The sense of touch, formally known as somatosensation, allows us to perceive the physical world, from a light breeze to intense heat or pain. This ability can be partially or completely compromised, a loss often experienced as numbness. Numbness is not a disease but a symptom, indicating damage somewhere along the network of nerves that transmits sensory information to the brain. Understanding this impairment requires looking into the biological mechanisms that govern touch and identifying where these processes have been disrupted.
The Biological Basis of Somatosensation
The process of feeling begins with specialized sensory receptors, such as mechanoreceptors, embedded in the skin. These receptors translate physical stimuli into electrical signals that travel along peripheral nerves toward the central nervous system. The nerve cell bodies for these primary neurons are located near the spinal cord in structures called dorsal root ganglia.
The signal enters the spinal cord and ascends toward the brain, often crossing over to the opposite side of the central nervous system in the brainstem. The impulse then passes through the thalamus, a relay station, before reaching its final destination. This crossover means sensation detected on the left side of the body is processed in the right side of the brain.
The conscious perception of touch is processed in the somatosensory cortex, located in the brain’s parietal lobe. This cortical area contains a spatial map of the entire body. Impairment can occur anywhere along this complex pathway, from the skin’s receptors to the final processing center.
Degrees of Altered Sensation
Touch impairment manifests in several ways, ranging from simple numbness to abnormal or painful feelings. The complete inability to feel any sensation, including pain and light touch, is medically termed anesthesia. A less severe presentation, where sensitivity is reduced or diminished, is known as hypoesthesia.
Other manifestations involve abnormal feelings rather than absent ones. Paresthesia describes a non-painful, spontaneous sensation, often felt as “pins and needles” or tingling. This sensation typically arises from temporary pressure on a nerve that disrupts its function.
A more concerning abnormal sensation is dysesthesia, defined as an unpleasant or painful feeling. Dysesthesia can present as a burning, stabbing, or electric shock sensation. Sometimes even light touch, such as clothing brushing the skin, can trigger severe pain.
Specific Causes of Touch Impairment
Damage leading to touch loss is categorized based on whether it affects the peripheral or central nervous system. Peripheral causes involve damage to nerves outside the brain and spinal cord, often resulting in peripheral neuropathy. Diabetes is the leading cause of this damage, as high blood sugar levels compromise the small blood vessels supplying the nerves.
Peripheral Causes
Other peripheral causes include physical trauma that crushes a nerve, certain chemotherapy drugs toxic to nerve cells, and nutritional deficiencies, particularly a lack of Vitamin B12. Infections like HIV or Lyme disease can also trigger nerve damage, sometimes leading to acute conditions like Guillain-Barré syndrome. Sensory loss in these cases usually starts in the longest nerves, affecting the feet and hands first in a “glove and stocking” pattern.
Central Causes
Central causes involve damage to the spinal cord or the brain itself. A stroke, especially one affecting the parietal lobe, can lead to a sudden loss of touch on the opposite side of the body. Conditions causing lesions in the central nervous system, such as Multiple Sclerosis or a spinal cord injury, interrupt ascending sensory pathways. This results in numbness or altered sensation below the level of the damage. Tumors or masses pressing on the spinal cord or brain can also disrupt sensory signal transmission.
Treatment Approaches and Prognosis
Diagnosis begins with a comprehensive neurological exam, testing the patient’s ability to detect light touch, pinprick, vibration, and temperature. Specialized tests assess peripheral nerve health. Nerve Conduction Studies (NCS) measure the speed and strength of electrical signals through the nerves. Electromyography (EMG) evaluates electrical activity within the muscles.
Imaging techniques like Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) scans check for central causes, such as stroke, tumor, or spinal cord compression. Treatment depends on addressing the underlying condition. For example, managing blood sugar levels is necessary for diabetic neuropathy, while surgery may be required to relieve a compressed nerve.
Physical and occupational therapies often include sensory re-education, a process designed to retrain the brain to interpret sensory input correctly. The prognosis for recovery varies significantly. If the underlying cause, such as temporary nerve compression, is relieved early, sensation may fully return. However, if there is severe, permanent damage to the central nervous system, recovery may be limited, though neuroplasticity can lead to some functional improvement.