Presentation:

38 year-old female with altered mental status after recent heroin overdose.

Key findings:

Diffuse increased signal intensity throughout the supratentorial white matter with sparing of the basal ganglia.
Relative sparing of the subcortical U-fibers.
Relative diffuse high DWI signal of involved structures, usually not representing true diffusion restriction.

Other findings can include symmetric butterfly wing pattern signal intensity of the cerebellar white matter

Diagnosis:

1. Diffuse supratentorial white matter disease with sparing of the grey structures with known clinical history suggest heroin-induced leukoencephalopathy.

High-yield Sequences:

FLAIR – Confluent and symmetric white matter abnormalities, often involving the corpus callosum.
DWI – Similar confluent and symmetric white matter abnormalities to FLAIR, may involve the corpus callosum.
SWI – Punctate microhemorrhages in the periventricular white matter occur in ~15% of patients with toxic leukoencephalopathy.

Discussion:

Heroin-induced leukoencephalopathy is a form of toxic leukoencephalopathy secondary to inhalation of heroin fumes, known as "chasing the dragon".

This results in symmetric spongiform degeneration, often of both supratentorial and infratentorial white matter and the corticospinal solitary tracts.

The above-mentioned subcortical U-fibers, also known as short association fibers, are connections between adjacent gyri along the very outer aspect of the subcortical white matter. These fibers are among the last parts of the brain to myelinate with very slow myelin turnover. Diseases altering myelin metabolism/turnover will therefore affect these last, resulting in relative "sparing" of the U-fibers.

Resources:

Different imaging presentations of common drug-induced encephalopathies