Radiological presentation of MELAS syndrome
Magnetic resonance imaging is the primary tool in the diagnosis of MELAS syndrome. Evaluation of serial scans reveals multiple hyperintense lesions that are localized predominantly in the cortico-subcortical structures. Their location may vary.
An essential diagnostic step is the use of diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps. In MELAS syndrome, the signal in these sequences has a heterogeneous character. This directly indicates the simultaneous presence of two pathological processes: vasogenic and cytotoxic edema.
Additionally, proton MR spectroscopy is used. It allows the detection of a characteristic lactate peak directly in the affected area (a classic example of localization is the occipital lobe).
Differential diagnosis: Wernicke Encephalopathy (Wernicke Encephalopathy)
When analyzing MRI scans, it is important to differentiate MELAS syndrome from other metabolic and toxic brain lesions, such as Wernicke encephalopathy.
This pathology is frequently associated with alcoholism, but it can develop against the background of prolonged starvation, malabsorption syndrome, massive glucose infusion without vitamin B1 in debilitated patients, as well as digitalis poisoning (digitalis). Treatment requires immediate thiamine infusion, which helps prevent mortality or irreversible Korsakoff dementia.
Clinical triad (occurs in only 30% of patients):
- Oculomotor disorders (nystagmus, conjugate gaze palsy, ophthalmoplegia).
- Ataxia.
- Altered mental status (confusion).
MRI semiotics of Wernicke encephalopathy:
- T2-WI: Bilateral symmetrical hyperintensity. Most commonly involves the medial thalami and periventricular zones of the third ventricle. The mamillary bodies (mamillary bodies), periaqueductal gray, pons, medulla oblongata, and basal ganglia are also affected. The cortex is rarely involved.
- DWI: Diffusion restriction due to cytotoxic edema, sometimes with increased diffusivity due to the vasogenic component.
- T1-WI / T2*: Microhemorrhages in the thalami and mamillary bodies are rarely seen (a sign of poor prognosis).
- Contrast enhancement: In 50% of cases, contrast accumulates in the periaqueductal regions. Marked enhancement in the mamillary bodies (up to 80% of cases) is a highly specific sign and may precede changes on T2-WI.
Upon chronic progression (Korsakoff syndrome), the signal on T2-WI decreases, and diffuse brain atrophy increases, especially noticeable in the mamillary bodies and at the level of the midbrain (mesencephalon).
Differential diagnosis: Osmotic Demyelination (Osmotic Demyelination)
Another condition with a specific radiological presentation is osmotic demyelination. It usually occurs due to overly rapid correction of hyponatremia. This leads to blood-brain barrier breakdown, accumulation of hypertonic fluid in the extracellular space, and non-inflammatory demyelination.
Clinically manifested by paralysis, dysphagia, dysarthria, and pseudobulbar palsy. Frequently fatal. Central pontine myelinolysis (involvement of pontine fibers) and, less commonly, extrapontine myelinolysis are distinguished.
MRI features:
- T2-WI: Hyperintense zone in the central pons, forming a symmetrical "trident" pattern (trident pattern). The corticospinal tracts and periphery remain intact.
- T1-WI: Moderately hypointense lesion.
- DWI / ADC: The lesion is hyperintense on DWI with low signal on ADC maps.
- Contrast: Peripheral enhancement (rare).
Briefly: answers to topical questions
What clinical symptoms are characteristic of patients with MELAS syndrome, in addition to radiological findings?
Patients with MELAS syndrome are characterized by the development of stroke-like episodes, encephalopathy, seizures, and myopathy. Additionally, patients exhibit:
- Lactic acidosis — a metabolic disorder.
- Cardiomyopathy — a comorbid pathology.
- Endocrinopathies — a comorbid pathology.
Acute metabolic decompensation can be triggered by factors that increase metabolic demands (e.g., fever).
In which brain structures are the lesions localized in extrapontine myelinolysis?
In extrapontine myelinolysis, lesions can be localized outside the pons.
Common localization:
- Cerebellum (cerebellum).
- Lateral geniculate body (lateral geniculate body).
Rare localization:
- Thalamus (thalamus).
- Subcortical white matter (subcortical white matter).
- Basal ganglia (basal ganglia).
Involvement of the thalami, basal nuclei, and corpus callosum trunk has also been described in osmotic demyelination syndrome.
The "trident" pattern on T2-WI is a classic marker of osmotic demyelination in the central pons.
Class and exam questions
Where is the lactate peak most frequently localized in MELAS syndrome?
During proton MR spectroscopy, a lactate peak is frequently detected in the occipital lobe.
Which MRI sign is most specific for Wernicke encephalopathy?
Marked enhancement of the mamillary bodies (occurs in 80% of cases). This sign may appear even earlier than typical T2-WI changes.
Which structures remain intact in central pontine myelinolysis?
Despite involvement of the central pons, the peripheral regions and corticospinal tracts remain intact.
What else to study on this topic
- Mechanisms of vasogenic and cytotoxic edema formation on DWI
- Proton MR spectroscopy: principles of lactate peak detection
- Korsakoff dementia as an outcome of untreated Wernicke encephalopathy
- Extrapontine myelinolysis: localization and radiological signs
- Digitalis poisoning as a rare cause of encephalopathy
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