Abnormal activation of excitatory amino acid receptors may play a key role in initiating cellular responses to metabolic insults in mitochondrial enceph-alopathy ⁽¹⁸⁾. The sensitivity of neurons to excitatory amino acids increases with energy deprivation and membrane depolarization. The distribution of lesions seems to be different from hypoxic-ischemic lesions or NMDA and AMPA receptor distributions. This implies that local energy generating capacity and demand is more relevant than the overall pattern of sensitivity of the brain to energy deprivation.

MR findings are variable, and show lesions similar to focal or generalized hypoxia-ischemia in the basal ganglia and cortex and rarely involve the white matter. DWI shows acute or subacute lesions as hyperintense with decreased ADC (Figure 6). MRS shows increased lactate and occasionally increased glutamate/glutamine in the areas of brain thought to be normal on MRI ^(19,20). Reduced NAA is thought to reflect continuing neuronal damage.

Figure 6. A 20 month-old female with clinically suspected mitochondrial encephalopathy

A. T2WI shows multiple hyperintense lesions in the basal ganglia, thalamus, and both hemispheres. The distribution of lesions seems to be neither related to the vascular territories nor similar to hypoxic-ischemic encephalopathy.

B. DWI shows these lesions as hyperintense with decreased ADC (not shown).

C. MRS (TE 135 ms, PRESS) shows inverted lactate peaks and a decreased NAA peak.