# Clinical Cases: Sensory Physiology ## Case 1: Diabetic Peripheral Neuropathy - Sensory Loss and Pain ### Clinical Image ![Diabetic Neuropathic Ulcer](case_01_image.jpg) *Source: [Wikimedia Commons - Neuropathic heel ulcer](https://commons.wikimedia.org/wiki/File:Neuropathic_heel_ulcer.jpg) - CC BY-SA 3.0* ### Patient Presentation A 62-year-old male with a 20-year history of poorly controlled type 2 diabetes mellitus presents for evaluation of numbness, tingling, and burning pain in his feet that has progressively worsened over 3 years. He describes the sensation as "walking on cotton" and has difficulty feeling the floor. The burning pain is worse at night and disturbs his sleep. He has noticed small wounds on his feet that he does not remember getting. His wife found a large blister on his heel from ill-fitting shoes that he never felt. He also reports difficulty maintaining balance in the dark. ### Demographics - Age: 62 years - Sex: Male - Past Medical History: Type 2 DM x 20 years, HbA1c 9.8%, hypertension, hyperlipidemia ### Chief Complaint Progressive numbness, tingling, burning pain in feet, and unnoticed wounds ### Physical Examination - General: Obese male in no acute distress - Feet examination: - Dry, cracked skin with absent hair on lower legs - 2 cm ulcer on right heel, Wagner grade 2 (deep to tendon level, no infection) - Calluses over metatarsal heads bilaterally - Deformed toes (claw toe deformity from motor involvement) - Neurological: - Sensory: Loss of light touch, pinprick, and temperature sensation in stocking distribution to mid-calf; severe loss of vibration sense at toes, reduced at ankles - 10-g monofilament test: Unable to perceive at 6/10 sites on each foot - Proprioception: Impaired at great toes - Motor: Mild weakness of toe extensors and intrinsic foot muscles - Reflexes: Absent ankle jerks, 1+ knee jerks - Gait: Wide-based, positive Romberg test (worsens with eyes closed) ### Workup - HbA1c: 9.8% (poor long-term control) - Nerve conduction studies: Reduced sensory nerve action potential amplitudes in sural nerve (axonal pattern); mildly reduced conduction velocities - EMG: Chronic denervation changes in intrinsic foot muscles - Semmes-Weinstein monofilament: Unable to perceive 10-g filament (high risk for ulceration) - Ankle-brachial index: 0.95 (rules out significant peripheral artery disease) - Wound assessment: No signs of osteomyelitis clinically (MRI if concern) ### Diagnosis Diabetic Peripheral Neuropathy (distal symmetric polyneuropathy) with neuropathic pain and loss of protective sensation; Wagner grade 2 neuropathic foot ulcer ### Treatment 1. Glycemic control optimization: Refer to endocrinology, adjust medications, goal HbA1c <7% 2. Neuropathic pain management: - First-line: Duloxetine (SNRI) or pregabalin/gabapentin - Alternatives: Tricyclic antidepressants (amitriptyline), topical capsaicin, lidocaine patches 3. Foot ulcer management: - Offloading: Total contact cast or removable cast walker - Wound care: Debridement, moist wound healing - Infection monitoring: Watch for signs of cellulitis, osteomyelitis 4. Foot care education: - Daily foot inspection (use mirror if needed) - Proper footwear (protective, well-fitted shoes) - Never walk barefoot - Regular podiatry visits 5. Fall prevention: Physical therapy for balance, home safety assessment 6. Cardiovascular risk reduction: Blood pressure and lipid control ### Physiological Principles Demonstrated - **Small vs. large fiber neuropathy**: Diabetic neuropathy often affects small fibers (C fibers for pain/temperature, autonomic fibers) early, causing burning pain and autonomic dysfunction. Large fiber involvement (Aβ for vibration/touch, Aα for proprioception) causes numbness and ataxia. - **Neuropathic pain mechanism**: Paradoxically, nerve damage can cause pain. Mechanisms include ectopic firing of damaged nociceptors, central sensitization, loss of inhibitory interneurons (gate control disruption), and ephaptic transmission (cross-talk between damaged fibers). - **Negative vs. positive sensory symptoms**: Negative symptoms (numbness, loss of sensation) result from nerve fiber loss. Positive symptoms (tingling, burning, electric shocks) result from abnormal spontaneous activity in damaged fibers. - **Loss of protective sensation**: The inability to perceive pain from injury removes the protective warning system. Patients with loss of 10-g monofilament perception have a 10-fold increased risk of foot ulceration. - **Stocking-glove distribution**: The longest nerve fibers (to toes/fingers) are most vulnerable to metabolic insults, explaining why symptoms begin distally and progress proximally in a length-dependent pattern. - **Romberg sign**: Proprioceptive loss from large fiber damage impairs