The exteroceptive sensations — pain, temperature, and light touch — are the sensations the patient most often complains about and the ones most likely to localize a lesion at the bedside. Their pathway is the spinothalamic tract, which crosses within the cord at or near the level of entry, ascends contralaterally, and reaches the somatosensory cortex via the thalamic VPL. The clinical content of exteroceptive testing is small-fiber function, the spinal cord at all levels, the brainstem laterally, the thalamus, and the parietal cortex. Few tests are as cheap and as informative as well-performed pinprick testing.

The Anatomy of Exteroceptive Sensation

Receptors and Fibers

Exteroceptive sensations are detected by free nerve endings in the skin. The fibers carrying these signals fall into two main groups:

  • Aδ fibers — thinly myelinated, fast-conducting (about 12-30 m/s) — carry sharp, localized pain (“first pain”) and cool temperature.
  • C fibers — unmyelinated, slow-conducting (about 0.5-2 m/s) — carry dull, burning, poorly localized pain (“second pain”) and warmth.

This division explains the dual perception of pain — the sharp first wave followed by a duller, sustained ache — and it has clinical consequences. Patients with small-fiber neuropathy may lose temperature and protopathic pain perception (C fibers) while preserving the sharp localized pain of Aδ fibers, or vice versa. The two are not always lost together, and the bedside exam should test both when small-fiber involvement is suspected.

The Central Pathway

The course of the spinothalamic tract is described in the Sensory Overview & Pathways page. Two clinically important features:

  • The tract decussates within the cord, at or one to three segments above the level of entry. Above the level of entry, spinothalamic information reflects the contralateral body.
  • The tract is somatotopically organized, with lower-body fibers (legs) running more laterally and upper-body fibers (arms) running more medially. This explains the “sacral sparing” of central cord lesions: a centrally placed cord lesion (syrinx, central cord stroke, ependymoma) damages the medial fibers first, sparing the lateral fibers carrying lower-body information. Conversely, a slowly compressive extramedullary lesion (e.g., a meningioma pressing in from outside) reaches the laterally placed lower-body fibers before the more medial upper-body fibers, producing a sensory level that “marches up” from the sacrum.

Testing Pain Sensation

How to Test

Use a sterile, disposable pin or the broken end of a wooden cotton swab. The point should be sharp enough to produce a clear, brief sharp sensation but not sharp enough to break skin. Apply each stimulus once — a single brief touch — and then move on. The patient is asked first to confirm that they feel something sharp, and then to compare sides.

The most informative way to elicit subtle pain loss is to ask the patient to rate the relative intensity: “If the sharpness on this side is 10, what is it on the other side?” A side rated 7 out of 10 indicates a meaningful asymmetry that a simple “yes I feel it” exam would miss. Asking “does it feel as sharp here as it did there?” gives more useful information than asking “do you feel this?”.

Test systematically. For a screening exam, pinprick at the dorsum of the great toe, the dorsum of the foot, the medial and lateral leg, the dorsum of the hand, the palm, the medial and lateral forearm, and the deltoid area on each side. When a complaint is focal, test the suspected territory in detail and compare to immediately adjacent territories above and below.

What Reduced Pain Sensation Means

Reduced pinprick sensation localizes to one of the following:

  • Peripheral nerve: a glove-and-stocking pattern points to length-dependent neuropathy; a discrete territory points to mononeuropathy or radiculopathy.
  • Dorsal root or root ganglion: dermatomal loss with associated radicular pain.
  • Spinal cord: a sensory level (loss below a defined dermatome on one or both sides) points to a cord lesion. Crossed loss (ipsilateral dorsal column, contralateral pain/temperature) points to hemisection (Brown-Séquard).
  • Brainstem: ipsilateral facial pain loss with contralateral body pain loss points to the lateral medulla (Wallenberg).
  • Thalamus or cortex: hemibody loss without a clear level.

Increased Pain Sensation

The opposite finding — hyperalgesia (exaggerated response to painful stimulus) or allodynia (pain produced by a non-painful stimulus) — is no less important. These are the hallmarks of small-fiber neuropathy, postherpetic neuralgia, complex regional pain syndrome, and the central pain syndromes (thalamic pain syndrome, central post-stroke pain). The bedside test is light brushing of the skin: a patient with allodynia describes burning or sharp pain from stimuli that should be entirely innocuous.

