In one breath
Sensation can be lost completely (anesthesia) or partly (hypesthesia), exaggerated (hyperesthesia), felt on the wrong side (alloesthesia) or turned unpleasant (paresthesia), and the cortical skills of reading figures drawn on the skin or recognizing objects by touch can fail (agraphesthesia, astereognosis). In dissociated anesthesia, pain and temperature are lost but touch survives, because a lesion around the central canal, classically syringomyelia, cuts the crossing spinothalamic fibers and spares the dorsal columns at the edge of the cord. Position sense can be lost on its own in posterior column disease such as tabes dorsalis and subacute combined degeneration, and the bedside sign is the Romberg sign: the patient cannot stand steady with the feet together once the eyes are closed.
Builds on: Ascending tracts: dorsal column and anterolateral system · Pathways for specific sensations · Leads to: Sensory lesions and sensory function tests · Thalamus: nuclei, functions and thalamic syndrome
Anesthesia
Definition
- Anesthesia: complete loss of every form of sensation.
- Hypesthesia: partial loss of sensation.
- Loss of one sensation alone is named with a prefix: thermoanesthesia for temperature, pallanesthesia for vibration.
Causes
| Type | Usual cause | Area affected |
|---|---|---|
| Hypesthesia | Central lesion: thalamus, internal capsule, cortex | Distal limbs most |
| Complete anesthesia | Peripheral nerve lesion, most often leprosy | The nerve’s own territory |
| Complete anesthesia | Complete transection of the cord | Limbs and trunk below the lesion |
Dissociated anesthesia
Definition
Pain and temperature are lost while touch is preserved.
Causes
- Syringomyelia
- Intramedullary tumors
- Brainstem lesions: syringobulbia
- Thrombosis of the posterior inferior cerebellar artery
Physiological basis
- These diseases damage the grey matter around the central canal.
- Spinothalamic fibers for pain and temperature cross to the other side close to the central canal, so the lesion destroys them.
- Dorsal column fibers lie at the periphery of the cord, so they escape and touch is kept.
Draw it: dissociated anesthesia in syringomyelia
Draw the cord in cross-section with the central canal widened into a cavity (the syrinx). Bring a pain fiber in by the dorsal root, relay it in the dorsal horn and send the second-order axon across just in front of the canal: show the cavity breaking it, on both sides. Shade the dorsal columns at the back as spared. Label the result: “pain and temperature lost, touch preserved”.
- Worth knowing, though not in your pages: only the fibers crossing at the diseased segments are cut, so the loss is bilateral and confined to those segments. With a cervical syrinx it lies over the shoulders and arms like a cape, with normal sensation below.
- Worth knowing, though not in your pages: thrombosis of the posterior inferior cerebellar artery gives the lateral medullary (Wallenberg) syndrome. Pain and temperature are lost on the same side of the face and the opposite side of the body, while touch, carried in the medial lemniscus, is spared.
Two opposite dissociations
In dissociated anesthesia pain and temperature go and touch stays. Dorsal column disease (tabes dorsalis, subacute combined degeneration) does the reverse: position and vibration sense go while pain and temperature stay. Don’t mix up the two.
Hemianesthesia
- Definition: loss of sensation over the face, arm, trunk and leg of one side of the body.
- Cause: a lesion of the thalamus, internal capsule or cortex on the opposite side.
Hyperesthesia
- Definition: an exaggerated response to sensory stimulation.
- The heightened sensitivity points to an overactive sensory apparatus, for instance receptors that have become more sensitive.
- Seen in thalamic lesions.
- Hyperpathia, an exaggerated perception of sensation, is used as another name for it.
- Worth knowing, though not in your pages: neurologists usually keep hyperpathia for an exaggerated, often explosive, painful reaction, especially to repeated stimuli, typically on a background of a raised threshold. It is a feature of the thalamic syndrome (see Thalamus: nuclei, functions and thalamic syndrome).
Alloesthesia
- Definition: a touch or a painful stimulus given on the side of hemisensory loss is felt at the matching spot on the opposite side. It is also spelled allesthesia.
- Causes:
- a lesion of the right putamen, usually a putamen hemorrhage;
- an anterolateral lesion of the cervical cord, perhaps because a few spinothalamic fibers do not cross.
Paresthesia
- Definition: sensations that are abnormal and distressing; even touch may feel unpleasant, close to pain. It is also called dysesthesia.
- It is felt as pricking, numbness, or a band-like feeling around the trunk.
- Causes:
- Nerve compression, the commonest: a peripheral nerve is stretched or pressed. The everyday example is the numbness and pricking after sitting cross-legged for a long time.
- Spinal tumors
- Subacute combined degeneration of the cord
- Disseminated sclerosis
- Thalamic lesions
Agraphesthesia
- Graphesthesia is the ability to localize a touch on the skin and to recognize figures written there. Your book counts it as a posterior column function.
- Agraphesthesia is its loss.
- Cause: usually a lesion of the sensory cortex.
Astereognosis
- Definition: inability to recognize a familiar object by handling it with the eyes closed, although basic touch is intact.
- Cause: cortical disease.
- Tell it apart from stereoanesthesia:
| Feature | Astereognosis | Stereoanesthesia |
|---|---|---|
| Lesion | Cortex | Spinal cord or brainstem |
| Touch and position input | Intact | Cut off |
Loss of proprioception
- Proprioception is the sense of joint movement and of where each part of the body lies in space. Vibration sense is closely linked to it.
- It can be lost on its own, with the other sensations intact, in posterior column lesions. Typical examples:
- tabes dorsalis
- subacute combined degeneration of the cord
- Lesions of the sensory cortex also impair proprioception and vibration severely.
Romberg sign
- The patient cannot keep a balanced stance with the feet together and the eyes closed. This is the Romberg sign.
- It shows that position sense is missing in both legs.
- Test the patient barefoot.
- The unsteadiness is a sensory ataxia. Cerebellar disease and vestibular disease also cause ataxia, and these must be told apart from a posterior column lesion.
- Worth knowing, though not in your pages: the patient with a posterior column lesion stands steadily while the eyes are open, because vision stands in for the missing position sense. A cerebellar patient is unsteady even with the eyes open, so closing them adds little.
Exam-answer skeleton: "Dissociated anesthesia" (short note)
- Definition: loss of pain and temperature with touch preserved.
- Causes: syringomyelia, intramedullary tumors, syringobulbia, thrombosis of the posterior inferior cerebellar artery.
- The anatomy behind it: spinothalamic fibers cross close to the central canal; dorsal column fibers run at the edge of the cord.
- Mechanism: a lesion of the grey matter around the canal cuts the crossing fibers, so pain and temperature go; the dorsal columns are spared, so touch stays.
- Diagram: cord section with a syrinx breaking the crossing fibers on both sides.
- The pattern: loss on both sides, limited to the diseased segments.
- Contrast: dorsal column disease (tabes dorsalis) loses position and vibration sense but keeps pain and temperature.