Hemispheres in the structure of topical diagnostics
Following the analysis of brainstem syndromes, it is logical to move on to lesions of the cerebral hemispheres. In terms of frequency, lesions are distributed as follows: brainstem lesions rank first, followed by hemispheric, and then spinal lesions. Before localizing a lesion in the hemisphere, it is necessary to exclude signs of brainstem involvement: alternating syndromes (Jackson, Millard-Gubler, Foville, Weber, Benedikt, Claude), bulbar syndrome, cranial nerve palsies, and gaze disorders.
If there are no brainstem signs, but there is a brachiofacial (cerebral pathway-type) hemiparesis, central paresis of the facial muscles and tongue, and higher mental function impairments, the lesion is located in the hemisphere.
Lateralization of the lesion
The first step in a hemispheric lesion is to determine the side. Motor and sensory disorders manifest on the side contralateral to the lesion: left-sided hemihypesthesia indicates a lesion in the right hemisphere. The presence of aphasia (motor, sensory, amnesic) in a right-handed person indicates involvement of the left, dominant hemisphere. The right hemisphere is more often responsible for visuospatial impairments and anosognosia.
Cortical lesion
A cortical lesion manifests primarily as impaired higher mental functions: aphasia, apraxia, agnosia. Motor disorders present as monoparesis (the hand is more frequently affected, reflecting the somatotopy of the precentral gyrus).
- Muscle tone in an isolated cortical lesion is not elevated, as the pyramidal system is predominantly affected, while tone is regulated by the extrapyramidal system.
- Sensory disturbance has a patchy (mosaic) type: deficits do not correspond to the innervation zones of peripheral nerves or roots.
- Cortical irritation causes Jacksonian epileptic seizures — focal motor paroxysms typical of tumors and traumatic brain injury.
Subcortical white matter (corona radiata)
A lesion of the corona radiata is more common than a cortical one. The mnemonic rule is «2 hemi»: hemiparesis and hemihypesthesia on the contralateral side. A characteristic feature is the uneven severity of the disorders: for example, muscle strength in the arm is 1 point and in the leg 4 points, or vice versa, depending on which fibers of the corona radiata are damaged.
Bilateral lesions of the corona radiata lead to pseudobulbar syndrome. Unlike true bulbar syndrome, in which oral automaticity reflexes are diminished, in pseudobulbar syndrome they are exaggerated. Characteristic features include:
- oral automaticity reflexes: palmomental (Marinesco-Radovici), snout, sucking, grasping reflexes;
- forced laughter and crying as an extreme manifestation of the disinhibition of subcortical automatisms;
- dysarthria and dysphagia with preserved soft palate reflexes.
Pseudobulbar syndrome frequently occurs in multiple sclerosis, recurrent lacunar infarctions, and dyscirculatory encephalopathy.
Internal capsule
A lesion of the internal capsule (capsula interna) produces the classic «three hemi» syndrome on the side contralateral to the lesion:
- Hemiplegia (or pronounced hemiparesis) due to damage to the pyramidal tract in the posterior limb;
- Hemihypesthesia due to involvement of thalamocortical fibers;
- Homonymous hemianopsia due to involvement of the optic radiation.
Difference from the corona radiata: in internal capsule lesions, motor and sensory deficits are evenly expressed in both the arm and the leg, muscle tone is spastic, and pathological plantar signs are present. Capsular syndrome may be accompanied by central paresis of the facial muscles and tongue on the same side as the hemiparesis.
Comparative summary of levels
- Cortex: monoparesis, HMF impairments, patchy hypesthesia, epileptic seizures, no increase in tone.
- Corona radiata: 2 hemis (paresis + hypesthesia) with uneven distribution; in bilateral processes — pseudobulbar syndrome.
- Internal capsule: 3 hemis (plegia + hypesthesia + hemianopsia), evenly distributed hemitype, spastic tone.
- Thalamus: all types of hemihypesthesia, thalamic pain, hemiataxia — discussed separately in the analysis of subcortical structures.
Key points
- The hemispheric level is established only after excluding brainstem syndromes.
- The side of the lesion is determined by contralateral symptoms; aphasia points to the dominant hemisphere.
- Cortex: monoparesis, mosaic hypesthesia, HMF impairments, focal epileptic seizures.
- Corona radiata: the «2 hemi» rule with uneven severity in the arm and leg.
- Bilateral corona radiata lesion — pseudobulbar syndrome with exaggerated oral automaticity reflexes.
- Internal capsule: the «3 hemi» rule — hemiplegia, hemihypesthesia, hemianopsia.
- Pseudobulbar syndrome differs from bulbar syndrome by reflex exaggeration and forced laughter and crying.