There are a large number of syndromes that may occur with focal lesions of the cerebellum, brainstem, or cerebrum. Focal lesions of the cerebellum or its connections often causes incoordination of walking or limb movements. Focal lesions of the brainstem or cerebrum may cause abnormalities of the lower functions (sensory, motor, or autonomic), or the higher functions (cognition, emotion, or consciousness), depending on the lesion location. Most of the motor abnormalities of focal lesions of the brainstem or cerebrum are usually related to upper motor neuron axons descending through the affected tissues. However, with some brainstem lesions there may also be lower motor neuron abnormalities of muscles connected to certain cranial nerves if their somas are involved.
A focal abnormality of the cerebrum often causes sensory or upper motor neuron abnormalities of the contralateral side, because many of these pathways cross from one side to the other in the central nervous system. In particular, vision, somatosensation, and upper motor neuron control of limb movements are usually crossed functions of the cerebrum, so that an abnormality of one cerebral hemisphere may cause contralateral visual loss, somatosensory loss, or upper motor neuron weakness. A few cognitive functions are also usually performed by one side of the cerebrum. For example, the left cerebral hemisphere performs most language functions in most people, so that an abnormality of that side may cause language dysfunction.
Many types of pathology often cause focal brain syndromes, including idiopathic, vascular, mechanical, immune, and neoplastic disorders. Epileptic disorders often cause focal syndromes of the cerebral cortex. Seizures that start in one focal area of cerebral cortex, which are called focal seizures, may stay in that place, or they may spread to most of the cerebral cortex on both sides, which is then called a focal seizure with secondary generalization. Many idiopathic disorders cause slow degeneration of focal areas of the brain, such as Parkinson’s disease. The vascular disorders ischemic and hemorrhagic stroke commonly cause focal brain syndromes. Ischemic stroke involves an area of insufficient blood flow, and hemorrhagic stroke involves an area of bleeding. Traumatic brain injury is a common mechanical disorder causing focal brain syndromes. The autoimmune disorder multiple sclerosis commonly causes focal demyelinating lesions of the brain. Neoplasia, particularly the spread of cancer from other tissues into the brain, often causes focal syndromes.