position sense. With eyes closed (removing visual compensation), patients become unsteady, indicating reliance on vision to maintain balance. --- ## Case 2: Sensorineural Hearing Loss - Cochlear Dysfunction ### Clinical Image ![Hearing Aid](case_02_image.jpg) *Source: [Wikimedia Commons - Hearing aid](https://commons.wikimedia.org/wiki/File:Hearing_aid_20080620.jpg) - CC BY-SA 3.0* ### Patient Presentation A 68-year-old male presents to his primary care physician with progressive difficulty hearing over the past 5 years. He reports trouble understanding speech, especially in noisy environments like restaurants, and frequently asks people to repeat themselves. His wife complains that he turns the television volume too loud. He has worked in construction for 40 years and was exposed to loud machinery without hearing protection. He denies vertigo, ear pain, discharge, or recent infections. He reports constant high-pitched ringing in both ears (tinnitus). ### Demographics - Age: 68 years - Sex: Male - Occupational history: 40 years in construction with significant noise exposure ### Chief Complaint Progressive bilateral hearing loss, difficulty understanding speech, and tinnitus ### Physical Examination - Otoscopy: External auditory canals clear, tympanic membranes intact bilaterally with normal landmarks - Whispered voice test: Unable to hear whispered words at arm's length - Weber test: Sound does not lateralize (midline) - symmetric hearing loss - Rinne test: Air conduction > bone conduction bilaterally (positive Rinne - consistent with sensorineural loss) - Cranial nerves: Otherwise normal including facial nerve function ### Workup - Pure tone audiometry: - Bilateral symmetric sensorineural hearing loss - High-frequency sloping pattern (characteristic of presbycusis with noise-induced contribution) - 4 kHz notch present bilaterally (acoustic notch - pathognomonic of noise exposure) - Speech discrimination: Reduced, especially in background noise - Tympanometry: Type A (normal) bilaterally, excluding conductive component - Acoustic reflex: Present but elevated thresholds - MRI internal auditory canals: No evidence of vestibular schwannoma (acoustic neuroma ruled out) ### Diagnosis Mixed Presbycusis and Noise-Induced Sensorineural Hearing Loss with Tinnitus ### Treatment 1. Hearing aids: Digital programmable bilateral hearing aids - Customized to audiogram pattern (more amplification at high frequencies) - Directional microphones to improve speech-in-noise understanding 2. Hearing assistive technologies: FM systems, captioned telephone 3. Communication strategies training: Face-to-face communication, quiet environments 4. Tinnitus management: - Hearing aids (often reduce tinnitus perception) - Sound therapy (white noise, environmental sounds) - Cognitive behavioral therapy if distressing 5. Hearing protection education: Prevent further damage (earplugs, avoid loud environments) 6. Aural rehabilitation: Consider speech-reading training 7. Consider cochlear implant evaluation if profound loss not benefiting from aids ### Physiological Principles Demonstrated - **Tonotopic organization**: The basilar membrane is organized by frequency - high frequencies at the base (near oval window), low frequencies at the apex. Noise damage and aging preferentially affect the high-frequency basal region, explaining the characteristic high-frequency hearing loss pattern. - **4 kHz notch**: The external auditory canal resonates at approximately 2-4 kHz, amplifying these frequencies. Combined with basilar membrane mechanics, the 4 kHz region is most vulnerable to noise damage, creating the characteristic audiometric notch. - **Hair cell mechanotransduction**: Sound vibrations deflect stereocilia on hair cells, opening mechanically-gated ion channels. The unusual high K+ concentration of endolymph creates the driving force for depolarization. Hair cells do not regenerate in humans; damage is permanent. - **Outer vs. inner hair cells**: Outer hair cells amplify sound through electromotility (cochlear amplifier). Their loss reduces sensitivity and frequency selectivity, explaining difficulty with speech discrimination in noise. Inner hair cells are the primary sensory receptors transmitting signals to the auditory nerve. - **Weber and Rinne tests**: In sensorineural loss, both air and bone conduction are impaired (inner ear or nerve problem), but air conduction remains better than bone (positive Rinne). Weber does not lateralize in symmetric loss. In conductive loss, Weber lateralizes to the affected ear, and bone conduction > air conduction (negative Rinne). - **Speech-in-noise difficulty**: High-frequency consonants (s, f, th, sh) carry much of the information distinguishing words. High-frequency hearing loss reduces