18.8 Somatic, Dissociative, Personality & Neurodegenerative Disorders

18.8 Somatic, Dissociative, Personality & Neurodegenerative Disorders

Somatic Symptom & Dissociative Disorders

• Somatic Symptom Disorder: One or more distressing physical symptoms accompanied by disproportionate, persistent anxiety, excessive time and energy devoted to health concerns, and high health service utilization.
• Illness Anxiety Disorder (formerly Hypochondriasis): Preoccupation with harboring or acquiring a serious undiagnosed medical illness; somatic symptoms are completely absent or minimal.
• Conversion Disorder (Functional Neurological Symptom Disorder): Sudden loss or alteration of voluntary motor or sensory function (blindness, limb paralysis, aphonia, pseudo-seizures) that is clinically incompatible with recognized neuroanatomical or medical pathophysiology. Typically triggered by acute severe stress or emotional conflict. Classically associated with la belle indifférence—a perplexing lack of emotional concern or distress regarding the severe physical disability.
• Factitious Disorder vs. Malingering:
  • Factitious Disorder (Munchausen Syndrome): Deceptive falsification or induction of physical or psychological symptoms solely to adopt the sick role, without external tangible incentives.
  • Malingering: Deliberate fabrication of illness motivated by external secondary gain (e.g., obtaining disability compensation, evading military service, avoiding criminal prosecution; not a mental disorder in DSM-5).
• Dissociative Disorders:
  • Dissociative Amnesia: Inability to recall important autobiographical memories (typically traumatic). May present with dissociative fugue (sudden, purposeful travel away from home accompanied by confusion about personal identity).
  • Depersonalization / Derealization Disorder: Persistent feelings of detachment from one's own mind or body (depersonalization) or surroundings (derealization: feeling the world is artificial or foggy), with intact reality testing.
  • Dissociative Identity Disorder (DID): Disruption of identity marked by two or more distinct personality states (alters) that recurrently control behavior, accompanied by dissociative amnesia; overwhelmingly rooted in chronic, severe early childhood abuse.

The Personality Disorders: Clusters A, B, and C

A Personality Disorder represents an enduring, rigid, pervasive pattern of inner experience and behavior that deviates markedly from cultural expectations, remains stable across adulthood, and produces significant impairment. Personality disorders are characteristically ego-syntonic—the patient perceives their maladaptive traits and behaviors as natural, correct, and acceptable parts of themselves (contrasting with ego-dystonic disorders like OCD).

Neurodegenerative Disorders: Alzheimer's vs. Parkinson's

MCAT questions frequently test the biochemical and neuropathological differences between the two most common neurodegenerative diseases:

1. Alzheimer's Disease (AD)

• Clinical Presentation: The leading cause of dementia in older adults. Insidious, progressive cognitive deterioration beginning with anterograde episodic memory impairment, advancing to aphasia (language deficit), apraxia (motor planning deficit), agnosia (sensory recognition deficit), and executive dysfunction.
• Neuropathological Hallmarks:
  1. Extracellular Amyloid-Beta ($A\beta_{42}$) Plaques: Dense spherical protein aggregates formed from abnormal sequential cleavage of the transmembrane Amyloid Precursor Protein (APP) by $\beta$-secretase and $\gamma$-secretase enzymes.
  2. Intracellular Neurofibrillary Tangles (NFTs): Filamentous bundles composed of hyperphosphorylated tau protein. Tau normally stabilizes axonal microtubules; hyperphosphorylation causes tau to detach and aggregate, disrupting axonal transport and inducing cell death.
  3. Cholinergic Depletion: Profound loss of cholinergic neurons in the Basal Nucleus of Meynert that project to the hippocampus and neocortex. Standard pharmacological therapy employs acetylcholinesterase inhibitors (e.g., donepezil) to boost synaptic acetylcholine.
  4. Gross Pathology: Widespread cerebral cortical atrophy, prominent sulcal widening, gyral narrowing, and compensatory enlargement of lateral ventricles.

2. Parkinson's Disease (PD)

• Clinical Presentation: A progressive motor system disorder characterized by the classic motor tetrad (TRAP):
  • T - Tremor: Resting tremor ("pill-rolling"), 4–6 Hz frequency, which attenuates during purposeful voluntary movement.
  • R - Rigidity: Increased muscle tone presenting as "cogwheel rigidity" during passive limb movement.
  • A - Akinesia / Bradykinesia: Slowness of voluntary movement, reduced arm swing, masked facies (hypomimia), and shuffling festinating gait.
  • P - Postural Instability: Impaired balance and righting reflexes, causing frequent falls (emerges in later stages).
• Neuropathological Hallmarks:
  1. Substantia Nigra Degeneration: Marked loss of dopaminergic neurons in the Substantia Nigra pars compacta (SNpc) of the midbrain. This depletes dopamine delivery to the striatum (caudate and putamen), disrupting the balance between basal ganglia direct (movement-promoting) and indirect (movement-inhibiting) motor pathways.
  2. Intracellular Lewy Bodies: Eosinophilic cytoplasmic neuronal inclusions composed of aggregated, misfolded $\alpha$-synuclein protein.
  3. Pharmacological Management: Dopamine cannot cross the blood-brain barrier. Therapy relies on L-DOPA (Levodopa), a dopamine precursor that crosses the blood-brain barrier via large neutral amino acid transporters, administered in combination with Carbidopa (a peripheral dopa-decarboxylase inhibitor that prevents premature conversion to dopamine in peripheral tissues, minimizing systemic side effects).