Understanding Tactile Sensation and Perception

Assessing Abilities and Capacities: Sensation

• The AOTA defines sensory-perceptual skills as actions or behaviors a client uses to locate, identify, and respond to sensations, interpret, organize, and remember sensory events. These sensations include visual, auditory, proprioceptive, tactile, olfactory, gustatory, and vestibular sensations.
• Tactile, thermal, and proprioceptive sensations are discussed in this lecture. Methods for evaluating these senses are described.
• Tactile sensation, especially in the hands, is crucial for competent occupational functioning throughout the body.
• After birth, infants experience a multitude of tactile sensations different from those in the womb.
• Infants quickly learn to interpret these tactile stimuli. This ability is vital for the development of skills like grasping objects, bringing hands together, and touching a parent's face.
• Loss of sensation without motor loss shows the close connection between the motor and sensory systems. Fine motor coordination and manipulative ability decrease with sensory loss in the hand.
• The force needed to maintain a grasp on an object depends on sensory feedback, typically just sufficient to overcome gravity and friction.
• Some activities entirely depend on touch: determining the temperature of food from a microwave, assessing if food is warm enough to carry, using haptic perception to discern the shape of objects in a pocket, fastening a necklace, or closing a zipper.
• Sensory evaluation purposes include assessing the type and extent of sensory loss, evaluating and documenting sensory recovery, assisting in diagnosis, determining impairment and functional limitation, directing occupational therapy intervention, determining the time to begin sensory re-education, determining the need for education to prevent injuries during occupational functioning, and determining the need for desensitization.
• Tactile receptors are found in skin, muscles, and joints. Each receptor type is specialized for specific sensory stimuli (e.g., touch, temperature, pain).
• Different types of tactile sensations include: constant touch or pressure, moving touch or vibration, proprioception and kinesthesia, pain (pinprick), chronic pain, and temperature.
• Sensory neurons form a sensory unit, with a receptive field encompassing the stimulation area. The center of the receptive field generally responds more readily to stimuli than the periphery. Adjacent receptive fields overlap.
• Innervation density (the number of sensory units) in a skin area is a factor in its sensitivity. Locations like the face, hand, and fingers show high sensitivity due to higher innervation density. These areas have large representations in the somatosensory cortex, the postcentral gyrus of the parietal lobe.
• Tactile stimuli of adequate strength activate different receptors, including constant and moving touch receptors, and potentially pain receptors.
• Temperature extremes activate pain receptors rather than temperature receptors.
• Sensitivity is based on input from multiple receptors for both joint position/movement (proprioception) and joint position (kinesthesia).
• The variation in the number of sensory units in a specific skin area is called innervation density. The face, hands, and fingers have high innervation densities and are highly sensitive, reflected in their substantial representation in the somatosensory cortex.

Role of Sensation in Occupational Functioning

• Ian Waterman Case: A 19-year old acquired a rare neurological illness, leading to the loss of all sensation below his neck. He had no bodily awareness of his limbs and lost the ability to control movement in any part of his body, but muscle movement wasn't directly affected. This case highlights how sensory input is crucial in complex activities involving control of body movement.

• Loss of sensation, particularly in the hands, can cause fine motor issues and reduced manipulative abilities

• The force to maintain a grip on objects depends on sensory feedback (i.e. friction from the grip surface).

• Inaccurate or excessive force usage in gripping can lead to objects slipping, delicate objects getting damaged, or muscle fatigue from overactivity.

• Some everyday activities rely completely on touch-based sensory feedback (e.g. sensing the warmth of food in a microwave or determining the weight of objects)

• Haptic perception is used for determining the size and shape of objects.

Purposes of Sensory Evaluation

• Assess the type and extent of sensory loss.
• Evaluate and document sensory recovery.
• Determine impairment and functional limitations.
• Assist in diagnosis.
• Provide direction for occupational therapy intervention.
• Determine time to begin sensory re-education.
• Determine need for education to prevent injury during occupational functioning.
• Determine the need for desensitization.

Neurophysiological Foundations of Tactile Sensation

• Tactile receptors are located in skin, muscles, and joints. Each specific receptor type is specialized for a particular type of sensory stimulation (like touch, temperature, or pain).

• Tactile sensation types include constant touch or pressure, moving touch or vibration, proprioception and kinesthesia, pain (pinprick), chronic pain and temperature.

• Sensory neurons and their terminations form a sensory unit.

• Stimuli in the receptive field can evoke a response; stimuli in the center of the field are easier to detect than those at the periphery.

• Adjacent receptive fields overlap, producing a multi-faceted response across sensory units when stimulated

• The degree of sensitivity is also affected by the density of sensory units in a region, high density regions demonstrating greater sensory sensitivity, such as face, hands, and fingers.

• Sensory units and their receptive fields' location in the cortex (specifically, the postcentral gyrus of the parietal lobe) influence tactile sensitivity.

