the submacular pigment epithelium, and invasion of the retina by macrophage-like pigment and cells.64 Retinal degeneration is associated with waxy pallor of the optic disc, narrowing of the retinal vessels, and clump or spot retinal pigment epithelium pigmentation rather than the bone corpuscle pigmentation usually seen in retinitis pigmentosa.116

Abetalipoproteinemia may also be complicated by subretinal neovascularization associated with retinal angioid streaks.90 Ocular motor abnormalities are also prominent. Yee et al358 first described an unusual form of internuclear ophthalmoplegia in which the adducting rather than the abducting eye showed nystagmus on sidegaze. Absence of adduction was accompanied by convergence insufficiency in these patients.167,358 Some patients may display angioid streaks radiating from the disc,165 producing a helicoid degeneration.354 Neurological abnormalities include loss of deep-tendon reflexes, followed by decreased distal lower extremity vibratory and proprioceptive senses and cerebellar signs such as dysmetria, ataxia, and spastic gait.255 Neuropathology reveals axonal degeneration of the spinocerebellar tracts and demyelination of the fasciculus cuneatus and gracilis.255

Total cholesterol levels are very low, and triglyceride levels are also very low, with little increase after ingestion of fat.255 There are no detectable plasma chylomicrons, very low-density lipoproteins (VLDLs), low-density lipoproteins (LDLs), or apolipoprotein B (apo B), the major structural apolipoprotein of these lipoproteins.255 An abnormality of LDLs or b lipoproteins, established the biochemical hallmark of this disease.272 Apo B is absent in the plasma of patients with abetalipoproteinemia, and it was thought that a molecular defect in the apo B gene may be responsible for this condition; however, the apo B gene was proven to be normal in genetic studies.186

Rather, a protein responsible for intracellular assembly and secretion of apo B-containing lipoproteins has been found to be deficient in abetalipoproteinemia.177,348 This defect leads to deficient fat absorption from the intestine, interfering with the absorption of all fat-soluble vitamins. The defect profoundly affects the metabolism of vitamin E, which relies on this lipoprotein not only for absorption from the intestine but also for transport to peripheral tissues from the liver.186 Vitamin E acts as a free radical scavenger and prevents oxidative injury to membrane lipids.289 Vitamin E deficiency has been implicated in retinal changes in abetalipoproteinemia. Oral vitamin E supplementation can prevent both the retinopathy of abetalipoproteinemia269 and other neurologic sequelae.177,228,255 Vitamin A and K supplements can adequately increase the plasma and tissue levels of these vitamins; however, very large oral doses of vitamin E are required to achieve adequate tissue levels of vitamin E. The recommended dosage is 150–200 mg/kg/day. Adults may require up to 20,000 mg/day (the recommended dietary allowance for normal people for vitamin E is 15 mg/day).

Children with pigmentary retinopathy and neurological degeneration and infants with malabsorption or failure to thrive should be screened for abetalipoproteinemia by performing a plasma cholesterol level. A level lower than 1.5 mmol/L (60 mg/dL) is suspicious for abetalipoproteinemia. Most patients who have very low cholesterol do not have abetalipoproteinemia but do have one of the more common syndromes, such as familial hypobetalipoproteinemia. Treatment with high doses of vitamin E can retard or halt progression of the neurological disease and, possibly, the retinal disease.269

Hypobetalipoproteinemia, a different disease, can be a phenocopy for abetalipoproteinemia.309 All cases of abetalipoproteinemia reported so far are due to mutations in the MTP gene, which encodes the microsomal triglyceride transfer protein (an 894 amino acid protein that is a component of a protein complex involved in the early stages of lipidation of apo B in liver and intestine). Most of these mutations result in truncated proteins devoid of function, but some missense mutations have been reported to be associated with a milder form of the disease.309

Mitochondrial Encephalomyelopathies

Mitochondrial encephalomyelopathies are relatively common neurometabolic disorders of childhood.76 Several neurodegenerative syndromes are caused by disorders of mitochondrial metabolism in children.79 These abnormalities produce defects in the energy cycle of susceptible cells, causing abnormal function and, ultimately, death of the cell. Nerve tissue and striated muscle are most commonly affected. The conditions included in this group of disorders are Alper disease, Menke disease, Leigh disease, and mitochondrial depletion syndrome (MDS), all manifesting their abnormalities in early childhood. A group of disorders with progressive neurological symptoms occurring later in life include chronic progressive external ophthalmoplegia (CPEO), KSS, MELAS, and myoclonic epilepsy with ragged red fibers (MERRF). Except for the syndrome of neurogenic weakness, ataxia, and retinitis pigmentosa (NARP) that can present in childhood,181 the fine or granular pigmentary retinopathy that accompanies these disorders differs from the bone-spicule pigmentation of retinitis pigmentosa.

