Upper Motor Neuron Lesion: This is the most common cause of hyperreflexia, which can result from a variety of conditions affecting the central nervous system, such as stroke, spinal cord injury, or multiple sclerosis. The upper motor neuron lesion leads to the loss of inhibitory control over the lower motor neurons, resulting in increased reflexes.

Anxiety or Stress: Anxiety and stress can cause an increase in reflexes due to the heightened state of arousal and increased muscle tone.
Thyroid Disorders: Hyperthyroidism can cause an increase in reflexes due to the increased metabolic state and heightened nervous system activity.
Electrolyte Imbalances: Certain electrolyte imbalances, such as hypocalcemia or hypomagnesemia, can affect nerve function and lead to increased reflexes.

Subarachnoid Hemorrhage: Although less common, a subarachnoid hemorrhage can cause sudden and severe hyperreflexia, and missing this diagnosis can be catastrophic.
Spinal Cord Compression: Compression of the spinal cord can cause hyperreflexia, and prompt diagnosis and treatment are crucial to prevent permanent damage.
Multiple Sclerosis: While not always immediately life-threatening, multiple sclerosis can have a significant impact on quality of life, and early diagnosis and treatment can help manage the condition.

Rabies: Rabies is a rare but potentially deadly viral infection that can cause hyperreflexia, among other symptoms.
Tetanus: Tetanus is a rare bacterial infection that can cause muscle stiffness and hyperreflexia.
Neurodegenerative Disorders: Certain rare neurodegenerative disorders, such as amyotrophic lateral sclerosis (ALS) or progressive supranuclear palsy, can cause hyperreflexia as part of their clinical presentation.