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Hyporeflexia is the condition of below normal or absent reflexes (areflexia). It can be tested for by using a reflex hammer. It is the opposite of a condition called hyperreflexia. Hyporeflexia is generally associated with a lower motor neuron deficit (at the alpha motor neurons from the 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 be inhibitory of 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 lead to hyporeflexia and/or areflexia.
Note that in spinal shock, which is commonly seen in transection of the spinal cord (Spinal cord injury), areflexia can occur transiently below the level of the lesion and after a period of time become hyperreflexic. Furthermore cases of severe muscle atrophy or destruction could render the muscle too weak to show a reflex at all, which should not be confused with a neuronal cause.
Hyporeflexia may have other causes including hypothyroidism, electrolyte imbalance (including 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
- Polyneuropathy (Achilles and plantar reflexes)
- Lambert-Eaton myasthenic syndrome
- Centronuclear myopathy
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