Peroxisomal Disorders Workup
- Author: Hoda Z Abdel-Hamid, MD; Chief Editor: Amy Kao, MD more...
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No single test is sufficient to diagnose all peroxisomal disorders. The selection of laboratory studies is based on the clinical presentation.
Four diagnostic groups have been described.
- PBD and POD: These include ZWS, NALD, IRD as part of PBD, and pseudo-NALD and D-bifunctional protein deficiency as part of POD.
- All 3 types of RCDP
- All types of X-ALD
- Refsum, AMACR, hyperoxaluria type I, glutaric aciduria type III, mevalonate kinase deficiency, acatalasemia, and mulibrey nanism
In group 1, VLCFA is abnormal in all patients, and normal VLCFA can rule out group 1 disorders. Elevation of C26:0 and C26:1 and the ratios C24/C22 and C26/C22 are consistent with a defect in peroxisomal fatty acid metabolism.
To further differentiate the disorders in this group, additional studies, including tests of erythrocyte plasmalogen, plasma bile-acid intermediates, pristanic acid, and phytanic acid, are recommended. If plasmalogen is deficient, the patient is likely to have PBD.
- A normal plasmalogen confirms POD. However, plasmalogen may be normal in mild forms of PBD (eg, IRD).
- Biochemical abnormalities detected in blood and/or urine should be confirmed in cultured fibroblasts.
In group 2, analysis of peroxisomal metabolites in plasma should not be used in the diagnostic work-up for RCDP. Instead, the first study should be an analysis of the plasmalogen level in erythrocytes, which is deficient in all cases of RCDP.
- After RCDP is confirmed, phytanic acid levels should be checked. If these are abnormally elevated, RCDP I is the likely diagnosis because the level is normal in types II and III.
- However, because phytanic acid is derived from dietary source, a normal level does not necessarily exclude type I and therefore does not distinguish the 3 types. In this case, enzymatic assay and DNA analysis are recommended to differentiate types II and III.
In group 3, elevated plasma VLCFA level is highly reliable in the diagnosis of ALD. Molecular studies are essential in determining a carrier state and especially in prenatal diagnosis, which can be performed in all peroxisomal disorders.
- Hubbard et al have reported on the validation of combined liquid chromatography–tandem mass spectrometric method for detecting X-ALD in newborns.
Group 4 is clinically heterogeneous. In Refsum disease, an elevated serum level of phytanic acid is reliable in diagnosis. However, enzymatic and molecular analyses are essential for confirmation.
- In hyperoxaluria type I, liver biopsy is required for enzymatic assay. Urinary excretion of glyoxylic and glycolic acids is increased.
- In glutaric aciduria type III, there is persistent elevation of glutaric acid excretion.
- Mevalonate kinase deficiency results in increased urinary excretion of mevalonic acid. However, enzymatic and molecular results may be normal in patients with hyperimmunoglobulin D (IgD).
- AMACR increases levels of pristanic acid, as detected on urine gas chromatography mass spectrometry, or levels of bile acid intermediates, as per tandem mass spectrometry. Further confirmation is obtained with enzymatic and molecular assays.
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MRI findings in ALD have been well correlated with the characteristic gross neuropathologic features of bilaterally symmetric demyelination in the parieto-occipital region with involvement of the splenium of the corpus callosum. The demyelination then spreads anteriorly and laterally.MRI of a patient with adrenoleukodystrophy showing the typical pattern of posterior white-matter involvement.See the list below:
- MRI shows accumulation of contrast material at the advancing margins. This finding is a predictor of disease progression and is consistent with the inflammatory response.
- Other patterns less characteristic than these occur. Examples are early frontal involvement, which is observed in approximately 15% of patients, and an asymmetric mass lesion.
- Milder ALD phenotypes and AMN commonly affect the brainstem corticospinal tracts and the cerebellar white matter.
- In the literature, 54% of 112 patients with AMN, brain MRIs were normal, and demonstrable neurologic involvement was confirmed in the spinal cord and peripheral nerves.
Magnetic resonance spectroscopy shows a diminution in N -acetyl aspartate due to axonal loss, an increase in choline peaks due to active demyelination, and a mild elevation in lactate peak due to inflammation. Spectroscopic changes precede those demonstrable on MRI.
MRI findings in ZWS include neocortical dysplasia (pachypolymicrogyria), germinolytic cysts, and delayed myelination. These finding are attributable to the disordered cytoarchitecture of the cerebral cortex due to neuronal migration defects.
- Colpocephaly (ie, enlargement of the occipital horns of the lateral ventricles) and hypointensity of white matter have been detected. These findings suggest delayed myelinogenesis.
- In late stages, cortical atrophy gradually appears.
In the remaining peroxisomal disorders, no characteristic findings on neuroimaging have been described.
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In patients with peroxisomal disorders, such as ZWS, EEG usually shows multifocal spikes in the early stages of disease.
In late stages, diffuse irregular high-voltage slow waves, correlated with atrophy, are common features.