Testing Temperature

Temperature sensation travels with pain in the spinothalamic system. The two are usually impaired together, but not always. In subtle spinothalamic dysfunction, temperature can be impaired before pain — the patient correctly identifies pinprick as sharp but cannot reliably distinguish cool from warm.

Formal temperature testing uses warm (40-45°C) and cool (20-25°C) tubes. At the bedside, the back (cold) side of a metal tuning fork or a tendon hammer is sufficient for cool testing. Apply briefly and ask the patient to identify the sensation as cold and to compare sides. For warmth, run a warm tap to a comfortable temperature and use a wet swab.

Temperature is particularly useful in:

  • Small-fiber neuropathy: diabetic painful neuropathy, amyloid neuropathy, alcoholic neuropathy. Temperature loss may be the only objective finding in pure small-fiber disease, where vibration and joint position sense remain normal.
  • Spinothalamic tract lesions: combined with pain loss, identifies the spinothalamic pattern.
  • Confirming dissociated sensory loss: in a patient with apparent crossed sensory findings or apparent suspended sensory loss, temperature confirms the spinothalamic involvement.

Testing Light Touch

Light touch is the least specific of the primary sensations because it is mediated by both the dorsal column and the spinothalamic systems. Lesions that severely impair one pathway often spare light touch on the other. The bedside test uses a wisp of cotton, the examiner’s fingertip, or a soft brush, applied lightly to the skin without dragging or moving.

Despite its lower diagnostic specificity, light touch is worth testing for two reasons:

  • When both pathways are severely affected, light touch is impaired. Profound bilateral light touch loss confirms severe sensory disease and may be the most striking finding in a patient with chronic sensory ataxic neuropathy.
  • The pattern of impairment may still be useful. A dermatomal pattern of light touch loss, although less sharp than a pinprick pattern, can still confirm a suspected radicular distribution.

Patterns and Their Meaning

Distal Symmetric Polyneuropathy

The most common pattern by far is a length-dependent stocking-and-glove loss of pinprick and temperature, beginning at the toes, progressing up to the ankle and then the calf, and (in advanced disease) extending into the fingers. Vibration is impaired earlier than pain in most polyneuropathies; pure small-fiber neuropathy is the exception, with selective impairment of pain and temperature and preserved vibration.

The distribution follows length: the longest axons are most vulnerable, so the toes are involved first. By the time the glove distribution reaches the wrists, the stocking distribution has typically reached the knees. The progression is symmetric. Asymmetric findings should prompt consideration of mononeuropathy multiplex (multiple discrete mononeuropathies), often vasculitic in etiology.

Mononeuropathies

Discrete sensory loss in the territory of a single peripheral nerve. The pattern is described in the Sensory Overview page; common examples include median nerve loss in carpal tunnel syndrome, ulnar nerve loss at the elbow, lateral femoral cutaneous loss (meralgia paresthetica), and peroneal nerve loss at the fibular head producing dorsum-of-foot sensory loss accompanying foot drop.

Radiculopathy

Dermatomal sensory loss, often combined with motor weakness and reflex changes in the same root distribution. The cervical and lumbosacral radiculopathies — C5, C6, C7, C8, L4, L5, S1 — are the ones to know precisely. Each has a characteristic territory:

  • C5: lateral upper arm, deltoid area.
  • C6: lateral forearm, thumb, index finger.
  • C7: middle finger.
  • C8: little finger, ulnar forearm.
  • L4: medial leg, medial foot.
  • L5: lateral leg, dorsum of foot, great toe.
  • S1: lateral foot, sole.

Cord Patterns

The major cord patterns of exteroceptive loss are described in detail on the Sensory Localization Patterns page. A summary:

  • Brown-Séquard (hemisection): ipsilateral dorsal column loss + contralateral pain and temperature loss starting a few segments below the lesion. Ipsilateral motor weakness.
  • Anterior cord syndrome: loss of pain and temperature bilaterally below the level (and motor function), with preserved dorsal column function. The classical cause is anterior spinal artery infarction.
  • Central cord (syringomyelia): “suspended” loss of pain and temperature in a cape-like distribution across the shoulders and arms, sparing the lower body and sparing dorsal column function. The fibers crossing through the central commissure are interrupted; long tract fibers passing through the lateral cord are spared.
  • Complete cord transection: loss of all modalities below the level.

Brainstem Patterns

The classical brainstem sensory pattern is the lateral medullary syndrome (Wallenberg), with ipsilateral facial pain and temperature loss and contralateral body pain and temperature loss, accompanied by the other features of the syndrome (ipsilateral Horner, ataxia, palatal weakness, vertigo). The dissociated facial-body pattern is the hallmark of the lateral medullary lesion.

