Temperature and pain sensations are conveyed via small myelinated and unmyelinated nerve fibers. These enter the spinal cord via the dorsal spinal root and synapse in the dorsal horn at the level of entry or ascend or descend ipsilaterally one to two spinal levels and then synapse in the dorsal horn. The second-order neurons then decussate at that level in the anterior commissure and ascend contralaterally through the spinal cord and brainstem to the thalamus. The third-order neurons from the thalamus synapse mainly in primary sensory cortex in the parietal lobe.
Clinically, pain sensation is assessed by gently applying a sharp stimulus, such as a pin, to the patient’s skin. Temperature sensation is usually assessed by applying a cold stimulus, such as the metal of the tuning fork, to the patient’s skin. When assessing temperature or pain sensation, it is important to assess for asymmetry and to compare proximal and distal sites.
A lesion involving the anterior commissure will cause bilateral reduction of pain and temperature sensation at that level due to disruption of the second-order neurons decussating through the anterior commissure. Pinprick and temperature sensation below the level of the anterior commissure lesion is spared, creating a so-called “suspended sensory level.” This differentiates it from bilateral anterolateral system lesions in which pinprick and temperature sensation are reduced at the level of the lesion and below. The most common cause of an anterior commissure lesion is hydromyelia, which is a cystic lesion involving the central canal, or syringomyelia (syrinx), which is a cystic lesion not connected to the central canal. A midline intrinsic spinal cord tumor such as an ependymoma or demyelination from multiple sclerosis can also involve the anterior commissure. If the sensory loss is over the neck or arms, a magnetic resonance image (MRI) of the cervical spine is indicated. If the sensory loss if over the abdomen, an MRI of the thoracic spine is indicated.
Since the second-order axons carrying pain and temperature sensations decussate in the spinal cord and the axons carrying proprioception and vibration sensations and the upper motor neurons decussate in the medulla, the combination of loss of pinprick and temperature sensations on one side and either loss of vibration and proprioception or upper motor neuron signs on the other side is diagnostic of a spinal cord lesion. When all three are present, the patient has a hemicord (Brown-Sequard) syndrome.
In addition to carrying pain and temperature sensations, small myelinated and unmyelinated peripheral nerves carry autonomic fibers. Patients with a small fiber dysfunction typically present with length-dependent burning and shooting pain, autonomic dysfunction such as decreased or increased sweating, dry eyes or mouth, orthostasis, urinary retention, constipation or gastroparesis, or a combination of both. Patients can have polyneuropathy with a combination of small and large fiber dysfunction or polyneuropathy with preferential involvement of the small fibers (i.e., small fiber polyneuropathy). Patients with a small fiber polyneuropathy will not have weakness or loss of vibration sensation above the toes or loss of reflexes above the ankles. Nerve conduction studies and electromyography (EMG) do not assess small fiber nerve function and thus are normal in small fiber polyneuropathy. Small fiber polyneuropathy is often diagnosed based on clinical findings; additional testing if there is diagnostic uncertainty can include assessing intraepidermal nerve fiber density on skin biopsy of the distal leg at the ankle, quantitative sensory testing, or autonomic testing. Common causes of small fiber polyneuropathy include diabetes, amyloidosis, HIV, and autoimmune disease such as systemic lupus erythematosus or Sjögren syndrome. About 30%–50% of small fiber polyneuropathy cases are idiopathic with a relatively stable course.
In the setting of unilateral focal loss of pain and temperature sensations, an attempt should be made to map this loss to the known sensory distribution of a nerve (e.g., lateral anterior thigh in meralgia paresthetica) or a dermatome. If this can be mapped to a particular nerve or nerve root and the patient does not have weakness, further diagnostic testing is usually not necessary. However, in many patients with mononeuropathies or radiculopathies the description of the distribution of numbness cannot be definitively ascribed to a single nerve or dermatome. In these cases, further diagnostic testing such as nerve conduction studies and EMG can be helpful.
Hemisensory pain and temperature disturbance affecting the face and body on the same side suggests a central (brain) lesion above the level of the medulla. Crossed sensory symptoms, with the face affected on one side and the body on the other, localize to the lateral medulla.