Weakness can occur due to lesions anywhere in the motor pathways: brain, brainstem, spinal cord, ventral roots, peripheral nerves, neuromuscular junction, and/or muscles. Sensory changes (loss of sensation, paresthesias, proprioceptive deficits) can occur due to lesions anywhere along the sensory pathways: peripheral nerves, dorsal root ganglia, dorsal roots, spinal cord, brainstem, or brain.
The clinical evaluation of weakness or sensory disturbances requires determining the distribution of the patient’s symptoms (i.e., unilateral, bilateral, proximal, distal) and the time course of the onset and evolution of the patient’s symptoms (i.e., sudden, acute, subacute, chronic, and whether there has been progression and/or fluctuation).
For weakness, the examination must also determine whether the weakness is consistent with localization to the CNS (brain, brainstem, or spinal cord; i.e., upper motor neuron signs on examination), PNS (roots, peripheral nerves; i.e., lower motor neuron signs on examination), neuromuscular junction (i.e., fatigability on examination; see Ch. 29), or muscles (often proximal and symmetric, although not always; see Ch. 30).
For sensory disturbances, the affected sensory modalities must be determined (i.e., pain/temperature and/or vibration/proprioception). Light touch has less localizing value than vibration, proprioception, pain, and temperature.
Understanding the localization of particular distributions of motor and/or sensory symptoms rests on the foundation of the pathways discussed in this chapter.
Involvement of the Motor and Sensory Pathways in the Brain
The motor and sensory cortices are adjacent to one another, and their white matter tracts in the brain (descending from the motor cortex; ascending from the thalamus to the sensory cortex) are also close to one another. Isolated weakness or isolated sensory loss caused by a lesion of the brain can occur if there is a small lesion affecting just one of these pathways (e.g., small embolic stroke affecting just one gyrus, or a small deep [lacunar] infarct in either the posterior limb of the internal capsule [motor] or the thalamus [sensory]). Lesions in the brain will cause contralateral motor and/or sensory deficits. Since the anterolateral and dorsal column pathways are adjacent from the level of the pons until the thalamus (where both synapse in the VPL nucleus), lesions above the level of the medulla usually cause sensory loss affecting all modalities (although very small brainstem lesions affecting just one of these pathways can sometimes occur). Isolated proprioception/vibration or isolated pain/temperature deficit generally suggests a lesion in the lower medulla, spinal cord, dorsal roots/ganglia, or peripheral nerves.
Since the face and arm are adjacent on the cortical surface and share the vascular supply of the middle cerebral artery (the leg region is supplied by the anterior cerebral artery; see Ch. 7), a common pattern of motor and/or sensory changes due to a lesion in the brain is deficits in the face and hand on one side. Since the hand and the the mouth are among the most sensitive and dexterous, they have substantial cortical representation. Therefore, the symptom of sensory changes and/or weakness in the hand and ipsilateral perioral region (cheiro-oral pattern) is highly suggestive of a lesion in the contralateral cerebral hemisphere.
Isolated lesions of the somatosensory cortex may not lead to sensory loss but may lead to higher level sensory deficits. For example, the patient may be unable to recognize a number or letter drawn on the hand (agraphesthesia). If the patient’s left and right sides of the body are touched simultaneously, the patient may not note the sensation in the limb contralateral to the lesion, despite being able to detect sensation when this limb is touched in isolation (extinction to double simultaneous stimulation).
Involvement of the Motor and/or Sensory Pathways in the Brainstem
In the brainstem, the corticospinal tracts travel in the anterior (ventral) portion, whereas the sensory pathways are predominantly dorsolateral (the medial lemniscus beginning in the anterior medial medulla is an exception to this generalization). Therefore, anterior brainstem lesions can affect the motor pathways, whereas dorsolateral lesions can affect the sensory pathways. Since all of the cranial nerve nuclei are also in the brainstem, isolated motor or sensory symptoms limited to the extremities from a brainstem lesion are rare, although they can occur with small anterior lesions affecting only the corticospinal tracts, or small dorsal lesions affecting only the sensory tracts.
As will be discussed in Chapter 9, unilateral brainstem lesions can cause crossed signs: weakness and/or sensory changes in the ipsilateral face, but in the contralateral body. This is because nearly all cranial nerves control ipsilateral functions in the head, whereas the not-yet-crossed corticospinal tracts and the already-crossed ascending sensory tracts at the level of the brainstem control contralateral functions in the body.
