Second Impact Syndrome

KEY FACTS

TERMINOLOGY

• Dysautoregulation/second impact syndrome

• Repeated head trauma within "window of vulnerability"

before brain recovers from initial concussive injury

• May cause catastrophic brain swelling

IMAGING

• Thin, acute subdural hematoma (aSDH)

○ Disproportionate mass effect

– Midline (subfalcine) shift more than accounted for by

thickness of aSDH

• Swollen, hypodense cerebral hemisphere under aSDH

○ Underlying sulci compressed, then obliterated

• Best imaging tools

○ NECT (initial screening) ± MR with T2*, DWI

TOP DIFFERENTIAL DIAGNOSES

• aSDH (without underlying dysautoregulation)

• Nonaccidental trauma (child abuse)

PATHOLOGY

• Thin subdural hematoma

• Underlying brain swollen, hyperemic 

○ May represent excitotoxic injury

○ ± secondary infarcts

○ ± uncal, central, tonsillar herniations

CLINICAL ISSUES

• Severe headache

• Vomiting

• May have dramatic, sudden collapse, coma

• Most common in

○ Adolescent male patients involved in high-impact sports

○ Abused infants

○ Elderly patients with repeated falls, head impacts

(Left) Axial NECT in an abused

infant with repeated inflicted

head trauma shows a mixeddensity right subdural

hematoma (SDH) ﬊. The

underlying right hemisphere is

diffusely swollen and

hypodense. Note

disproportionate mass effect,

subfalcine herniation ſt

compared with relatively thin

SDH. (Right) Axial NECT in an

elderly patient with repeated

falls and head trauma shows a

thin acute SDH ﬈. Underlying

cortex is swollen ﬇ with

disproportionate mass effect

on left lateral ventricle.

(Left) Axial NECT in an abused

infant shows mixed acute,

subacute/chronic SDH

overlying a swollen,

hypodense hemisphere. (Right)

Axial fat-suppressed T2WI in

the same case 2 days later,

after evacuation of the SDH,

shows swollen hyperintense

cortex ﬇ underlying bilateral

SDHs. Note hyperintensity in

the corpus callosum splenium

st. Findings are consistent

with excitotoxic injury.

Trauma, and Stroke

Brain: Pathology-Based Diagnoses: Malformations,

51