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Autosomal recessive axonal neuropathy with neuromyotonia

Autosomal recessive axonal neuropathy with neuromyotonia



Overview

Autosomal recessive axonal neuropathy with neuromyotonia is a rare inherited disorder that damages peripheral nerves, causing muscle weakness and delayed relaxation.

Symptoms

• Muscle Weakness and Wasting (Atrophy): Affects the lower legs, feet, and hands, often starting in childhood or adolescence.

• Gait and Mobility Issues: Leads to an unusual walking style (abnormal gait), foot drop, and frequent falls.

• Exercise Intolerance: Muscle weakness becomes much more noticeable during physical exertion.

• Joint Deformities (Contractures): Causes tightening of the tendons in the hands and feet.

• Neuromyotonia (Isaac Syndrome): Results in nerve overactivation, causing delayed muscle relaxation after tensing, painful muscle cramps, and muscle stiffness.

• Myokymia: Involuntary, rippling, or twitching movements under the skin of the muscles at rest.

• Sensory Impairment: Occasionally causes a mild reduction in the ability to feel touch, heat, or cold in the limbs.

• Reflex Changes: Deep tendon reflexes are often decreased or completely absent.

Causes

• HINT1 Gene: Encodes the histidine triad nucleotide-binding protein 1, which is found in peripheral nerves and involved in cell signaling pathways.

• Mutation Type: Recessive loss-of-function variants or homozygous/compound heterozygous mutations (such as the common founder mutation R37P).

• Axon Damage: Loss of HINT1 protein function leads to damage of the axons (nerve fibers transmitting signals) primarily in motor peripheral nerves.

• Nerve Hyperexcitability: The damage leads to peripheral nerve hyperexcitability, causing neuromyotonia (delayed muscle relaxation, cramps, and continuous muscle twitching/myokymia)

Diagnosis

• Clinical Evaluation: Assessment begins with patient history, family history, and physical examination focusing on progressive distal muscle weakness, muscle wasting in the hands and feet, delayed muscle relaxation (pseudomyotonia), and muscle twitching (myokymia).

• Electrophysiological Studies: Needle electromyography (EMG) and nerve conduction velocities (NCV) are performed. They typically reveal a chronic motor-predominant axonal neuropathy alongside characteristic continuous, high-frequency neuromyotonic or myokymic discharges.

• Molecular Genetic Testing: Diagnosis is confirmed by identifying disease-causing homozygous or compound heterozygous mutations in the HINT1 gene (histidine triad nucleotide-binding protein 1), which accounts for the vast majority of inherited axonal neuropathy cases with neuromyotonia.

Treatment 

Sodium-channel blockers: Medications like carbamazepine or phenytoin help reduce muscle stiffness, cramps, and delayed relaxation caused by nerve hyperexcitability.

• Physical therapy: Regular exercise routines help maintain muscle strength and mobility.

• Orthotic devices: Ankle-foot orthoses (AFOs) or specialized shoes assist with gait impairment and prevent foot drop.

• Orthopedic management: Surgeries or interventions may be used to address severe joint contractures in the hands and feet

Type of Doctor Department : A neurologist

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