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Complex II Deficiency

Complex II is, as in Complex I, caused by mutations in nDNA. This mutation is defined as a “direct hit" in the genes that encode subunits of respiratory chains complexes. It affects the enzyme succinate CoQ reductase which is responsible for the transfer of electrons by the reduction of succinate to fumarate in the electron chain pathway (see Table C-2). Deficiency of complex II is characterized by highly variable phenotypic expression. The clinical features include encephalomyopathy, failure to thrive, severe developmental delay, muscle hypotonia, lethargy, respiratory failure, ataxia, and myoclonic seizures. The presence of lactic acidosis is common. The most frequent clinical condition is Leigh syndrome. See Table C-4.

Table C-4 Summary of Mitochondrial Syndromes and Common Clinical Manifestations
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Table C-4 Summary of Mitochondrial Syndromes and Common Clinical Manifestations
SyndromeCommon Clinical Manifestations
Coenzyme Q deficiencyFamilial mitochondrial encephalomyopathy, ataxia, seizures, mental retardation, proximal muscle weakness, pyramidal signs, exertional fatigue with lactic acidosis (ATT: administration of coenzyme Q10 may contribute to control of seizure activities)
MELASShort stature, sensorineural deafness, stroke-like episodes that most often are occipital and not conforming to metabolic territories, seizures, exercise intolerance, asthenia, severe muscle weakness, diabetes mellitus, cerebral atrophy
NARPNeuropathy, ataxia, retinitis pigmentosa
MNGIEGastric hypomotility, PEO, muscle wasting and weakness, deafness
KSSPEO with onset before 30 years of age, retinal pigmentary degeneration, high cerebrospinal fluid protein, heart block (always present before 50 years of age), identification of white matter abnormalities on magnetic resonance imaging, intracranial calcification, often raised intracranial pressure
PEOPtosis and progressive complex external ophthalmoplegia, limb muscle weakness and wasting, exercise intolerance, intracerebral calcification, white matter abnormalities
LSPsychomotor retardation, reduced ability to suck and swallow in infancy leading to failure to thrive, signs of brainstem dysfunction (respiratory abnormalities, sudden death in infancy, eye movement disturbance, nystagmus), peripheral neuropathy, dystonia, other movement disorders
LHONSubacute visual failure (particularly in males; male-to-female ratio 9:1), dystonia

NARP, Neuropathy, ataxia, retinitis pigmentosa; MELAS, mitochondrial encephalopathy, lactic acidosis, and stroke; MERRF, myoclonic epilepsy, ragged-red fibers; MNGIE, myopathy and external ophthalmoplegia, neuropathy, gastrointestinal, encephalopathy; LHON, Leber hereditary optic atrophy, neuropathy; KSS, Kearns-Sayre syndrome; PEO, progressive external ophthalmoplegia; LS, Leigh syndrome.

Modified from Thyagarajan D, Byrne E; Mitochondrial disorders of the nervous system: Clinical, biochemical, and molecular genetic features. Int Rev Neurobiol 53:93, 2002.

Succinate CoQ Reductase Deficiency; Succinate Dehydrogenase.

If we consider only cases with documented mutations, complex II deficiency appears to be a rare cause of mitochondrial disorders.

Inheritance is usually autosomal recessive. Deficiency of complex II is characterized by highly variable phenotypic expression. It can be caused by mutation in the nuclear-encoded SDHA gene on chromosome 5p.

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Cheam EW, Critchley LA. Anesthesia for a child with complex I respiratory chain enzyme deficiency. J Clin Anesth 10:524, 1998.  [PubMed: 9793822]

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