Hyporeflexia

Hyporeflexia is usually the result of damage to the motor neurons in the central nervous system. Motor neurons are responsible for transmitting signals from the brain to the rest of the body to produce muscle movement. Damage or changes to the motor neurons can be caused by underlying central nervous system disorders. 

More specifically, hyporeflexia is linked with lower motor neuron (LMN) lesions, which are lesions associated with damage to the motor neurons that send messages between the spine and the rest of the body. In contrast, damage to the motor neurons that send signals from the brain to the spine are known as upper motor neuron (UMN) lesions and they can produce hyperreflexia. 

As such, hyporeflexia can be caused by lower motor neuron diseases, such as spinal muscular atrophy (SMA), Guillain-Barré syndrome (GBS), and chronic inflammatory demyelinating polyneuropathy (CIPD); mixed motor neuron conditions like amyotrophic lateral sclerosis (ALS); and spinal cord injuries. Hypothyroidism (i.e., low levels of thyroid hormones) can also cause hyporeflexia due to muscle weakness and slow nerve conduction. 

Signs and symptoms of hyporeflexia usually appear gradually and include muscle weakness that can increase over time. At first, an individual with hyporeflexia may only notice a slight decrease in muscle strength; over time, however, they may have difficulty holding objects, walking, and standing up straight. Additionally, individuals may experience muscle atrophy.  

Other signs and symptoms may be present depending on the underlying cause. For instance, individuals with ALS often also experience slurred speech, difficulty walking, and, eventually, difficulty breathing. Those with GBS usually present with a rapid onset numbness and paralysis of the arms, legs, face, and breathing muscles. CIDP often presents with weakness as well as numbness and tingling in the arms and legs and can eventually progress to a complete loss of normal muscle reflexes. Finally, with hypothyroidism, individuals may also experience fatigue, changes to body temperature, constipation, dry skin, weight gain, slowed heart rate, as well as pain and stiffness in the muscles and joints.  

Hyporeflexia is diagnosed through a neurological examination that evaluates the deep tendon reflexes (DTRs) using a reflex hammer. Some of the common reflexes tested include the biceps, brachioradialis, and triceps reflexes in the arm, as well as the patellar and Achilles reflexes in the leg. Normally, tapping the reflex hammer at specific tendon should elicit an immediate contraction in the relevant muscle, thereby assessing the integrity of the associated reflex arc. With hyporeflexia, there is a weakened or diminished muscle contraction in response to the hammer tap. No contraction at all is termed areflexia.  

Upon diagnosis of hyporeflexia, additional tests may be performed in order to determine the underlying cause. Blood tests, nerve conduction studies, magnetic resonance imaging (MRI), and muscle biopsies may be required. A diagnostic test called electromyography (EMG) may also be used to evaluate the muscle response and the electrical activity of the muscles.