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Hyporeflexia refers to below normal or absent reflexes (areflexia). It can be detected through the use of a reflex hammer. It is the opposite of hyperreflexia. Hyporeflexia is generally associated with a lower motor neuron deficit (at the alpha motor neurons from spinal cord to muscle), whereas hyperreflexia is often attributed to upper motor neuron lesions (along the long, motor tracts from the brain). The upper motor neurons are thought to inhibit the reflex arc, which is formed by sensory neurons from intrafusal fibers of muscles, lower motor neurons (including alpha and gamma motor fibers) and appurtenant interneurons. Therefore, damage to lower motor neurons will subsequently result in hyporeflexia and/or areflexia.
Note that, in spinal shock, which is commonly seen in the transection of the spinal cord (Spinal cord injury), areflexia can transiently occur below the level of the lesion and can, after some time, become hyperreflexic. Furthermore, cases of severe muscle atrophy or destruction could render the muscle too weak to show any reflex and should not be confused with a neuronal cause.
Hyporeflexia may have other causes, including hypothyroidism, electrolyte imbalance (e.g. excess magnesium), drug induced (e.g. the symptoms of benzodiazepine intoxication include confusion, slurred speech, ataxia, drowsiness, dyspnea, and hyporeflexia).
Diseases associated with hyporeflexia include
- Centronuclear myopathy
- Guillain–Barré syndrome
- Lambert-Eaton myasthenic syndrome
- Polyneuropathy (Achilles and plantar reflexes)
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