Hyporeflexia is the reduction or absence of normal bodily reflexes (areflexia). It can be detected through the use of a

reflex hammer A reflex hammer is a medical instrument used by practitioners to test deep tendon reflexes. Testing for reflexes is an important part of the neurological physical examination in order to detect abnormalities in the central or peripheral nervous s ...

and is the opposite of

hyperreflexia Hyperreflexia is overactive or overresponsive bodily reflexes. Examples of this include twitching and spastic tendencies, which indicate upper disease of the upper motor neurons and the lessening or loss of control ordinarily exerted by higher ...

. Hyporeflexia is generally associated with a deficit in the lower motor neurons (at the

alpha motor neurons Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their ...

from spinal cord to muscle), whereas hyperreflexia is often attributed to

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classif ...

s in the upper motor neurons (along the long, motor tracts from the brain). The upper motor neurons are thought to inhibit the

reflex arc A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons w ...

, 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. In

spinal shock Spinal shock was first explored by Whytt in 1750 as a loss of sensation accompanied by motor paralysis with initial loss but gradual recovery of reflexes, following a spinal cord injury (SCI) – most often a complete transection. Reflexes in the sp ...

, which is commonly seen in the transection of the spinal cord (

Spinal cord injury A spinal cord injury (SCI) is damage to the spinal cord that causes temporary or permanent changes in its function. Symptoms may include loss of muscle function, sensation, or autonomic function in the parts of the body served by the spinal cor ...

), areflexia can transiently occur below the level of the lesion and can later become hyperreflexic. Cases of severe muscle atrophy or destruction may 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), and drug use (e.g. the symptoms of benzodiazepine intoxication include confusion, slurred speech, ataxia, drowsiness, dyspnea, and hyporeflexia). Diseases associated with hyporeflexia include *

Centronuclear myopathy Centronuclear myopathies (CNM) are a group of congenital myopathies where cell nuclei are abnormally located in the center of muscle cells instead of their normal location at the periphery. Symptoms of CNM include severe hypotonia, hypoxia-requi ...

Guillain–Barré syndrome Guillain–Barré syndrome (GBS) is a rapid-onset muscle weakness caused by the immune system damaging the peripheral nervous system. Typically, both sides of the body are involved, and the initial symptoms are changes in sensation or pain oft ...

* Lambert-Eaton myasthenic syndrome * Polyneuropathy (Achilles and plantar reflexes) *

Friedreich's Ataxia Friedreich's ataxia (FRDA or FA) is an autosomal-recessive genetic disease that causes difficulty walking, a loss of sensation in the arms and legs, and impaired speech that worsens over time. Symptoms generally start between 5 and 20 year ...

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References