Several unique features of mitochondrial functioning account for the genetic and clinical features of these syndromes. The mitochondrial encephalomyopathies have only recently begun to be understood on a molecular level, and a detailed classification system has yet to be worked out.156 A thorough understanding of these conditions is made difficult by the complexity of mitochondrial energy metabolism, which is controlled by both nuclear DNA and mitochondrial DNA (mtDNA) and by the characteristics of

mitochondrial inheritance and deterioration of mitochondrial function with aging.134,340

Mitochondria are the major supplier of adenosine triphosphate for cellular energy metabolism. The mitochondrial metabolism itself can be disturbed in any of four major steps.19 The complexity of the interplay between these steps of mitochondrial metabolism and other cellular functions can be illustrated by the fact that abnormalities of the different steps in the mitochondrial energy chain can result in the same phenotype, whereas identical genetic defects can cause different phenotypic expression.134,156,318,319,340

The complexity of mitochondrial diseases becomes more readily apparent when one considers that the circular mtDNA containing 16,500 base pairs works in concert with nuclear DNA to build and execute the energy-producing function of the subcellular organelle. Each circular mtDNA has 37 genes encoding 22 transfer RNAs, two ribosomal RNAs, and 13 proteins essential to oxidative phosphorylation. Nuclear DNA encodes for 56 subunits of the mitochondrial electron transport chain, and the expression of the mtDNA genes requires replication, transcription, and translation, most of which is encoded by nuclear DNA. Nuclear mutations have been found to be responsible for a number of recessive mitochondrial disorders. Oxidative phosphorylation alone requires hundreds of nuclear, mitochondrial, and cytoplasmic genes.19

Mitochondria are the only subcellular organelles to have their own DNA, and this DNA differs from nuclear DNA in several important ways. First, it is circular and has no enterons (the noncoding sequences common to nuclear DNA). The genetic code used by mtDNA is also different from the nuclear DNA code. Mitochondria divide in a manner similar to the budding of bacteria. On cell division, mitochondria are randomly divided into each daughter cell. During fertilization, the human sperm cytoplasm has very few mitochondria and does not contribute significantly to the mitochondrial content of the zygote; therefore, all offspring inherit the female parent’s mitochondrial genotype. While nuclear DNA is inherited in a Mendelian fashion, mtDNA is entirely maternally inherited. The mitochondrial function is not controlled exclusively by the mtDNA present in the organelle, but rather, most mitochondrial functions are still under the control of nuclear DNA. However, mtDNA encodes for 13 components of the electron transport chain, most importantly, complex I, III, IV, and V. Ribosomal and transfer RNA are also encoded by the mtDNA. Abnormalities in these RNAs produce multiple defects in oxidative phosphorylation.

Mitochondrial disorders are caused by mutations of nuclear or mtDNA-encoded genes involved in oxidative phosphorylation.134 Because mitochondria are present in many of our organs and play a key role in energy metabolism, mitochondrial encephalomyopathies often present as

multisystem disorders that may manifest with neurologic, cardiac, endocrine, gastrointestinal, hepatic, renal, and/or hematologic involvement. The clinical recognition of mitochondrial disorders as a group is impeded by the enormous variability in their phenotypic expression.134

There are hundreds of mitochondria per cell and thousands of copies of mtDNA, which leads to a mixture of normal mtDNA and mutant DNA, a phenomenon called heteroplasmy. Furthermore, a cell may drift toward the expression of more normal or more mutant DNA with cell replication, a phenomenon called mitotic segregation. Whether a cell’s energy metabolism reflects the abnormal DNA present in a cell may be influenced by a threshold effect in which a certain percentage of abnormal DNA is required before energy metabolism is affected. Finally, the degree to which a particular cell depends on mitochondrial energy metabolism may vary, thus explaining why muscle, brain, and heart, with their very high energy demands, may be particularly vulnerable to these abnormalities.