Thalamic Pain Syndrome (Dejerine-Roussy)

A small thalamic infarct involving the VPL nucleus produces an initial hemisensory loss of all modalities. Over weeks to months, this evolves into a peculiarly distressing syndrome: spontaneous burning or aching pain in the previously numb hemibody, with marked hyperalgesia and allodynia. The pain is often severe and resistant to most analgesics; antidepressants (amitriptyline, duloxetine) and antiepileptics (gabapentin, pregabalin) are the standard treatments.

Cortical Sensory Loss

Lesions of the primary somatosensory cortex produce contralateral hemibody loss of cortical sensory functions (two-point discrimination, stereognosis, graphesthesia, tactile localization) with relative preservation of primary sensations. The pattern is the inverse of peripheral sensory loss: the patient can detect a pin and feel cold, but cannot identify objects placed in their hand or interpret figures drawn on their palm.

Suspected Small-Fiber Neuropathy

Small-fiber neuropathy is a relatively recently recognized clinical entity in which only the small (Aδ and C) fibers are involved. The patient describes burning pain in the feet, often worse at night, sometimes spreading proximally; allodynia and autonomic symptoms (sweating changes, vasomotor changes, sometimes orthostasis) may be prominent. The exam shows pinprick and temperature loss in the affected territory, with normal vibration and joint position sense, normal reflexes, and normal nerve conduction studies. Diagnosis is supported by quantitative sensory testing, skin biopsy showing reduced intraepidermal nerve fiber density, and sometimes autonomic testing.

Causes include diabetes (the most common), prediabetes, amyloid neuropathy, Sjögren syndrome and other autoimmune diseases, sodium channel mutations (SCN9A, SCN10A, SCN11A), and idiopathic disease. Recognition matters because the condition is often initially diagnosed as “no neuropathy” or “idiopathic pain” because EMG/NCS — which test only large fibers — are normal.

🔍 Did You Know?

The classical “first pain” and “second pain” demonstration — pinch a fingertip and feel two distinct waves of sensation — illustrates the dual fiber system. The sharp first pain travels by Aδ fibers in about one to two seconds from finger to brain. The duller second pain travels by C fibers and lags by several seconds. The same patient can preserve one wave while losing the other in selective small-fiber disease.

Pitfalls and Pearls

  • Pinprick should be a single brief touch. Repeated application produces summation and falsely increased response.
  • The pin should not break skin. A pin that draws blood produces an exaggerated response and is unkind. A wooden swab broken in half is a humane and effective tool.
  • “Yes I feel it” is the least useful patient response. Ask for relative intensity comparisons between sides.
  • Test pinprick and temperature together when small-fiber involvement is suspected. Either may be lost preferentially.
  • Always test perianal pinprick in suspected cauda equina or conus syndromes. Saddle anesthesia is the cardinal feature; missing it has major consequences.
  • Allodynia and hyperalgesia point to peripheral sensitization or central pain. They are not malingering, and they deserve specific pharmacological management.
  • A sensory level on the trunk should be tested both anteriorly and posteriorly. The cord lesion may produce different levels on the two sides, and the position of the level helps with the surgical planning if intervention is required.
  • Functional sensory loss has characteristic patterns: midline-splitting on the trunk, non-anatomic distributions, glove or stocking with a sharp proximal boundary, fluctuation with attention. The diagnosis is supported by the constellation, not by any single finding.
  • The thalamic pain syndrome is treatable. Tricyclic antidepressants, SNRIs, and gabapentinoids are the first-line agents. Do not under-treat a patient with severe central pain.

References

  1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 32.
  2. Devigili G, Tugnoli V, Penza P, et al. The diagnostic criteria for small fibre neuropathy: from symptoms to neuropathology. Brain. 2008;131(7):1912-1925.
  3. Themistocleous AC, Ramirez JD, Serra J, Bennett DL. The clinical approach to small fibre neuropathy and painful channelopathy. Pract Neurol. 2014;14(6):368-379.
  4. Treede RD, Jensen TS, Campbell JN, et al. Neuropathic pain: redefinition and a grading system for clinical and research purposes. Neurology. 2008;70(18):1630-1635.
  5. Klit H, Finnerup NB, Jensen TS. Central post-stroke pain: clinical characteristics, pathophysiology, and management. Lancet Neurol. 2009;8(9):857-868.
  6. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.