Patients with Late-onset Friedrich’s Ataxia May Have Different States of Sensory Nerve Conduction

Patients with late-onset Friedrich’s ataxia (LOFA) may present variable sensory nerve conduction, according to new research.

The study, “Variable Sensory Nerve Conduction Parameters In Late Onset Friedreich Ataxia,” was published in the journal Muscle & Nerve.

Nerve conduction studies are commonly performed by neurology specialists to assess the ability of the nervous system to conduct electrical impulses and to evaluate nerve and muscle function to determine if neuromuscular disease is present.

Friedrich’s ataxia (FA) is characterized by reduced levels of the protein frataxin (FTX) due to anomalies in the gene encoding this protein. These anomalies consist in repeats of DNA sequences within the gene, and the higher the number of repeats, the sooner the onset of FA and associated cardiomyopathy, or chronic disease of the heart muscle, occurs in the patients.

There is increasing recognition of late-onset Friedreich’s ataxia (LOFA), a form of the disease that develops later than those 25 years old. Researchers reported the case of three patients with genetic mutations leading to FA with no cardiomyopathy, but with variable sensory nerve conduction.

Case 1 was a 49-year-old man with difficulty walking and upper limb (arms) incoordination for five years. A few months before his evaluation in the hospital, his speech had changed. After being examined, the patient was found to have mild optic disc pallor (abnormal coloration of the optic disc), cerebellar dysarthria (motor speech disorder), and gait and lower limb ataxia.

The patient had no limb weakness or spasticity. However, vibration sense was impaired in the sternum, and an analysis of the motor nerve conduction revealed limited activation of the upper surface of the foot.

Case 2 was the 45-year-old sister of patient 1, who had balance problems since the age of 39. The patient also had optic disc pallor, cerebellar dysarthria, and gait and limb ataxia. Like her brother, she did not have limb weakness or spasticity. Her motor nerve conduction was normal.

Case 3 was a 68-year-old woman with a 10-year history of a slowly progressive gait disorder with subsequent cerebellar dysarthria and upper limb incoordination. She also had mild cerebellar dysarthria and limb and gait ataxia. There was no weakness or spasticity. Motor nerve conduction was normal. The patient subsequently developed a myocardial infarction.

“The classic nerve conduction findings in FA are of an early and severe sensory neuronopathy, and significantly reduced or absent sensory nerve action potentials (SNAPS) are also reported in LOFA,” the authors wrote.

The authors believe the difference in terms of sensory nerve conduction among these patients are due to larger repeat expansions in the frataxin gene.

“In our patients, the two with the larger … expansions had abnormal [sensory nerve conduction], consistent with previous reports,” the authors wrote. “Overall, in LOFA, [sensory nerve conduction] may be variable, and clinicians should consider genetic testing in patients with late onset ataxia and normal nerve conduction studies.”