Mitochondrial hepatopathy: Respiratory chain disorders- ‘breathing in and out of the liver’

Table 3 Stepwise evaluation of mitochondrial hepatopathies (respiratory chain disorder/non- respiratory chain disorders)

Steps	Description	Additional action
Level-1 (body fluids)	Basic: CBC, INR, AFP, CPK, NH3, sugars, phosphorous, urine ketones. Advanced: Lactate: Pyruvate (1 h post feeds); Ketone Body ratio, 3OH-butyrate: Acetoacetate; Serum acylcarnitine profile; Urine organic acidogram; Serum aminoacidogram; 3 Methyl Glutaconic acid in serum/urine; CSF lactate: Pyruvate, CSF alanine, protein; Plasma thymidine (MNGIE); Leucocyte CoQ levels	Parallel level-1: Evaluate other involved systems: CNS: MRI/MR-Spectroscopy, EEG; Eye: Fundus evaluation, clinical evaluation for ophthalmoplegias; Hearing screen; Heart: 2D-Echo, ECG; Renal: urine electrolytes, proteins, amino acids; Muscle: Muscle biopsy (Level-1 in case of primary muscle involvement, level-3 otherwise); Endocrine: HbA1c, 8 AM cortisol; Pancreas: Fecal elastase
Level-2 (genetics)	Common genes genotyping: POLG-1; DGUOK; MPV-17; SUCLG-1; TRMU; C10ORF2/Twinkle; CPT-1; mtDNA point mutations	Alternative level-2: Next generation sequencing/clinical exome sequencing for simultaneous evaluation of all mitochondrial DNA and nuclear DNA
Level-3 (invasive)	Tissue diagnosis: (1) Liver biopsy: Light microscopy including oil red O stain for steatosis; Electron microscopy for structural mitochondrial alterations; Frozen tissue analysis for respiratory chain enzymes, DNA quantification. (2) Muscle biopsy: Frozen tissue analysis as above; Blue native page analysis. (3) Skin biopsy: Same as muscle biopsy	Key points to note during level-3 evaluation: Biopsy specimens for electron microscopy need to be preserved in glutaraldehyde and not formalin; It is possible that one invasive test may not give a clue and one has to proceed for an additional invasive test. This is usually because of heteroplasmy. Often liver biopsy molecular analysis provides a final definitive answer; Combination of level-1, level-2 and level-3 studies are sometimes needed to provide comprehensive management and for prognostication

2D Echo: Two-dimensional echocardiography; AFP: Alpha-fetoprotein; CBC: Complete blood count; CNS: Central nervous system; CoQ: Coenzyme Q; CPK: Creatine phosphokinase; CSF: Cerebrospinal fluid; EEG: Electroencephalogram; HbA1c: Glycosylated hemoglobin; INR: International normalized ratio; MRI: Magnetic resonance imaging; NH3: Serum ammonia levels; POLG: DNA polymerase subunit gamma; RCD: Respiratory chain disorders.