Chronic Progressive External

Ophthalmoplegia (CPEO)

Chronic progressive external ophthalmoplegia has been divided into many subsets according to clinical findings. The most well known of the syndromes considered to be a subset of CPEO is Kearns–Sayre syndrome. Its unique phenotype not withstanding, Kearns–Sayre syndrome may be one particular manifestation of a larger group of abnormalities, all caused by deletions of mtDNA. These deletions lead to similar biochemical abnormalities that produce clinical syndromes that differ because of the phenomena previously noted. MtDNA deletions of varying sizes have been demonstrated in patients with CPEO, but to date, no correlation between the size of the deletion and the severity of symptomatology has been described.

Most cases of mitochondrial disease associated with CPEO arise sporadically.34 In sporadic cases, it is likely that the rearrangements occurred during embryogenesis. Autosomal recessive and autosomal dominant inheritance have also been demonstrated, implicating nuclear DNA abnormalities.134 Confirmation of the diagnosis usually requires fresh muscle biopsy for histopathological examination (using cytochrome oxidase stain with electron microscopy to look for “parking lot” inclusions) and Southern blot analysis to look for deletions. MtDNA analysis of skeletal muscle tissue of some CPEO patients reveals rearrangements of segments of mtDNA in the form of deletions and duplications. Largescale mtDNA rearrangements are commonly found in CPEO and Kearns–Sayre syndrome. These rearrangements have been found in over 90% of Kearns–Sayre syndrome patients,

compared with about 50% of CPEO patients.34 Patients are typically heteroplasmic for these rearrangements, and the mutant mtDNA accounts for 20–90% of the total skeletal muscle mtDNA. Kearns–Sayre syndrome patients typically have a greater percentage of mutant mtDNA in their tissues than patients with less severe CPEO syndromes.34

More than 90% of patients with Kearns–Sayre syndrome and about 50% of patients with CPEO have a single large deletion in mtDNA. Blood mtDNA analysis is usually normal. Patients with mtDNA deletions present as sporadic cases, whereas other patients with CPEO show a maternal pattern of inheritance in which mtDNA point mutations are found. Large-scale deletions resulting in CPEO are almost always heteroplasmic – the more tissue with the deletions (i.e., the greater the degree of heteroplasmy), the more likely the phenotype will be severe (i.e., more toward the Kearns–Sayre syndrome phenotype than the simple CPEO).14 The risk of developing a severe phenotype (i.e., additional CNS symptoms with neurological manifestations) is higher when the age of onset is before age 9 and lower when the onset is after age 20.14

Complete Kearns–Sayre syndrome is characterized by onset of clinical abnormalities in the first or second decade of life, with progressive ptosis and external ophthalmoplegia. A characteristic retinal abnormality occurs in patients with Kearns–Sayre syndrome, consisting of widespread salt-and- pepper retinal pigment epithelial mottling, seen most strikingly in the macula, together with a discrete halo associated with peripapillary pigmentary atrophy118 (Fig. 10.13). Cardiac conduction defects due to degeneration of the HIS Purkinje system begin with a partial block but lead to a complete heart block with or without an associated cardiomyopathy. The cerebrospinal fluid (CSF) protein is found to be elevated to greater than 100 mg/dL, and many patients demonstrate cerebellar ataxia.

A history must be obtained regarding other symptoms or signs of mitochondrial disease, including ptosis, deafness, weakness, ataxia, malabsorption syndromes, palpitations, syncope, respiratory insufficiency, diabetes, and tetany.34 Routine laboratory testing for mitochondrial disease is limited. Serum lactate elevation, especially after exercise, is a variable finding in patients with CPEO, MELAS, and Leigh syndrome. Neuroimaging is mandatory to rule out associated CNS lesions, with diffusion-weighted imaging and spectroscopy MRI sometimes providing supportive information. CSF analysis may reveal high lactate levels and elevated protein. Skeletal muscle biopsies (with examination by a laboratory that is equipped for mitochondrial analysis to perform enzymatic assays to measure biochemical deficiencies) can be examined to look for ragged red fibers. Genetic analysis is best performed on skeletal muscle biopsies, especially if rearrangements of mtDNA are suspected. Point mutations in mtDNA can be detected using polymerase chain reaction

amplification techniques on whole blood samples or any tissue that contains mitochondria. Avoiding agents that might stress mitochondrial energy production is a nonspecific recommendation with no confirmed benefit.

