The sensory system is one of the most powerful localizing tools at the bedside, but it is also one of the most challenging — sensation is subjective, the modalities are many, and the somatotopy of the sensory pathways is fundamentally different from the motor system. A careful, systematic sensory examination interpreted in light of the anatomy of the somatosensory tracts can localize a lesion from cortex to peripheral nerve. This page covers the principles that make sensory localization possible: which modalities travel where, how patterns of sensory loss point to specific levels, and the recurring patterns that come up at the bedside.

The Two Great Sensory Tracts

Almost the entire localizing power of the sensory examination rests on the fact that different sensory modalities travel in different pathways from periphery to cortex. Two large tract systems are central:

The Dorsal Column–Medial Lemniscal (DCML) System

Carries vibration, proprioception, and discriminative touch. Course:

• Large-diameter primary afferents enter the spinal cord at the dorsal root entry zone.
• They ascend ipsilateral to their entry, in the dorsal columns (fasciculus gracilis from lower body, fasciculus cuneatus from upper body).
• First synapse occurs in the dorsal column nuclei at the cervicomedullary junction (nucleus gracilis and nucleus cuneatus).
• Second-order axons immediately decussate as the internal arcuate fibers, forming the medial lemniscus.
• The medial lemniscus ascends through the brainstem to the ventral posterolateral (VPL) nucleus of the thalamus.
• Third-order neurons project from VPL to the primary somatosensory cortex (S1, Brodmann areas 3, 1, 2 in the postcentral gyrus).

Key localizing principle: vibration and proprioception travel ipsilateral in the cord and cross in the medulla. Below the medulla, a unilateral lesion produces ipsilateral loss of these modalities; above the medulla, it produces contralateral loss.

The Spinothalamic Tract (Anterolateral System)

Carries pain and temperature (and crude touch). Course:

• Small-diameter primary afferents enter the cord.
• First synapse occurs immediately, in the dorsal horn at the level of entry (sometimes traveling one or two segments up or down in Lissauer tract before synapsing).
• Second-order axons decussate within one to two segments via the anterior white commissure.
• They ascend in the contralateral anterolateral cord as the spinothalamic tract.
• The tract ascends through the brainstem (lateral medulla, pons, midbrain) to VPL of the thalamus.
• Third-order neurons project from VPL to S1.

Key localizing principle: pain and temperature cross within one to two segments of entry into the cord. A cord lesion above this crossing affects the contralateral body; a lesion at or just above the crossing can spare or selectively involve the dermatomes at the affected level.

Practical Consequence: Dissociated Sensory Loss

Because the two systems take different paths from cord entry to thalamus, a single lesion can affect one and spare the other — producing the very characteristic pattern of dissociated sensory loss. This is one of the most powerful localizing patterns in neurology. Examples:

• Lateral medullary (Wallenberg) syndrome: ipsilateral face pain/temperature loss (from spinal trigeminal nucleus and tract injury) + contralateral body pain/temperature loss (from spinothalamic tract injury), with sparing of vibration and proprioception (the medial lemniscus is medial and is preserved).
• Brown-Séquard syndrome (hemisection of the cord): ipsilateral vibration/proprioception loss below the lesion (uncrossed dorsal columns) + contralateral pain/temperature loss below the lesion (already crossed spinothalamic).
• Syringomyelia: cape-like loss of pain and temperature over shoulders/arms with sparing of vibration and proprioception, from interruption of the decussating spinothalamic fibers at the anterior white commissure.
• Tabes dorsalis: selective dorsal column dysfunction from neurosyphilis — sensory ataxia, loss of vibration and proprioception, with relatively preserved pain and temperature.

Patterns of Sensory Loss and Where They Localize

Stocking and Glove

Symmetric distal sensory loss to all modalities in feet (and later hands), most pronounced in the toes. This is the signature of a length-dependent peripheral neuropathy — the longest axons are affected first, so deficits begin in the feet and ascend up the legs. By the time the loss reaches the knees, the fingertips are usually beginning to be affected (the second longest axons). Causes include diabetes, alcohol, B12 deficiency, chemotherapy, hereditary neuropathies, paraneoplastic syndromes.

Single Nerve (Mononeuropathy)

Sensory loss confined to the cutaneous distribution of one peripheral nerve. Distribution maps precisely onto the territory of that nerve — for example, the median nerve covers the palmar surface of the thumb, index, middle, and lateral half of the ring finger; the ulnar nerve covers the medial half of the ring finger and the entire fifth finger. Recognizing nerve distributions distinguishes mononeuropathy from radiculopathy.

Multiple Nerves (Mononeuritis Multiplex)

Sensory and motor loss in the distribution of two or more named nerves, in different parts of the body. This pattern points to multifocal nerve infarction — most often vasculitis (polyarteritis nodosa, ANCA vasculitides), diabetes, leprosy, sarcoidosis, sometimes lymphoma.