Neuropathologic lesions in the peroxisomal disorders can be divided into 3 major classes: (1) abnormalities in neuronal migration or differentiation, (2) abnormalities in myelination (defects in the formation or maintenance of myelin in the central white matter and/or in the peripheral nerves), and (3) postdevelopmental neuronal degeneration.
Abnormalities in neuronal migration and differentiation
These abnormalities are most prominent in ZWS and ZWS-like disorders, which are characterized by a unique combination of centrosylvian pachygyria-polymicrogyria. Migration of all neuronal classes, particularly those destined for the outer layers of the cortex, appears to be affected. Less severe cerebral-migration abnormalities, usually in the form of polymicrogyria, are seen in NALD as diffuse, focal, or multifocal lesions that may be associated with subcortical heterotopias; they also are seen in thiolase deficiency and in bifunctional enzyme deficiency.
Cerebral neuronal migration problems have not been identified in IRD, classical adult Refsum disease (ARD), hyperpipecolic acidemia, acyl-CoA oxidase deficiency (pseudo-NALD), ALD, or AMN. In rare reports, RCPD was associated with abnormalities of cerebral neuronal migration. Other subtle neuronal migration problems, apparently asymptomatic, are found in the form of heterotopic Purkinje cells. Defects in neuronal differentiation or terminal migration are common and usually involve the principal nuclei of the inferior medullary olives and, infrequently, the dentate nuclei and claustra. The malformations of these structures consist of dysplasia and simplification. Apparent neuronal loss has also been reported.
Abnormalities in myelination
Lesions of the peripheral nerve can be seen in all peroxisomal disorders with neurologic involvement. Involvement of the peripheral nerves has not been studied sufficiently, except in ARD, which typically results in hypertrophic (ie, onion-bulb) demyelinating neuropathy. Central demyelination and dysmyelination are typically noted in these disorders.
Postdevelopmental neural degeneration
Degenerative changes in the CNS white matter can be classified as 1 of 3 types of lesions: inflammatory demyelination, noninflammatory demyelination, and nonspecific reduction in myelin volume or myelin staining with or without reactive astrocytes.
Inflammatory demyelination, for which ALD is the prototype, is characterized by confluent and bilaterally symmetric loss of myelin in the cerebral and cerebellar white matter. The cerebral lesions usually begin in the parieto-occipital regions and progress asymmetrically toward the frontal or temporal lobes. Arcuate fibers generally are spared, except in chronic cases. The loss of myelin exceeds that of axons, but axonal loss may be considerable. On occasion, lesions involve the brainstem, particularly the pons.
The spinal cord is usually not involved except for bilateral degeneration of the corticospinal tract. Characteristic lamellar and lamellar-lipid inclusions typical of ALD are found in the cytoplasm of Schwann cells or in endoneural macrophages when the peripheral nerves are damaged. Inclusions also may be seen in CNS macrophages but not in oligodendrocytes. Spicular or trilaminar inclusions may be found in the CNS.
The sequence of demyelination in ALD is as follows: enlargement of the extraneural space; vacuolization and myelin swelling with reactive astrocytes and macrophage infiltration; perivascular lymphocytic infiltration and increased permeability of the blood-brain barrier. There is loss of myelin with lipophage formation, loss of oligodendroglia and axons, and dystrophic mineralization.
Inflammatory demyelinating lesions may also be seen in AMN, NALD, thiolase deficiency, and some cases of bifunctional enzyme deficiency. Areas of myelin pallor or oligodendroglial loss with or without reactive astrocytes have been seen in ZWS, NALD, IRD, and probably oxidase deficiency.
Noninflammatory dysmyelination is seen mostly in AMN. Myelin pallor is noted with scant interstitial periodic acid-Schiff (PAS)–positive macrophages but no lymphocytes or reactive astrocytes. Dysmyelination has been found in the early stages of the disease; in advanced stages, inflammatory lesions may supervene.
In nonspecific reduction in myelin volume or myelin staining with or without reactive astrocytes, specific neuron or myelinated fiber tracts show major postdevelopmental noninflammatory abnormalities. The first of these is associated with a progressive loss of hearing that has been classified as sensorineural. It can be seen in ZWS, NALD, IRD, ARD, RCDP, and acyl-CoA oxidase deficiency. The second lesion is retinal pigmentary degeneration, which has been reported in ZWS, NALD, IRD, and ARD. In both lesions, the pathologic changes appear to reside in specialized sensory neurons. The third site of pathogenic change is another restricted one in which the neurons of the dorsal nuclei of Clarke and the lateral cuneate nuclei accumulate lamellar lipids containing VLCFA; this has been seen only in ZWS.
Major neuronal and/or axonopathic degeneration is seen in AMN. Patients with AMN have degeneration of the ascending and descending tracts of the spinal cord, especially the fasciculus gracilis and the lateral corticospinal tracts; the pattern is that of wallerian degeneration. The final lesion is cerebellar atrophy, which has been noted in a few patients with RCDP. It is due to loss of Purkinje and granule cells with focal depletion of basket cells. Cerebellar atrophy has been noted in IRD.
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