Abstract
The first step in the management of the neurological patient is to localize the lesion. While taking the history, the neurologist generates an anatomical hypothesis, which subsequently can be confirmed or rejected during the bedside examination. Following this, a working diagnosis is established and ancillary tests are chosen accordingly. Although the anatomy of the nervous system is highly complex, distinct anatomical entities have characteristic features. For instance, the hallmark of a myopathy is symmetric proximal weakness without sensory disturbances. Fatigability together with proximal weakness, including bulbar and oculomotor features, is typical for a disorder of the neuromuscular junction. In contrast, diseases of peripheral nerves, the brachial and lumbosacral plexus, as well as nerve roots usually lead to both motor and sensory deficits. Further, injury to the spinal cord is associated with a triad of paraparesis, a sensory level of the trunk, and sphincter disturbances. Brainstem processes often produce ipsilateral cranial nerve deficits and contralateral sensorimotor signs. While damage of the cerebellar hemispheres causes ataxia and intention tremor of the ipsilateral extremity, lesions of the midline region mainly lead to gait ataxia and truncal instability. Movement disorders due to disease involving the basal ganglia can be divided into hypo- and hyperkinetic disorders. Lesions involving the subcortical white matter frequently induce visual field deficits, complete hemiplegia, and dense numbness. Impairment of higher cognitive function, incomplete hemiparesis (sparing the leg), and epileptic seizures are common signs of cortical disease. This chapter reviews the relevant neuroanatomy from a clinical viewpoint and provides the reader with the tools to perform a competent clinical history.
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Notes
- 1.
Samual A. K. Wilson, 1878–1937; British neurologist and describer of hepatolenticular degeneration (Wilson’s disease).
- 2.
A patient with a recent history of head dropping has a high likelihood of having myasthenia gravis, but other differential diagnoses to keep in mind include amyotrophic lateral sclerosis (motor neuron disease), isolated neck extensor myopathy (including radiation-induced myopathy following neck radiation for malignancy), and multiple system atrophy (neurodegenerative disorder with atypical parkinsonism).
- 3.
From the primary motor cortex in the cerebrum to the great toe, there are only two motor neurons. Thus, in a six-foot-tall man, these upper and lower motor neurons measure three feet each on average. “Corticospinal tract,” “pyramidal tract,” “first motor neuron,” and “central motor neuron” are often used synonymously (but not 100% correctly) for “upper motor neuron” (UMN). Likewise, “second motor neuron,” “alpha motor neuron,” and “peripheral motor neuron” are frequently used instead of “lower motor neuron” (LMN). The UMN cell bodies are found in the motor cortex. The LMN cell bodies are situated in the ventral horns in the spinal cord and in the motor cranial nerve nuclei in the brainstem. Damage to the UMN is “supranuclear”; damage to the LMN “infranuclear.” UMN weakness is characterized by spasticity, hyperreflexia, Babinski sign, and a typical pattern of weakness distribution that will be discussed later. LMN weakness is associated with hyporeflexia, decreased muscle tone, atrophy, and fasciculations.
- 4.
“Aids to the examination of the peripheral nervous system” by O’Brien M. (Saunders Ltd. 5th ed.) is a tremendously helpful pocket manual.
- 5.
Pain and temperature are transmitted by thin type III sensory fibers and unmyelinated type IV fibers. Both fiber types have very small diameters (0.2–5 μm). In contrast, proprioception and vibration are transmitted by large-diameter myelinated type Ia, Ib, and II sensory fibers (6–20 μm).
- 6.
Ataxia due to lost proprioception is called sensory ataxia and must be distinguished from ataxia of cerebellar and vestibular origin. Characteristically, and in contrast to cerebellar ataxia, visual information can compensate for sensory ataxia.
- 7.
Similar to CMT disease type 1A, HNPP is due to mutation of the PMP22 gene. HNPP is caused by a deletion or nonsense mutation of the PMP22 gene in chromosome 17p11.2, the same region that is duplicated in CMT 1A.
- 8.
Lead poisoning also leads to microcytic anemia, abdominal pain, and gingival discoloration at the interface of the teeth and gums. Exposure to lead may be due to lead paint found in older housings or glazed ceramic coffee mugs.
- 9.
As stated earlier, the cell bodies of the UMNs are found in the primary motor cortex in the frontal lobe, located on the anterior wall of the central sulcus. The major part of the corticospinal tract crosses the midline at the pyramidal decussation of the medulla oblongata.
- 10.
The axons that build up the posterior column pathway synapse in the medulla oblongata with second-order sensory neurons, the cell bodies of which are found in the gracile nucleus and the cuneate nucleus. The axons of the second sensory neurons cross the midline at the level of the medulla and form the contralateral medial lemniscus pathway. They synapse in the thalamus with third-order neurons, the axons of which end in the parietal lobe’s postcentral gyrus, the primary sensory cortex.