Dermatomal

Sensory loss in the distribution of one or more nerve roots. Dermatomes have substantial overlap with neighboring roots, so a single-root lesion may show only mild or partial sensory loss. A vesicular rash in a dermatomal distribution is shingles (herpes zoster). Radicular pain often radiates along the dermatome.

Plexus

Sensory loss involving distributions of multiple nerves derived from one plexus — broader than a single nerve, broader than a single root. Pattern is shaped by which trunks or cords of the plexus are involved. Brachial plexopathy from radiation can give a more proximal pattern; lower trunk involvement (true neurogenic thoracic outlet) gives medial arm and hand sensory changes.

Spinal Cord Sensory Level

A sensory level — abrupt change in sensation at a horizontal line on the trunk — is a specific finding for spinal cord injury. The level identifies the upper extent of the lesion. Pain and temperature levels often appear one to two segments below the actual cord lesion (because the spinothalamic tract has just crossed within a couple of segments of entry). Vibration levels match more closely. A clear sensory level demands urgent MRI imaging of the cord.

Suspended Sensory Loss

Sensory loss in a band-like distribution on the trunk with preserved sensation above and below — the cape distribution of syringomyelia is the prototype. This pattern reflects involvement of the central cord, where decussating spinothalamic fibers are interrupted at the level of the syrinx, sparing the long tracts entering the cord at higher and lower levels.

Hemibody (Brainstem or Thalamus)

Loss of all sensory modalities affecting one entire side of the body (face, arm, trunk, leg) points to a lesion at or above the brainstem where all modalities have come together. A thalamic VPL lesion is the classic substrate — gives dense contralateral hemisensory loss to all modalities, sometimes accompanied by thalamic pain syndrome. Pontine or midbrain lesions in the medial lemniscus and spinothalamic tract can produce similar hemibody loss.

Crossed (Face vs Body)

Loss of pain and temperature on one side of the face combined with the opposite side of the body is the signature of a lateral medullary lesion (Wallenberg syndrome). The spinal trigeminal nucleus/tract carries face pain/temperature ipsilaterally and is hit; the spinothalamic tract carries body pain/temperature from the contralateral side (already crossed) and is also hit. Vibration and proprioception are typically preserved.

Cortical Sensory Loss

Lesions in S1 cortex produce a different sensory picture than those at lower levels. Basic primary modalities (pain, temperature, light touch, vibration) are often relatively preserved, but discriminative or “cortical” sensory functions are lost. These include:

• Two-point discrimination: the ability to recognize two nearby stimuli as distinct.
• Stereognosis: identifying an object placed in the hand by feel alone.
• Graphesthesia: recognizing letters or numbers drawn on the palm.
• Joint position sense for fine discrimination.
• Sensory extinction: with double simultaneous tactile stimulation, the side opposite a parietal lesion is not perceived even though each side alone is recognized.
• Tactile localization: precisely indicating where on the body one was touched.

A parietal lobe lesion may give intact pinprick and touch on a screening exam — only careful testing of discriminative functions reveals the deficit. This is why a normal screening sensory exam does not exclude a parietal lesion.

Examining Sensation: Practical Principles

The sensory examination is among the most subjective and patient-dependent parts of the neurologic examination. A few principles help maximize its localizing value:

• Have a hypothesis before testing. Rather than “checking sensation everywhere,” form a hypothesis from history and other findings, then test focally. Is this a hemibody loss? A dermatomal loss? A length-dependent loss? Test what your hypothesis predicts.
• Test both spinothalamic and dorsal column modalities. Pinprick (or temperature) tests spinothalamic; vibration (and joint position) tests dorsal columns. Disagreement between them — dissociated loss — is one of the most powerful localizing patterns and is missed if only one system is examined.
• Compare side-to-side and ask “is this the same?” rather than asking the patient to grade severity. Subjective comparisons are more reliable.
• For suspected cord level, examine the trunk carefully. Run pinprick or cold-tuning-fork up the trunk from belly to nipples — the change is often obvious.
• For suspected radiculopathy, test the dermatome edge. A C7 dermatome is often densest at the third finger; a L5 dermatome is often densest at the dorsal foot.
• For suspected length-dependent neuropathy, test the toes versus more proximally. Look for a smooth gradient.
• For cortical sensory loss, test discriminative functions. If primary sensation is intact but stereognosis, graphesthesia, or extinction is abnormal, think parietal cortex.
• Validate the sensory exam against the motor exam. A lesion almost always affects motor and sensory together if the anatomy puts them together. Pure sensory complaints with motor exam findings in the same distribution should match the same lesion.