Body Parts Sensory Representation in the Postcentral Gyrus

• Sensory representation in the postcentral gyrus illustrates that specific body parts are represented in specific locations of the brain. Regions with higher density of sensory representation areas are more sensitive.
• Body parts like fingers, hands, lips, and face have an extremely large sensory representation while other body parts have a smaller one.

Somatosensory Deficit Patterns

• Sensory pathway disruptions or damage to sensory areas can diminish or completely eliminate sensation.
• Sensory deficit severity and extent can usually be predicted based on the injury location and type.
• Sensory impairment patterns reflect the damaged neuroanatomical structures (either in the central or peripheral nervous system)

Cortical Injury

• Patients with stroke or brain injuries demonstrate sensory loss based on specific neurons' function within the central nervous system.

• Fine touch and proprioception are primarily affected, with the sensitivity of temperature and pain being affected less.

• Sensory effects from strokes vary depending on blood vessel occlusions, usually impacting the body side opposite to the affected blood vessel.

• Occlusions in specific critical arteries can lead to loss of several sensations, with the anterior cerebral artery blockage causing more loss in the leg than in the face or arm because its distribution targets the medial aspect of the cerebral cortex.

• Sensory loss in pain and proprioception perception tends to occur more frequently following right-side strokes than left-side strokes, while impaired visual function has been associated with stroke in research.

• Sensory impairments associated with head injuries are usually heterogeneous due to the diffuse brain damage pattern.

• Recovery from cortical injury-related sensory deficits may result from reduced swelling (edema), improved blood flow, the brain's ability to reorganize (cortical plasticity), and learning.

• Pain and temperature receptors recover before light touch and proprioception.

• Strategies for assessing sensory function following cortical injury should focus on areas that are less likely to be affected first, and move on to more sensitive, possibly compromised, areas as assessment progresses.

Guidelines for Planning Assessments After Cortical Injuries

• Rapid initial assessment of areas where sensation is predicted to be intact, followed by more detailed assessments of any deficit areas in a more concentrated way helps with time optimization, effort, and the overall efficiency of assessing patients.

• Assess the affected side more thoroughly, which is almost always the opposite of the site of the causative lesion.

• Sensory evaluation approaches vary; initially focus on evaluating touch/proprioception if those areas are likely intact.

• Assess pain/temperature if touch/proprioception is reduced or absent during sensory function assessment.

Spinal Cord Injury

• Complete spinal cord lesions lead to a complete absence of sensation in dermatomes below the lesion level.
• The sensory loss extent strictly depends on the level of damage in the spinal cord.
• Paresthesia or tingling/pins-and-needles sensations are frequent symptoms at the level of injury or the dermatomes associated with it.
• Incomplete lesions produce sensory loss depending on particular spinal tracts affected, where injury to the posterior portion usually leads to a diminished sense of light touch/vibration but may not initially affect the sensation of pain/temperature

Brown-Sequard Syndrome

• Damage to one side of the spinal cord leads to loss of touch, vibration, and proprioception on the damaged side.
• The opposite side of the spinal cord shows loss of temperature and pain sensation.
• Temperature and pain sensations cross over to the opposite side of the spinal cord quickly (as compared to touch sensations, which generally cross over the spinal cord in the medulla), while touch sensations ascend to the medulla before crossing over.

Spinal Cord Injury

• Damage to the central spinal cord typically causes bilateral loss of pain and temperature sensation below the injury level.
• Spinal cord nerve fibers cross to the opposite side within the cord, resulting in potentially bilaterally affected pain/temperature sensations.
• Mild to moderate spinal cord compression can lead to decreased or absent sensation, potentially affecting dermatomes both at the compression site and below it.
• Sensory recovery following traumatic spinal cord injury is usually within the first year and most noticeable during the first 3-6 months, particularly in incomplete injuries.
• Reduced congestion/swelling and improved blood flow in the injured areas are contributory to recovery mechanisms within the spinal cord.

Dermatomes

• A dermatome is a specific body region served by sensory fibers of each spinal nerve, used for neurological mapping.

Guidelines for Planning Assessments for Patients with SCI

• Sensory assessments of spinal cord injury patients should use pinprick and cotton ball testing to locate the sensory level and/or the zones of preservation.

• Testing should comprehensively assess and evaluate the sensory levels bilaterally (and from a rostral to caudal direction) within each dermatome for accurate evaluations of the affected body region.

• Areas of sparing/partial preservation should be identified and mapped for better information and treatment planning.

Guidelines for Planning Assessments for Patients with SCI

• Multiple sensory modalities (e.g., pain, temperature, touch, vibration, proprioception) should be thoroughly evaluated, especially in patients with incomplete or unknown lesions.
• Tests for assessing spinothalamic (pain and temperature) function must include thermal threshold testing (using a Medoc Thermal Sensory Analyzer, for example).
• Determining posterior column function involves assessing vibration thresholds (using a Bio-Thesiometer).
• Tests of both spinothalamic and posterior column functions can include Quantitative Sensory Testing.