The sense of touch, formally known as somatosensation, is a complex sensory input that allows the body to interact with its environment and monitor its internal state. This system perceives stimuli such as pressure, vibration, temperature, and pain. When somatosensation is disrupted, the body can experience a loss of feeling. This impairment can manifest as a partial reduction of feeling, known as hypoesthesia, or as the complete inability to sense stimuli, termed anesthesia.
The Somatosensory System
The process of feeling begins with specialized sensory receptors embedded within the skin, muscles, and joints. These receptors convert physical stimuli into electrical signals that the nervous system interprets. Mechanoreceptors detect physical deformation like touch and pressure, thermoreceptors respond to temperature changes, and nociceptors signal painful stimuli.
These electrical signals travel along peripheral nerves, which are bundles of nerve fibers extending from the spinal cord and brain. The signals enter the central nervous system through the dorsal root ganglia. They then ascend the spinal cord toward the brain, following distinct pathways based on the type of sensation.
The destination for conscious perception of touch is the somatosensory cortex, located in the parietal lobe. Before reaching the cortex, signals pass through the thalamus, a relay center that filters and directs sensory information. Damage to any point along this chain—from the receptors to the peripheral nerves, spinal cord, or brain centers—can result in an altered or lost sense of touch.
A temporary loss of sensation, such as a limb “falling asleep,” occurs when a nerve is briefly compressed, interrupting signal transmission. Chronic sensory loss, however, involves actual damage to the structure or function of these neural components. The location and extent of the damage determine whether the loss is partial or complete, isolated to a small area, or spread across large sections of the body.
Common Causes of Sensory Loss
Damage to the somatosensory system often starts in the peripheral nerves, a condition categorized as peripheral neuropathy. The most frequent chronic cause is diabetes mellitus, where high blood glucose levels gradually injure nerve fibers. This leads to a progressive loss of sensation, commonly beginning in the feet and hands in a characteristic “stocking-glove” pattern.
Other systemic issues can trigger peripheral neuropathy, including nutritional deficits like vitamin B12 deficiency, which is necessary for nerve health. Exposure to toxins, such as excessive alcohol or certain chemotherapy agents, can also lead to widespread nerve damage. In these scenarios, the impairment results from the nerves deteriorating, preventing the transmission of sensory signals.
Sensory loss can also arise from problems within the central nervous system, involving the spinal cord or the brain. Spinal cord injuries can sever ascending nerve tracts, causing a complete loss of sensation below the level of damage. A stroke affecting the parietal lobe or the thalamus can impair the brain’s ability to process touch signals, often resulting in sensory loss on one side of the body.
Acute and localized causes involve direct trauma or mechanical compression of a specific nerve. Carpal tunnel syndrome occurs when swelling or displaced tissue puts pressure on the median nerve in the wrist. Infections such as herpes zoster (shingles) can also cause painful, acute sensory loss by attacking the sensory neurons of a particular nerve root.
Diagnosis and Management
Identifying the cause and location of sensory loss begins with a detailed sensory examination conducted by a healthcare professional. This process includes tests like two-point discrimination, which determines the smallest distance a person can feel two distinct points of touch. Doctors also use a monofilament test, applying a fine nylon fiber to the skin to assess the ability to feel light pressure.
To confirm nerve involvement and pinpoint the site of the problem, electrodiagnostic tests are employed. Nerve conduction studies (NCS) measure how quickly electrical signals travel through a nerve, identifying demyelination or axonal damage. Imaging techniques, such as magnetic resonance imaging (MRI), are used when central damage, like a stroke or spinal cord compression, is suspected.
Management focuses on addressing the underlying condition responsible for the sensory impairment. For diabetic neuropathy, rigorous control of blood sugar levels is the primary goal to halt further nerve deterioration. If nerve compression is the cause, such as carpal tunnel syndrome, treatment may range from splinting and physical therapy to surgical intervention to relieve pressure.
Medications are used to manage symptoms, particularly the neuropathic pain that can accompany sensory loss, using agents such as anticonvulsants or certain antidepressants. While sensory loss caused by transient compression may resolve completely, recovery from significant nerve or central nervous system damage is often limited. Physical and occupational therapy can help individuals adapt to their sensory deficits and maximize remaining function.