- 11.
Axons forming the spinothalamic tract synapse in the thalamus with third-order neurons which send their axons to the primary sensory cortex.
- 12.
CN I and CN II do not possess nerve nuclei.
- 13.
Please bear in mind that dysphagia, dysarthria, and diplopia are also often due to diseases of the muscle, neuromuscular junction, or cranial nerves. Non-neurological causes are common as well, e.g., dysphagia and dysarthria due to tonsillitis. Also note that distinguishing disturbed speech (dysarthria) from abnormal language (dysphasia) is crucial. The latter is not a brainstem sign; it is a cortical phenomenon.
- 14.
“Supranuclear” means “above the cranial nerve nuclei,” in this case the nuclei of CN III, IV, and VI.
- 15.
“External” and “internal” refer to the external eye muscles, respectively, the muscles of the iris. Thus, an external ophthalmoplegia leaves the pupillary function intact, whereas with an internal CN III paresis, there is mydriasis.
- 16.
Parinaud’s syndrome (also known as dorsal midbrain syndrome) includes supranuclear gaze palsy, pupillary dysfunction characterized by poor reaction to light (but normal to accommodation) and convergence-retraction nystagmus.
- 17.
CN IV is unique in that its axons cross the midline before emerging from the brainstem. Thus, a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei (CN III, CN V, CN XII) affect the ipsilateral side.
- 18.
However, it is not uncommon for neurologists to be asked to evaluate an alert patient after surgery or trauma with a large pupil; these patients often turn out to be friendly and unconcerned people without any other focal signs than habitual anisocoria.
- 19.
The mnemonic Standing Room Only may assist in remembering that V1 passes through the superior orbital fissure, V2 through the foramen rotundum, and V3 through the foramen ovale.
- 20.
Lyme disease is a common cause of isolated (unilateral or bilateral) peripheral nerve palsy in children, whereas in adults, neuroborreliosis almost always leads to other symptoms as well.
- 21.
Some myopathies (e.g., FSH) and myasthenia gravis also frequently lead to bilateral orofacial weakness.
- 22.
In contrast to true vertigo, dizziness is often described as “a lightheaded feeling as if I was about to faint” or as “a spaced out feeling as if I wasn’t quite there.” The latter description usually points to a functional complaint, whereas the former may also suggest a vasovagal or orthostatic event.
- 23.
Vestibular schwannoma is also known as “acoustic neurinoma.” However, this is a misnomer because, first, the vestibular nerve tends to be affected prior to the cochlear nerve and, second, the tumor is a schwannoma, not a neurinoma.
- 24.
The carotid bodies are small clusters of chemoreceptors at the carotid bifurcation and are sensitive to changes in the composition of the arterial blood, e.g., alterations of partial pressure of oxygen and carbon dioxide.
- 25.
The hypothalamus produces vasopressin, oxycontin, and growth hormone-inhibiting hormone, as well as releases hormones for corticotropin, gonadotropin, thyrotropin, and growth hormone.
- 26.
An exception is falx meningioma compressing bilaterally the areas of the motor cortex that represent the lower extremities, thus leading to paraparesis. This is an important, but rare, differential diagnosis to paraplegia of the UMN type.
- 27.
Even in two-thirds of left-handed people, language function is localized in the left hemisphere. Thus, when informed that a patient has dysphasia and left-sided hemiparesis, the first question to ask is whether the patient is left-handed. Often it turns out that he is right-handed, which makes dysphasia very unlikely. In most cases, dysarthria or confusion has then been mistaken for dysphasia.
- 28.
Occasionally, personality changes can be hard to detect for those who have not known the patient prior to the neurological event. A man in his 70 s suffered from a right-sided frontal infarction, but on admission to the hospital, he had no focal deficits. Cheerful and with a pleasant manner, he soon became a favorite with the nurses. The real impact of the infarct did not become clear until a few days later when his wife came for a visit and was totally bewildered that this “miserable old man” had turned into such a lovely person.
- 29.
The neocortex, which represents the vast majority of the cerebral cortex, has six layers and is phylogenetically younger than the archicortex of the three-layered hippocampus.
- 30.
An anatomical variant, both ACA may also arise from a single stem, and occlusion of this stem then causes bilateral infarction of the anterior and medial aspects of both hemispheres. As a result, patients may develop paraparesis, incontinence, and abulia.
- 31.
Conditions such GBS, ALS, or myasthenia gravis may occasionally be associated with afferent denervation that is severe enough to mimic the locked-in syndrome.
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Kondziella, D., Waldemar, G. (2023). Clinical History and Neuroanatomy: “Where Is the Lesion?”. In: Neurology at the Bedside. Springer, Cham. https://doi.org/10.1007/978-3-031-43335-1_2
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