Involvement of the Motor and/or Sensory Pathways in the Spinal Cord
The anatomic arrangement of the long tracts in the spinal cord gives rise to several possible clinical syndromes, which are discussed in detail in Chapter 5. Since the spinal cord is relatively small, it is common for disease processes affecting it to cause bilateral symptoms and signs, although small lesions within the spinal cord (intramedullary lesions) or unilateral external compression of the spinal cord (extramedullary lesions) can lead to unilateral symptoms.
Involvement of the Motor and/or Sensory Pathways in the Peripheral Nervous System
As discussed in Chapter 27, peripheral neuropathies can be motor, sensory, or mixed, and can affect an individual nerve (mononeuropathy), multiple nerves (mononeuropathy multiplex), or all nerves (polyneuropathy), leading to varying patterns of weakness and/or sensory changes. When an individual limb is weak and/or has changes in sensation such as numbness or pain, the problem may be referable to an individual root or nerve, multiple roots or nerves, or to a nerve plexus (see Chs. 16–17).
If there are sensory and/or motor changes in the face (with or without deficits in the extremities), this requires a lesion at the level of the brainstem or in the cerebral hemisphere. (Evaluation of facial weakness and sensory changes is discussed in further detail in Chapter 13.)
If the arm and leg are affected on one side without the face being involved, the lesion is most likely in the medulla or cervical spinal cord.
If one limb is affected in isolation, a lesion at nearly any level of the nervous system is possible: a small cortical or subcortical lesion, a spinal cord lesion, or a lesion at the level of the nerve roots, plexus, or one or more peripheral nerves. A brainstem lesion causing unilateral involvement of one limb is unlikely since the fibers of the motor and sensory pathways are packed so closely in the brainstem.
Isolated bilateral weakness suggests a process affecting the spinal cord, peripheral nerves, muscles, or neuromuscular junction. To have bilateral weakness from a brain or brainstem lesion would require bilateral lesions, which will usually cause symptoms/signs beyond just motor problems (e.g., changes in mental state if there are bilateral cerebral lesions or cranial nerve deficits if there are bilateral brainstem lesions). An exception is a very medial lesion affecting the bilateral leg areas of the motor cortices, which can lead to paraplegia mimicking a spinal cord lesion. This can be caused by a parasagittal meningioma or bilateral anterior cerebral artery strokes (which can occur when both anterior cerebral arteries arise from a common trunk, called azygous anterior cerebral arteries; see Ch. 7). In the case of bilateral anterior cerebral artery strokes, there are usually cognitive deficits in addition to the motor findings. Isolated bilateral sensory changes generally suggest a process affecting the spinal cord, dorsal roots, dorsal root ganglia, or peripheral nerves.
Schematic showing patterns of weakness and/or sensory disturbances caused by lesions at different levels of the nervous system. Note that a myopathy will not cause sensory deficits. Adapted with permission from Aminoff M, Greenberg D, Simon R: Clinical Neurology, 9th ed. New York: McGraw-Hill Education; 2015.
With respect to time course of symptom onset and evolution:
Sudden-onset weakness and/or sensory changes. Common causes of sudden-onset weakness and/or sensory changes localizing to the brain include stroke, seizure, or migraine. The most common cause of sudden-onset weakness and/or sensory changes localizing to the brainstem is stroke. Common causes of sudden-onset weakness and/or sensory changes localizing to the spinal cord include acute disc prolapse or vertebral collapse (e.g., due to trauma or metastatic malignancy to the vertebrae), which are typically painful. Spinal cord infarct occurs rarely (see Ch. 19). Sudden-onset weakness and/or sensory changes localizing to a particular nerve may be caused by a nerve infarct (as can be seen in vasculitic neuropathies; see Ch. 15)
Acute- to subacute-onset (over hours to days) weakness and/or sensory changes. Acute- to subacute-onset weakness and/or sensory changes at any level of the nervous system can be due to inflammatory conditions (e.g., Guillain-Barré syndrome (see Ch. 27), transverse myelitis, or an acute demyelinating lesion in multiple sclerosis; see Ch. 21) or infectious processes (e.g., cerebral or epidural abscess; see Ch. 20).
Subacute- to chronic-onset (over weeks to months to years) weakness and/or sensory changes. Subacute- to chronic-onset weakness and/or sensory changes can be caused by neoplasm, chronic inflammatory disease (e.g., chronic inflammatory demyelinating polyneuropathy [CIDP]; see Ch. 27), or degenerative disease (e.g., in the case of isolated weakness, motor neuron disease; see Ch. 28).