Segmentally dissociated sensory disorders
This type occurs when the gray matter of the spinal cord is damaged. Its main characteristic is sensory dissociation (separation): certain types of perception are lost while others remain intact in the same area.
- Mechanism of development: Pain and temperature fibers terminate in the posterior horns (cornu posterius). When these are destroyed, superficial sensation is lost. Deep sensation fibers (vibration, muscle-joint sense) do not enter the gray matter, but ascend via the posterior funiculus (funiculus posterior), and therefore remain intact.
- Clinical presentation: A unilateral lesion forms a «half-jacket»-type area of anesthesia. Bilateral damage (e.g., in syringomyelia at the cervical and thoracic segments) results in a symmetrical «jacket»-type area.
- Lesion of the anterior gray commissure (commissura grisea anterior): In this structure, the axons of second-order neurons decussate to form the spinothalamic tract (tractus spinothalamicus). Destruction of the commissure interrupts pathways from both sides of the body, producing bilateral segmental anesthesia.
Spinal tract sensory disorders
Develops when the white matter (neural pathways) is damaged. It is characterized by a decrease (hypesthesia) or complete loss (anesthesia) of sensation strictly below the level of the lesion.
- Complete transverse lesion: Total loss of all sensory modalities below the lesion. In thoracic segment trauma, this combines with lower central paraplegia. Cervical cord damage causes flaccid paralysis of the arms and spastic paralysis of the legs. Pelvic organ function is also impaired due to the interruption of afferent and efferent pathways.
- Brown-Séquard syndrome (hemisection): Occurs, for example, due to spinal cord compression by a tumor. On the side of the lesion (ipsilaterally), deep sensation is lost. On the opposite side (contralaterally), pain and temperature sensation are lost, beginning 1–2 segments below the lesion (due to fiber decussation in the anterior white commissure).
- Isolated lesion of the spinothalamic tract: A focus in the lateral funiculus (funiculus lateralis) causes the loss of superficial sensation on the contralateral side. Important topical sign: the actual level of the lesion is 1–2 segments higher than the upper border of cutaneous anesthesia, since the axons ascend before crossing (decussatio).
Cerebral and brainstem syndromes
Damage to brain structures creates extensive sensory loss zones, often involving half of the body.
- Alternating hemianesthesia: Characteristic of unilateral brainstem lesions (most commonly due to strokes). Sensation is impaired in a crossed pattern: on the face—on the side of the lesion (damage to the trigeminal nerve nucleus), and on the trunk and limbs—on the opposite side (damage to the spinothalamic pathway).
- Cerebral tract sensory disorder: Manifests as hemihypesthesia or hemianesthesia on the side contralateral to the lesion. It occurs when the parietal lobe (lobus parietalis), internal capsule (capsula interna), corona radiata, or thalamus is damaged.
- Thalamic syndrome: A lesion of the thalamus (thalamus) causes not only hemihypesthesia but also central neuropathic pain. The pain is burning and stabbing in character. Allodynia (pain triggered by light touch) and hyperpathia (an elevated perception threshold where a stimulus causes a delayed, agonizing, and prolonged pain response) are observed.
- Cortical sensory disorder: A lesion in the postcentral gyrus produces localized deficits (affecting only the head, arm, or leg) on the contralateral side.
- Psychogenic disorders: In hysterical (conversion) hemihypesthesia, the border of the sensory deficit runs strictly along the midline of the body. In organic lesions, such a sharp boundary does not occur due to the overlap of innervation zones.
Briefly: answers to topical questions
Which pathways form the posterior funiculus of the spinal cord?
The posterior funiculus of the spinal cord is formed by the ascending somatosensory pathways:
- Gracile fasciculus (fasciculus of Goll, fasciculus gracilis) — located medially.
- Cuneate fasciculus (fasciculus of Burdach, fasciculus cuneatus) — located laterally.
They conduct proprioceptive (muscle-joint), vibratory, tactile, and discriminative sensations. The fibers of these fasciculi terminate in the corresponding nuclei of the lower medulla oblongata, where the second-order neurons of the sensory pathway are located.
Which thalamic nuclei serve as subcortical sensory centers?
The subcortical centers for somatosensory perception are the ventral posterior nuclei of the thalamus:
- Ventral posterolateral nucleus (VPL) — the termination site of the medial lemniscus; housing the third-order neurons of the proprioceptive pathway.
- Ventral posteromedial nucleus (VPM) — the sensory nucleus of the thalamus; damage to it can cause sensory disturbances and thalamic pain.
Which sensory modalities belong to deep sensation (proprioception)?
Deep sensation (proprioception) includes:
- Muscle-joint sense — the sensation of body and limb position in space.
- Vibratory sense.
- Weight sense.
- Sense of skin fold movement.
Receptors for deep sensation are located in muscles, tendons, ligaments, joints, the labyrinth, and semicircular canals.
How are sensory loss zones on the face distributed in lesions of the trigeminal nerve nucleus?
When the sensory nucleus of the trigeminal nerve is damaged, sensory disturbance zones are distributed in a segmental pattern forming concentric brackets (Zelder's zones).
- Upper part of the nucleus (nucleus tractus spinalis n. trigemini) — causes hypesthesia in the inner segment of the face, including the perioral and perinasal areas.
- Middle part of the nucleus (nucleus tractus spinalis n. trigemini) — causes sensory impairment in the middle part of the face, including the eye region.
- Lower part of the nucleus (nucleus tractus spinalis n. trigemini) — leads to hypesthesia in the outer segment of the face (lateral regions).
The lesion is accompanied by segmentally dissociated loss of pain and temperature sensation.
The «jacket» rule: damage to the spinal cord gray matter (posterior horns) puts a «jacket» or «half-jacket» on the patient—only pain and temperature sensations are lost, while deep sensation (vibration, joints) remains intact.
Class and exam questions
Why is temperature sensation lost on the healthy side in Brown-Séquard syndrome?
Because the fibers of the spinothalamic tract, which carry pain and temperature information, decussate within the spinal cord. Damage to one half of the spinal cord interrupts pathways that have already crossed over from the opposite side of the body.
How to distinguish psychogenic sensory loss from organic sensory loss?
In psychogenic (hysterical) disorders, the boundary of sensory loss runs perfectly straight along the body's midline. Anatomically, this is impossible due to the natural overlap of adjacent nerve innervation zones.
What is sensory dissociation?
This is a pathological condition in which one sensory modality (e.g., pain and temperature) is completely lost, while another (muscle-joint sense) remains preserved in the exact same area.
Why does the cutaneous border of anesthesia not coincide with the level of the lesion in spinothalamic tract damage?
The axons of second-order neurons, emerging from the posterior horns, ascend 1–2 segments before decussating. Therefore, the actual lesion in the spinal cord is always located above the cutaneous boundary of the deficits.
What else to study on this topic
- Topical diagnosis of internal capsule and corona radiata lesions
- Mechanisms of allodynia and hyperpathia in thalamic syndrome
- Clinical presentation of syringomyelia and anterior gray commissure lesions
- Differential diagnosis of cervical and thoracic spinal cord injuries
- Anatomy of the spinothalamic tract and posterior funiculi decussation
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