Areflexia is an absence or a loss of tendon reflexes. This may be physiological, in that some individuals never demonstrate tendon reflexes, or pathological, reflecting an anatomical interruption or physiological dysfunction at any point along the monosynaptic reflex pathway, which is the neuroanatomical substrate of phasic stretch reflexes. Sudden tendon stretch, as produced by a sharp blow from a tendon hammer, activates muscle spindle Ia afferents which pass to the ventral horn of the spinal cord, there activating α-motor neurones, the efferent limb of the reflex, so completing the monosynaptic arc. Hence, although reflexes are typically regarded as part of the examination of the motor system, reflex loss may also occur in "sensory" disorders, affecting the Ia afferents from the muscle spindle. It is often possible to "hear" that reflexes are absent from the thud of tendon hammer on tendon.
Areflexia is most often encountered in disorders of lower motor neurones, specifically radiculopathies, plexopathies and neuropathies (axonal and demyelinating). Areflexia may also occur in neuromuscular junction disorders, such as the Lambert-Eaton myasthenic syndrome, in which condition the reflexes may be "restored" following forced muscular contraction (facilitation). Transient areflexia may be seen in central nervous system disorders, such as cataplexy, and in acute spinal cord syndromes ("spinal shock", e.g., acute compression, acute inflammatory myelopathy).