What are the characteristic MRI findings in a pediatric patient with suspected cretinism?

MRI Findings of Cretinism

The characteristic MRI finding in cretinism (congenital hypothyroidism) is delayed myelination for chronological age, which appears as persistent T2 hyperintensity in white matter tracts that should have already myelinated, often accompanied by cortical atrophy and other structural brain abnormalities.

Primary MRI Finding: Delayed Myelination

• Delayed myelination is the hallmark neuroradiologic finding in untreated or inadequately treated congenital hypothyroidism, assessed by evaluating seven key neuroradiologic landmarks including the forceps minor, which is critical for accurate diagnosis 1
• The myelination pattern should be expressed as a "myelination age equivalent" (similar to bone age assessment) to quantify the degree of delay, as this increases diagnostic reliability 1
• White matter that should be myelinated for the patient's chronological age will show:
  • Persistent T2 hyperintensity (bright signal on T2-weighted sequences)
  • Lack of expected T1 hyperintensity (normal myelin appears bright on T1)
  • Failure to achieve age-appropriate myelination milestones 1

Associated Structural Abnormalities

• Cortical atrophy is the most common accompanying structural abnormality, present in the majority (10 of 14 patients) with delayed myelination 1
• Other CNS structural abnormalities frequently coexist with delayed myelination in this population 1
• The severity of MRI abnormalities correlates with the clinical presentation of severely stunted physical and mental growth 2

Clinical-Radiologic Correlation

• Developmental delay is documented in approximately 86% of patients with delayed myelination (12 of 14 patients), making it the most common clinical correlate 1
• The radiologic findings of delayed myelination directly reflect the irreversible mental and growth retardation characteristic of untreated cretinism 2, 3
• MRI is superior to CT for assessing myelination patterns in infants and young children with suspected congenital hypothyroidism 1

Imaging Protocol Recommendations

• Obtain brain MRI with dedicated sequences for myelination assessment, including:
  • T1-weighted sequences (to assess for expected hyperintensity of myelinated white matter)
  • T2-weighted sequences (to identify persistent hyperintensity in unmyelinated regions)
  • Evaluate all seven neuroradiologic myelination landmarks systematically 1
• Age 0-36 months is the critical window for myelination assessment, as this is when normal myelination progression should be most evident 1

Common Diagnostic Pitfalls

• Lack of familiarity with normal myelination milestones is the most common reason for misdiagnosis of delayed myelination, occurring in approximately 15% of cases 1
• Failure to recognize the forceps minor as a key landmark leads to missed diagnoses of delayed myelination 1
• Do not rely on CT scanning, as it is inadequate for assessing myelination patterns in this age group 1
• The diagnosis requires comparison to established age-specific myelination criteria rather than subjective assessment 1

Neurological Pattern in Endemic Cretinism

While the question focuses on MRI findings, it's important to note that in endemic (iodine-deficiency) cretinism, the neurological pattern differs somewhat from congenital hypothyroidism due to thyroid aplasia:

• Endemic cretinism shows spasticity (particularly proximal lower extremities), deaf-mutism, and characteristic mental deficiency 4
• Cerebellar function is largely spared in endemic cretinism, which may influence the distribution of MRI abnormalities 4
• This contrasts with the more global developmental impact seen in thyroid aplasia cases 2