Patients may use the word numbness to describe an alteration in sensation (ie, paresthesia or sensory loss), strength (ie, weakness), or coordination (ie, clumsiness). Therefore, the approach to the patient presenting with numbness begins with clarifying what the patient means by the word. This chapter will focus on the evaluation of the patient with alteration in sensation.
Differential diagnosis in neurology requires determining the localization of the problem within the nervous system (brain, brainstem, spinal cord, nerve roots, peripheral nerves, neuromuscular junction, or muscle), the time course over which the problem has arisen (acute, subacute, or chronic), and any associated symptoms that accompany the chief complaint (eg, if the chief complaint is numbness, is there associated weakness or pain?).
OVERVIEW OF SENSORY PATHWAYS
Sensory information from the body travels in peripheral nerves to dorsal root ganglia, then enters the spinal cord through the dorsal roots (FIGURE 98-1). After entering the spinal cord, different types of sensory information travel in different pathways en route to the brain. Pain and temperature sensation travel in the spinothalamic (anterolateral) tracts, whereas proprioception and vibration travel in the dorsal (posterior) columns. Light touch sensation travels to some extent in both pathways and is therefore of less precise localizing value.
Dorsal columns, spinothalamic tracts, and basic trigeminal pathway.
The fibers destined for the spinothalamic tracts cross directly to the contralateral anterolateral spinal cord after entering the spinal cord, and ascend through the brainstem to the thalamus, and from the thalamus to the somatosensory cortex housed in the postcentral gyrus. A unilateral spinal cord lesion affecting the spinothalamic tract at any level therefore causes contralateral loss of pain and temperature below the level of the lesion. Since the entering fibers cross over several spinal levels, if this pathway is affected in the spinal cord, there may also be a small patch of ipsilateral sensory loss at and above the level of the lesion.
The dorsal column pathways remain ipsilateral in the spinal cord and cross at the level of the lower brainstem (the medulla), where it then ascends to the thalamus en route to the somatosensory cortex like the spinothalamic tract. Therefore, a unilateral spinal cord lesion affecting the dorsal column pathway causes ipsilateral loss of proprioception and vibration sensation below the level of the lesion, whereas a lesion superior to the medulla causes contralateral loss of proprioception and vibration sense.
Sensation on the face is subserved by the trigeminal nerves (cranial nerve V), which transmits information to the brainstem, where it ultimately crosses to join the corresponding somatosensory pathways from the body (ie, information from the left trigeminal nerve crosses to the right side of the brainstem to join the already-crossed ascending sensory pathways ...