SENSORY SYSTEM

This chapter covers sensory modalities, ascending sensory pathways, dermatomes, and cortical sensory functions. Clinical patterns of sensory loss are explained to help localize lesions in peripheral nerves, spinal cord, brainstem, and cortex. A structured sensory examination approach is emphasized for bedside accuracy.

The sensory system allows perception of touch, pain, temperature, position, and vibration. Careful sensory examination is crucial because sensory findings often provide precise anatomical localization, sometimes more accurately than motor signs.

The sensory system is broadly divided into:

Primary sensory modalities
Cortical sensory functions

Sensory Modalities

Primary Sensory Modalities

Primary sensations include:

Pain
Temperature
Light touch
Vibration
Joint position sense (proprioception)

These sensations are transmitted via specific ascending pathways in the spinal cord and brain.

Pain and Temperature

Conducted by spinothalamic tract
Crosses within 1–2 spinal segments
Lesions produce contralateral loss below the level

Pain should be tested using a pin; temperature using cold and warm objects.

Light Touch

Travels through both posterior columns and spinothalamic tracts
Often preserved even when pain is lost

Vibration Sense

Tested using a 128 Hz tuning fork
Assessed over bony prominences
Loss suggests posterior column involvement

Joint Position Sense

Tested by moving distal joints
Loss produces sensory ataxia

Posterior Column Functions

Posterior columns convey:

Vibration sense
Proprioception
Fine touch

Clinical Features of Posterior Column Lesions

Loss of vibration and position sense
Positive Romberg sign
Sensory ataxia
Preservation of pain and temperature

Common Causes

Vitamin B12 deficiency
Tabes dorsalis
Multiple sclerosis
Spinal cord compression

Cortical Sensory Functions

Cortical sensations require integration at the parietal cortex.

These include:

Stereognosis
Graphesthesia
Two-point discrimination
Tactile localization

Testing Cortical Sensation

These tests are performed only when primary sensations are intact.

Astereognosis

Inability to recognize objects by touch
Indicates contralateral parietal lobe lesion

Agraphesthesia

Inability to recognize numbers written on skin

Tactile Extinction

Failure to perceive bilateral simultaneous stimuli
Suggests parietal lobe dysfunction

Dermatomes

A dermatome is the area of skin supplied by a single spinal nerve root.

Clinically Important Dermatomes

Dermatomal sensory loss suggests radiculopathy.

Patterns of Sensory Loss

Understanding patterns is essential for localization.

Peripheral Nerve Lesion

Sensory loss in nerve distribution
Often accompanied by motor weakness

Root Lesion (Radiculopathy)

Dermatomal sensory loss
Radicular pain
Reflex loss at that level

Spinal Cord Lesion

Sensory level
Bilateral involvement
Below the level of lesion

Brown–Sequard Syndrome

Ipsilateral loss of vibration and proprioception
Contralateral loss of pain and temperature

Dissociated Sensory Loss

Loss of pain and temperature with preserved touch
Seen in syringomyelia

Sensory Ataxia

Sensory ataxia occurs due to loss of proprioceptive input.

Clinical Features

Unsteady gait
Stamping gait
Worsening with eye closure
Positive Romberg sign

Differentiation from Cerebellar Ataxia

Pain Syndromes and Clinical Correlation

Neuropathic Pain

Burning, shooting pain
Seen in diabetes, post-herpetic neuralgia

Radicular Pain

Sharp, shooting along dermatome
Exacerbated by movement

Thalamic Pain Syndrome

Severe, persistent pain
Occurs after thalamic stroke
Difficult to treat

Approach to Sensory Examination

Compare side to side
Test distal before proximal
Identify modality involved
Look for sensory level
Correlate with motor findings

A systematic approach ensures accurate localization.