Special Sensory Patterns

Pure Sensory Stroke

A lacunar infarct in the VPL nucleus of the thalamus produces contralateral hemibody sensory loss to all modalities without motor signs — the “pure sensory stroke” of Fisher. The face, arm, trunk, and leg are typically all affected together because of the close packing of the thalamic somatotopy. Recovery may be partial and sometimes accompanied by central neuropathic pain (Dejerine-Roussy syndrome).

Thalamic Pain Syndrome (Central Post-Stroke Pain)

After a thalamic stroke, a burning, dysesthetic pain may develop on the affected side, sometimes weeks to months after the stroke. This is one form of central neuropathic pain — refractory to most analgesics, sometimes helped by anticonvulsants (gabapentin, pregabalin) or tricyclics. It is a sobering complication that demonstrates how the central nervous system can become a generator of pain.

Sensory Ataxia

Loss of proprioceptive input — from large-fiber neuropathy, dorsal column disease (tabes dorsalis, B12 deficiency, copper deficiency), or large primary afferent sensory neuronopathy — produces an ataxic gait worse with eyes closed. The Romberg test (steady with eyes open, falls with eyes closed) is positive because the patient relies on visual input to compensate for absent proprioception. Distinguish from cerebellar ataxia, where eye closure does not dramatically change the gait.

Pseudo-radicular Sensory Loss

Cortical lesions in the postcentral gyrus can produce sensory loss that mimics a radicular distribution — for example, isolated thumb-index numbness from a small cortical lesion. The clue: vibration and proprioception may be preserved, while two-point discrimination and stereognosis are out. A radicular pattern with motor weakness in the same root distribution is more likely true radiculopathy.

Functional (Non-Organic) Sensory Loss

Sensory loss that doesn’t fit any organic distribution — for example, a sharp midline cutoff exactly at the midline (when the dermatomes overlap a few centimeters across the midline), changing the side of vibration loss when the tuning fork is moved across the sternum (where bone conduction does not differ between sides), or sensory loss in a “stocking glove” that ends abruptly at a joint line rather than gradually. Recognition matters because workup for organic disease may be unnecessary and time spent communicating the diagnosis is well spent.

🔍 Did You Know?

The crossed sensory pattern of Wallenberg syndrome — pain and temperature loss on one side of the face and the other side of the body — is so characteristic that recognizing it at the bedside immediately suggests a lateral medullary stroke. The anatomy is satisfyingly precise. The spinal trigeminal nucleus and tract, which carry pain and temperature from the ipsilateral face, descend through the lateral medulla on the side of the lesion. The lateral spinothalamic tract, which carries pain and temperature from the contralateral body (having already crossed at cord level), also runs through the lateral medulla on the same side. A single lateral medullary lesion catches both — interrupting face sensation ipsilateral and body sensation contralateral. Meanwhile, the medial lemniscus (carrying vibration and proprioception from the contralateral body) runs more medially and is spared, so vibration and proprioception remain intact. Combine the crossed sensory pattern with ipsilateral Horner syndrome, ipsilateral ataxia, vertigo, dysphagia, hoarseness, and ipsilateral facial weakness of pain/temperature — and the diagnosis of Wallenberg syndrome should be apparent within seconds of the bedside examination.

Sensory Localization by Level — Quick Summary

Pitfalls and Pearls

• Test both modalities — spinothalamic and dorsal column. Dissociated loss is a powerful localizing pattern that is missed if only pinprick or only vibration is tested.
• Pain-temperature levels can appear one to two segments below the cord lesion, while vibration levels match. Don’t be misled.
• A normal screening sensory exam does not exclude a parietal lesion. Test discriminative functions (stereognosis, graphesthesia, extinction).
• Cortical sensory loss usually involves discriminative functions disproportionately.
• Wallenberg syndrome gives crossed face/body pain-temp loss; this is one of the most specific sensory patterns in neurology.
• Pure sensory stroke is a lacunar VPL infarct. Hemibody loss to all modalities without motor or cortical signs.
• Romberg positive (sensory ataxia) points to dorsal columns or large-fiber neuropathy, not cerebellum.
• Be alert for non-organic patterns: midline cutoff exactly at the midline, vibration loss changing sides across the sternum.
• Sensory complaints out of proportion to findings can still represent small-fiber neuropathy, early myelopathy, or central sensitization — don’t dismiss them as non-organic without thoughtful examination.
• The thalamic pain syndrome can develop weeks to months after a thalamic stroke — patients may describe a burning sensation in the affected hemibody and need to be asked about it specifically.

References

1. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.
2. Campbell WW. DeJong’s The Neurologic Examination. 8th ed. Wolters Kluwer; 2019.
3. Patten J. Neurological Differential Diagnosis. 2nd ed. Springer; 1996.
4. Caplan LR. Caplan’s Stroke: A Clinical Approach. 5th ed. Cambridge University Press; 2016.
5. Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 11th ed. McGraw-Hill; 2019.