The Sensory Examination

The sensory examination is at once the most anatomically elegant and the most clinically treacherous part of the neurologic exam. Done well, it can pinpoint a lesion to a single nerve root, a precise spinal cord level, or a thalamic nucleus. Done carelessly, it generates noise — patients want to please you, fatigue quickly, and will report "differences" that mean nothing. The secret to mastering it is not technique alone but anatomy: once you internalize where each great sensory pathway crosses the midline, the bewildering patterns of sensory loss resolve into a small set of recognizable syndromes.

Two Great Ascending Systems — and Why Decussation Matters

Somatic sensation from the limbs and trunk reaches the cortex by two largely separate highways. They differ not just in what they carry, but — crucially — in where along the neuraxis they cross to the opposite side. That single difference is the engine behind every dissociated sensory syndrome you will ever see.

• Spinothalamic tract (anterolateral system) — pain, temperature, and crude (non-discriminative) touch. The first-order neuron enters the cord and synapses in the dorsal horn. The second-order neuron crosses within the cord, in the anterior (ventral) white commissure, within about one to two segments of the level of entry, and then ascends contralaterally to the thalamus. Practical consequence: a lesion of the spinothalamic tract within the cord produces loss of pain and temperature on the opposite side of the body, beginning a segment or two below the lesion.
• Dorsal column–medial lemniscus system — vibration, proprioception (joint position sense), and fine discriminative touch. The first-order neuron enters the cord and ascends ipsilaterally, uncrossed, in the dorsal columns (fasciculus gracilis for the legs, fasciculus cuneatus for the arms) all the way to the lower medulla, where it synapses in the gracile and cuneate nuclei. Only there does the second-order neuron cross — as the internal arcuate fibers — to form the contralateral medial lemniscus. Practical consequence: a lesion of the dorsal columns within the cord produces loss of vibration and proprioception on the same side as the lesion.

The whole point: in the spinal cord, pain/temperature has already crossed, while vibration/proprioception has not. A single cord lesion therefore hits these two modalities on opposite sides of the body — the anatomical basis of dissociated sensory loss.

Testing the Primary Modalities

Test each modality deliberately, compare side-to-side and proximal-to-distal, and keep the patient's eyes closed. Show the patient the normal stimulus first so they know what they are judging against.

• Pinprick (sharp/dull) — spinothalamic. Use a disposable sharp (a clean pin or a snapped tongue-blade), never a reusable point. Ask the patient to report "sharp" or "dull," not merely "do you feel it." Map borders by moving from an abnormal area toward a normal one.
• Temperature — spinothalamic. A cold tuning fork or a cold metal surface usually suffices; cold is more reliably perceived than warm. Useful to confirm a suspected pain/temperature deficit when pinprick is equivocal.
• Light touch — both systems. A wisp of cotton or a light fingertip touch. Because crude touch travels with the spinothalamic tract and fine touch with the dorsal columns, light touch is the least localizing modality — it is often preserved even when one system is damaged.
• Vibration — dorsal column. Use a 128 Hz tuning fork placed over a bony prominence (great toe interphalangeal joint, medial malleolus, then move proximally; finger, ulnar styloid). Ask the patient to say when the buzzing stops. Vibration sense is frequently the earliest casualty in large-fiber length-dependent neuropathy. Note the caveat: in small-fiber neuropathies the deficit falls preferentially on pain and temperature, and vibration and proprioception may be entirely preserved — a normal vibratory exam does not exclude a small-fiber process.
• Joint position sense (proprioception) — dorsal column. Grasp the sides of the distal phalanx of the great toe or a finger, move it a few degrees up or down, and ask "up or down?" Grasping the top and bottom gives away the answer through pressure cues. The Romberg test is the integrated bedside test: loss of balance on closing the eyes (when vision can no longer substitute) implicates inadequate proprioceptive and/or vestibular input — most often the dorsal columns or proprioceptive afferents, though vestibular failure can also unmask it. It is not a dorsal-column-only sign, nor a primary test of the cerebellum.

Cortical / Discriminative Sensation

These are functions of the parietal lobe (primary somatosensory cortex and association areas). They are higher-order integrations and are only interpretable if the primary modalities are intact. If pinprick and proprioception are already absent in a hand, you cannot blame the cortex for a failure of stereognosis. When primary sensation is normal but discriminative sensation is impaired, the lesion is cortical (contralateral parietal).

• Stereognosis — identifying a common object (coin, key, paperclip) placed in the hand with eyes closed. Failure = astereognosis.
• Graphesthesia — recognizing a number traced on the palm. Failure = agraphesthesia.
• Two-point discrimination — the minimum distance at which two simultaneous points are felt as two; normal thresholds are small on the fingertips and much larger on the back.
• Extinction / double simultaneous stimulation — touch both sides at once; a patient who feels each side alone but neglects one side when both are touched together shows extinction, a sign of a contralateral (usually right) parietal lesion and a feature of the neglect syndrome.

Patterns of Sensory Loss and Their Localization

The shape of the deficit is more diagnostic than its severity. Train your eye to recognize these patterns.

• Sensory level — a horizontal cutoff across the trunk, with normal sensation above and abnormal below. This is the signature of a spinal cord lesion (myelopathy). Always check the back and look for the level.
• Stocking-glove, length-dependent — symmetric loss starting in the toes, ascending, with the hands affected only once the legs are involved to about the knees. This is polyneuropathy, the longest axons failing first.
• Dermatomal — a band of loss following a single dermatome points to a nerve root (radiculopathy).
• Single peripheral nerve territory — loss confined to the cutaneous distribution of one named nerve indicates a mononeuropathy (e.g., median nerve at the wrist).
• Hemisensory loss (face + body, same side) — when the face and body are affected together, the lesion is above the cord: thalamic (often dense, all modalities) or cortical (often with prominent discriminative loss and extinction).
• Dissociated loss — one modality lost while another is spared in the same region, the fingerprint of a cord or brainstem lesion:
  • Syringomyelia (central cord). An expanding central cavity first interrupts the decussating spinothalamic fibers in the anterior commissure, producing a "cape" or suspended dissociated loss of pain and temperature across the shoulders and arms, while vibration and proprioception (dorsal columns) are spared.
  • Brown-Séquard syndrome (cord hemisection). Below the lesion: ipsilateral loss of vibration/proprioception and ipsilateral weakness (dorsal columns and corticospinal tract, neither yet crossed), with contralateral loss of pain and temperature (spinothalamic, already crossed in the cord).

Pitfalls and Pearls

• It is the most subjective exam. Sensory testing depends on patient attention and honesty more than any other part of the neurologic exam. Treat a single inconsistent finding with skepticism; demand reproducibility before you believe it.
• Corroborate with objective signs. A claimed proprioceptive loss should be backed by a positive Romberg or sensory ataxia; a claimed pinprick level should be sharp and reproducible on repeat testing. Anchor soft sensory complaints to reflexes, strength, and gait.
• Map from abnormal to normal. Borders are perceived more sharply when you move from the deficient region into the intact one.
• Mnemonic — "Dorsal columns Don't Decussate (in the cord); they cross Down in the medulla." Spinothalamic, by contrast, crosses immediately at the cord. Keep these straight and dissociated loss interprets itself.
• Don't over-test. Screen efficiently, then concentrate detailed testing where the history predicts a deficit. A fatigued patient gives unreliable answers.