CNS trauma

Primary Brain Injury - The First Impact

• Definition: Immediate, irreversible damage from mechanical forces at the moment of impact.
• Mechanisms:
  • Contact: Leads to focal injuries (e.g., skull fractures, contusions).
  • Inertial (Acceleration-Deceleration): Causes coup-contrecoup injuries and diffuse axonal injury (DAI).
• Pathology Types:
  • Contusions: Bruising of brain parenchyma. Most common at frontal and temporal poles.
  • Lacerations: Tearing of brain tissue.
  • Diffuse Axonal Injury (DAI): Widespread shearing of axons from rotational forces; often leads to coma.

⭐ Coup-Contrecoup Pattern: A coup injury occurs at the site of impact, while a contrecoup injury develops on the opposite side as the brain rebounds against the skull.

Intracranial Hemorrhage - Brain Bleed Breakdown

• Epidural Hematoma (EDH)
  • Vessel: Middle meningeal artery, often from temporal bone fracture.
  • CT: Biconvex (lentiform) hematoma. Does not cross suture lines.
  • Assoc.: Trauma, initial loss of consciousness, lucid interval, then decline.
• Subdural Hematoma (SDH)
  • Vessel: Tearing of bridging veins.
  • CT: Crescent-shaped hematoma. Can cross suture lines.
  • Assoc.: Elderly, alcoholics (brain atrophy), shaken baby syndrome.
• Subarachnoid Hemorrhage (SAH)
  • Vessel: Ruptured berry (saccular) aneurysm or AVM.
  • Clinical: Sudden, severe "worst headache of my life."
  • Dx: Xanthochromia (yellow CSF) on lumbar puncture.

⭐ The classic "lucid interval"-temporary improvement after head trauma before rapid deterioration-is a hallmark of an expanding epidural hematoma requiring emergent neurosurgical intervention.

Secondary Injury & Herniation - The Pressure Cooker

• Pathophysiology: A cascade of delayed damage following primary trauma, driven by excitotoxicity, inflammation, and edema, leading to neuronal death.
• Raised Intracranial Pressure (ICP): Normal <15 mmHg. Sustained ICP >20 mmHg is pathologic, causing decreased Cerebral Perfusion Pressure (CPP).
  • Cerebral Perfusion Pressure: $CPP = MAP - ICP$.

• Herniation Syndromes: Brain displacement from mass effect.
  • Subfalcine: Cingulate gyrus herniates under falx cerebri.
  • Transtentorial (Uncal): Medial temporal lobe (uncus) herniates past tentorium cerebelli → CN III palsy (fixed, dilated pupil).
  • Tonsillar: Cerebellar tonsils herniate through foramen magnum → brainstem compression → cardiorespiratory arrest.

⭐ Cushing's Triad: Indicates severely high ICP and impending herniation. Presents as:

1. Hypertension
2. Bradycardia
3. Irregular Respirations

Spinal Cord Injury - Cord in Crisis

• Primary Injury: Initial mechanical trauma (e.g., fracture-dislocation) causing cord compression, contusion, or transection.
• Secondary Injury: Cascade over hours-to-days post-injury.
  • • Ischemia, vasogenic edema, excitotoxicity (glutamate).
  • • Central hemorrhagic necrosis, followed by Wallerian degeneration.
• Spinal Shock: Transient loss of spinal reflexes below the lesion → flaccid paralysis, areflexia. Lasts hours to weeks.
• Neurogenic Shock: Loss of sympathetic tone in lesions above T6 → unopposed vagal activity causing hypotension & bradycardia.

⭐ Autonomic Dysreflexia: A medical emergency in lesions above T6. Noxious stimuli below the lesion (e.g., full bladder) trigger an imbalanced sympathetic response → severe hypertension, headache, sweating.

High‑Yield Points - ⚡ Biggest Takeaways

• Epidural hematoma: Middle meningeal artery rupture, classic lucid interval, and lentiform (biconvex) CT shape; does not cross suture lines.
• Subdural hematoma: Tearing of bridging veins, crescent-shaped hematoma on CT that crosses suture lines; common in elderly and alcoholics.
• Subarachnoid hemorrhage: Ruptured berry aneurysm presents as the "worst headache of my life"; look for xanthochromia on LP.
• Diffuse axonal injury: Shearing forces lead to coma; see punctate hemorrhages at the gray-white junction.