Current criteria for diagnosis include two obligatory features: early-onset CPEO (prior to age 20) and retinal pigmentary degeneration, plus one of the following three: heart block, CSF protein greater than 100 mg/dL, or cerebellar syndrome.77 However, a large number of systemic, neurologic, and laboratory abnormalities have been noted in Kearns–Sayre syndrome (Table 10.6). The use of systemic corticosteroid therapy in these patients can precipitate hyperglycemic acidotic coma and death.15

The characteristic MR imaging abnormalities in CPEO include abnormal hyperintensities in the deep gray matter nuclei (particularly the thalamus and globus pallidus) on T2-weighted images and patchy white matter involvement.19,72,81,94,173,179,231 The white matter involvement is predominantly peripheral with early involvement of the subcortical U fibers sparing of the periventricular fibers (Fig. 10.14). Other disorders involving myelin, such as lysosomal disorders and peroxisomal abnormalities, tend to spare this subcortical myelin and affect the older central myelin first.17

The finding of little or no reduction in extraocular muscle volume may help distinguish CPEO from the other forms of ophthalmoplegia, such as congenital fibrosis syndrome.238 The brain ultimately undergoes a spongy degeneration affecting both gray and white matter, and these patients may eventually become demented. Muscle biopsy shows ragged red fibers as it does in patients with the other mitochondrial encephalomyopathies.

Leigh Subacute Necrotizing

Encephalomyelopathy

Leigh disease is probably the most severe manifestation of mitochondrial encephalomyelopathy.342 Onset is often within the first year of life but may rarely develop in later childhood or adulthood. Affected children exhibit hypotonia, loss of verbal and motor milestones, a waxing and waning course of vomiting, weight loss, stupor, and seizures. The striking resemblance to the pathological abnormalities of thiamine deficiency (Wernicke encephalopathy) led to the early suggestion that Leigh disease is secondary to an inborn error of thiamine metabolism. However, a variety of energy metabolism abnormalities have been found, all of which impair mitochondrial DNA production.180

Children with Leigh disease may develop a variety of unusual brainstem motility abnormalities, including horizontal gaze palsies, internuclear ophthalmoplegia, dorsal midbrain syndrome, and a condition initially resembling spasmus

nutans.279 Although primarily a gray matter disease, white matter is eventually involved, and optic atrophy may develop late in the course of the disease.

A characteristic symmetrical pattern of neuroimaging abnormalities is now known to be highly characteristic for Leigh disease. MR imaging shows prolonged T1 and T2 relaxation times in the basal ganglia, periaqueductal region, and cerebral peduncles. Involvement of cerebral white matter may also occur17,213 (Fig. 10.15). Serum and CSF lactate levels may be elevated. Proton spectroscopy may be useful in delineating Leigh disease from other diseases primarily affecting basal ganglia as it is the only disorder to date to show elevated lactate levels in these areas by this study.83

The clinical features of Leigh disease may be caused by several biochemical defects, including pyruvate dehydrogenase deficiency (X-linked inheritance), COX deficiency (autosomal recessive), and OXPHOS deficiency (mtDNA mutations). Most Leigh disease results from nuclear gene defects.256 Approximately 20% of patients with Leigh disease have the T-to-G or T-to-C mtDNA mutation at np8993, within the ATPase 6 gene of complex V of the electron transport chain. A third of patients with NARP carry the Leigh mutation and present with variable combinations of ataxia, seizures, sensory neuropathy, dementia, and retinitis pigmentosa.181,237 Heteroplasmic levels greater than 90% are seen

Fig. 10.15  Leigh disease. T2-weighted MR image shows increased signal intensity in lentiform nuclei (large arrows) and medial thalamic nuclei (small arrows) bilaterally. Courtesy of Charles M. Glasier, M.D.