speech discrimination, especially in background noise where the auditory system cannot use frequency-selective filtering. --- ## Case 3: Complex Regional Pain Syndrome - Central Sensitization ### Clinical Image ![Complex Regional Pain Syndrome](case_02_image.jpg) *Source: Clinical illustration of pain pathway sensitization and CRPS features* ### Patient Presentation A 45-year-old female presents to the pain clinic with persistent, severe pain in her right hand 4 months after a Colles fracture that was treated with casting. Although the fracture has healed with good bony alignment on X-ray, she describes burning, throbbing pain that far exceeds what would be expected from the original injury. The slightest touch to her hand causes excruciating pain (allodynia), and even air currents are painful. Her hand is swollen, discolored (alternating between red and blue), and sweats excessively. She cannot use the hand and keeps it wrapped and protected. ### Demographics - Age: 45 years - Sex: Female - Precipitating event: Right distal radius fracture 4 months ago ### Chief Complaint Severe persistent burning pain in right hand disproportionate to original injury, swelling, color changes, and hypersensitivity to touch ### Physical Examination - Right hand/wrist: - Severe allodynia: Light touch elicits intense pain (cotton swab causes severe discomfort) - Hyperalgesia: Pinprick causes disproportionate pain - Skin: Edematous, alternating red/dusky discoloration (vasomotor instability) - Temperature: Cooler than left hand by 2C (measured with thermometer) - Sweating: Hyperhidrosis compared to left hand - Hair/nail changes: Increased hair growth, brittle nails - Motor: Weakness and tremor, limited range of motion due to pain - Contracture beginning in fingers - Left hand: Normal examination ### Workup - X-ray right hand: Healed distal radius fracture, patchy osteoporosis (spotted demineralization characteristic of CRPS) - Three-phase bone scan: Increased uptake in late (delayed) phase in affected hand - consistent with CRPS - Thermography: Temperature asymmetry >1C between hands - Labs: Normal inflammatory markers (rules out infection, inflammatory arthritis) - EMG/NCS: Normal (CRPS Type I - no identifiable nerve injury; Type II has confirmed nerve lesion) ### Diagnosis Complex Regional Pain Syndrome Type I (CRPS-I, formerly reflex sympathetic dystrophy) ### Treatment 1. Early aggressive physical and occupational therapy (most important intervention) - Mirror therapy: Patient performs movements with unaffected hand while watching mirror reflection, tricking brain into perceiving affected hand moving painlessly - Graded motor imagery - Desensitization exercises 2. Medications for neuropathic pain: - First-line: Gabapentin, pregabalin, or duloxetine - Topical agents: Lidocaine patches, capsaicin - Low-dose naltrexone (emerging evidence) 3. Interventional approaches if refractory: - Sympathetic nerve blocks (stellate ganglion block for upper extremity) - Spinal cord stimulation - Intrathecal drug delivery 4. Psychological support: CBT, coping strategies, treatment of depression/anxiety 5. Vitamin C 500 mg daily (may reduce risk of CRPS after fractures) 6. Avoid immobilization; encourage functional use within pain tolerance ### Physiological Principles Demonstrated - **Central sensitization**: After injury, repeated nociceptor activation causes hyperexcitability of spinal cord dorsal horn neurons. This "wind-up" phenomenon involves NMDA receptor activation and leads to amplification of pain signals. - **Allodynia mechanism**: Central sensitization causes normally innocuous Aβ fiber input (light touch) to activate pain circuits, producing pain from non-painful stimuli. This represents a failure of the gate control mechanism. - **Hyperalgesia**: Enhanced pain from painful stimuli results from both peripheral sensitization (lowered nociceptor thresholds from inflammatory mediators) and central sensitization (amplified spinal cord responses). - **Autonomic dysregulation**: The vasomotor instability (color changes), temperature changes, and sweating abnormalities reflect disordered sympathetic function. Whether this is cause or effect remains debated. - **Neuroplasticity in pain**: Chronic pain can cause maladaptive reorganization of sensory cortex. The affected limb's representation may shrink, and the brain's body schema becomes distorted. Mirror therapy attempts to normalize this cortical representation. - **Gate control theory application**: The theory predicts that enhancing large fiber (Aβ) input should reduce pain transmission. In CRPS, central sensitization disrupts this mechanism, and stimulation (e.g., spinal cord stimulation) at specific frequencies and intensities attempts to restore inhibitory control.