Hypoesthesia and Anesthesia
Depending on the scale of sensory pathway involvement, two main forms of reduced or lost sensitivity are distinguished:
- Total Form. Occurs with extensive damage (nerve roots, medial lemniscus, internal capsule, postcentral gyrus). The patient completely loses the ability to perceive all types of stimuli.
- Partial Form. Develops when the posterior horns, dorsal columns of the spinal cord, or structures of the medulla oblongata are affected. Only specific modalities of sensation are impaired (e.g., pain or tactile sensation).
Specific partial disorders include astereognosis (the inability to recognize objects by touch) and topoanesthesia (the loss of ability to localize where a stimulus was applied).
Mechanisms and Etiology
Sensory loss can develop due to receptor dysfunction, a reduced number of receptors on cell membranes, or blocked signal conduction (e.g., in diabetic polyneuropathy or lead poisoning). In some cases, the central perception apparatus (thalamus, cortex) is affected.
Main causes include:
- Nervous tissue trauma.
- Strokes and circulatory disorders.
- Degenerative diseases (syringomyelia).
- Central nervous system tumors.
- Encephalitis and developmental anomalies of sensory pathways.
Hyperesthesia and Dysesthesia
Hyperesthesia (hyper — over, aisthesis — feeling) is an exaggerated reaction to a stimulus. It can be receptor-based (e.g., in burns or herpes) or central (in neuroses, where cortical and hippocampal excitability is increased).
Dysesthesia is an abnormal, unpleasant perception of a stimulus. Its main variants include:
- Thermalgesia: heat or cold is perceived as pain.
- Polyesthesia: a single stimulus is perceived as multiple distinct stimuli (a single pinprick feels like burning and pressure).
- Allodynia: a non-painful stimulus evokes pain.
- Hyperpathia: an exaggerated, poorly localized pain response, even to a light touch.
- Paresthesia: "pins and needles" or numbness occurring without external stimulation (during ischemia or nerve root compression).
Levels of Sensory Disturbances
All sensory disorders can be divided into three levels depending on the localization of the pathology:
- Receptor Level. The sensitivity threshold changes (hypo- or hypersensitization) or the number of receptors changes. Causes include hypoxia, hypothermia, and sodium-potassium ion imbalances.
- Pathway Level. Occurs due to the inhibition or blockade of impulses within nerve trunks and the spinal cord.
- Central Level. The thalamic nuclei and cerebral cortex are damaged, distorting the actual formation of the sensory experience.