impairment and sensory-motor connectivity defects, suggesting the presence of axonal sensory neuropathy, or damage to the axons (fibers) of sensory nerves.
“However, it is still not clear whether sensory neuropathy in SMA 1 is an age and/or [nerve] length-dependent process,” the researchers wrote.
To find out, a team of researchers at the Bambino Gesù Children’s Hospital, in Rome, evaluated the velocity and intensity of electrical signals transmitted by sensory nerves of different lengths in 28 SMA type 1 patients, aged 0 to 16 years, and in 93 healthy individuals, who served as controls.
These parameters were obtained through nerve conduction studies in two sensory nerves: one located in the leg and the other in the arm.
Notably, lower velocities typically suggest damage to myelin — the fatty sheath surrounding axons that allows them to transmit electric signals at high speed — while weak electric signals support the presence of axon degeneration.
The children with SMA type 1 — 20 girls and eight boys — had a mean age of 6.73 years, while the healthy participants, which included 58 girls and 35 boys, had a mean age of 7.03 years.
To help in the comparative analysis, the participants were divided into three age groups: 6–24 months, 3–6 years, and 7–16 years.
Sensory nerve damage due to other potential causes was ruled out in all patients, and healthy participants had no abnormal neurological signs or symptoms.
Up until age 6, the velocities and intensities of electrical signals transmitted by sensory nerves did not differ between healthy children and those with SMA type 1.
However, in the older group, with participants ages 7–16 years, the signal intensity from both leg and arm sensory nerves was significantly lower in SMA type 1 patients as compared with healthy individuals.
Notably, no electric signal intensity was detected in the leg sensory nerve of two 12-year-old patients, nor in both the leg and arm sensory nerves in two other adolescent patients, ages 14 and 16.
SMA type 1 patients older than 7 also showed significantly slower conduction velocities in the sensory nerve of the leg, but not of the arm, relative to healthy participants of the same age group.
These findings support the presence of an age-dependent axonal sensory neuropathy in SMA type 1 patients that affects both arms and legs.
When comparing these results with those of an external study in a healthy pediatric population, the researchers found that children with type 1 disease have normal nerve signal intensity until age 6, after which it declines. Signal velocity, meanwhile, begins to slow at age 3.
“Our study reveals a connection between the development of an axonal sensory neuropathy and increasing of age in SMA 1 patients,” the team wrote, adding that monitoring sensory nerve signal intensity “could therefore provide further information on the evolution of the disease.”
They also proposed that nerve conductions studies of sensory nerves be included in the evaluation of patients with this severe type of SMA, which may help “the development of new therapeutic strategies that will benefit and support all SMA patients.”
Still, further studies including a larger number of patients, who are followed over time are needed to confirm these findings, the researchers noted.
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