In retrospect, the first cases of X-ALD were probably described in the late 19^th century. In 1897 Heubner described a young boy with rapidly progressive neurologic deterioration consistent with X-ALD, classified as having “diffuse sclerosis” on autopsy 10. This designation was used at the time for any disease of the white matter causing hardening of the tissue. Cases of “diffuse sclerosis” that resemble X-ALD were also described in 1899 by Ceni 11 and in 1910 by Haberfield and Spieler 12. Shortly thereafter Schilder suggested that “diffuse sclerosis” was too vague and proposed a more accurate pathological classification of the leukodystrophies 131415. He described several cases with lesions in the cerebral white matter and perivascular inflammation that he named “encephalitis periaxialis diffusa”. The syndrome of rapidly progressive cerebral demyelination with inflammatory changes in the white matter on autopsy became known as “Schilder’s disease”. Many cases were described in the following fifty years 16. Nevertheless, Siemerling and Creutzfeldt are often credited for having described the first case of X-ALD because they were the first to describe the association of acute cerebral demyelination with clinical and pathological signs of Addison’s disease 17.

X-ALD has remained an enigmatic disease for most of the 20^th century, although some suspected it might be a metabolic disorder affecting both the central nervous system and adrenal glands 1819. Based on the presence of lipid inclusions in adrenal glands, testis, brain and Schwann cells, Schaumburg and Powers suggested the term adrenoleukodystrophy for the disorder and speculated it might be a lipid storage disorder 20. This hypothesis was confirmed when post-mortem biochemical analysis of brain, adrenal glands and serum of seven patients revealed the presence of cholesterol esters with high amounts of fatty acids longer than C22. These fatty acids were referred to as very long-chain fatty acids (VLCFA) 21. X-linked adrenoleukodystrophy now became a distinct clinical entity with an associated biochemical marker that could be used to confirm the diagnosis in readily accessible materials like blood cells and plasma 3.

Already in 1910 Addison’s disease associated with spastic paraplegia was described 22. Familial Addison disease with or without spastic paraplegia was usually considered to be a variant of hereditary spastic paraplegia 2324. In 1976, an X-linked adult onset progressive myelopathy that was often associated with Addison’s disease was reported 25. A year later, five more cases were described and the term adrenomyeloneuropathy (AMN) was proposed because of the involvement of the adrenal cortex, spinal cord and peripheral nerves 26. With plasma VLCFA analysis as newly identified biomarker, AMN was soon found to be a form of X-ALD thus extending the phenotypic spectrum of X-ALD 3. Now, we classify the symptomatology of X-ALD in children and adults in several phenotypes Table 1. They will be discussed here in more detail.

- = absent; + = present; CCALD = childhood cerebral ALD; AdolCALD = adolescent cerebral ALD; ACALD = adult cerebral ALD; AMN = adrenomyeloneuropathy; AD = Addison-disease.

Adrenocortical insufficiency (or even an Addisonian crisis) can be the presenting symptom of X-ALD in boys and men, years or even decades before the onset of neurological symptoms. X-ALD is a frequent cause of Addison’s disease in boys and adult males 2728, in particular when circulating adrenocortical autoantibodies are absent 29. A percentage as high as 35 has been reported 27. However, this figure may be overestimated. Indeed, in a study of 40 Dutch men with adrenocortical insufficiency, no patients with X-ALD were identified (B.M. van Geel BM and J. Assies unpublished data). Recognizing that Addison’s disease is due to X-ALD has important implications for genetic counseling but also management. It is therefore important to consider X-ALD in any boy or adult male presenting with Addison’s disease. Adrenocortical insufficiency initially affects the glucocorticoid function of the adrenal, but the mineralocorticoid function is ultimately deficient in approximately half of the X-ALD patients 27.

These phenotypes are the most rapidly progressive and devastating phenotypes of X-ALD. They most frequently present in childhood (childhood cerebral ALD; CCALD), however never before the age of 2.5 years 3031. The onset of CCALD is insidious, with deficits in cognitive abilities that involve visuospatial and visuomotor functions or attention and reasoning. In boys and adolescents it initially results in a decline of school performance. These early clinical symptoms are often misdiagnosed as attention deficit hyperactivity disorder and can delay the diagnosis of CCALD. As the disease progresses, more overt neurologic deficits become apparent, which include withdrawn or hyperactive behavior, apraxia, astereognosia, auditory impairment ("word deafness'' reflecting impairment in acoustic analysis of word sounds), decreased visual acuity, hemiparesis or spastic tetraparesis, cerebellar ataxia and seizures. At this stage progression is extremely rapid and devastating. Affected boys can lose the ability to understand language and walk within a few weeks. Eventually, patients are bedridden, blind, unable to speak or respond, requiring full-time nursing care and feeding by nasogastric tube or gastrostomy. Usually death occurs two to four years after onset of symptoms, or - if well-cared for - patients may remain in this apparent vegetative state for several years 30.

The rapid neurologic decline of CCALD is caused by a severe inflammatory demyelination process primarily affecting the cerebral hemispheres. Postmortem histopathological examination of brain tissue reveals extensive demyelination with perivascular infiltration of lymphocytes and macrophages that resembles, to some extent, the demyelinating lesions seen in multiple sclerosis 32. In 80% of patients the initial demyelinating lesion is localized in the splenium of the corpus callosum and progresses to involve the adjacent parieto-occipital white matter 33. Alternatively, the initial demyelinating lesions may occur in the genu of corpus callosum and then progress symmetrically or asymmetrically to the white matter of the frontal lobes 33. The lesions may also initially involve the pyramidal tracts within the pons or the internal capsules and then extend into the white matter of the centrum semiovale. Brain MRI shows abnormal signal intensities (increased signal on T2 and FLAIR sequences, decreased signal on the T1 sequence) in the corpus callosum, parieto-occipital or frontal white matter or pyramidal tracts within the brainstem, pons and internal capsules (Figure 1). Atypical patterns of demyelination, for instance highly asymmetrical disease have been described 34. Initially, the demyelinating lesions do not show enhancement on T1 sequences after intravenous gadolinium administration. The presence of gadolinium enhancement of demyelinating lesions occurs usually in a second stage when the disease starts to progress rapidly, reflecting severe inflammation and disruption of the blood brain barrier. This transition to the rapidly progressive neuroinflammatory stage may occur very early, even when the demyelinating lesions are restricted to the corpus callosum or pyramidal tracts or later, once the demyelinating lesions have already significantly extended into the cerebral white matter. In the absence of biomarkers to predict this evolution, brain MRI remains the only tool to detect this evolution in an early stage. A scoring system to grade the demyelinating abnormalities on brain MRI has been developed by Loes 35. This score correlates well with neurologic symptoms when the demyelinating lesions involve the parieto-occipital white matter. However, lesions in the frontal white matter can produce severe symptoms (especially behavioral), while the Loes score is still low. Less frequently, cerebral demyelination as the presenting phenotype of X-ALD occurs in adolescence (AdolCALD) or adulthood (ACALD). The symptomatology in these patients strongly resembles CCALD, but the initial progression of symptoms usually is slower. In adults, the early cognitive decline is rarely recognized by their families and friends or at work. As the disease progresses, psychiatric disturbances mimicking schizophrenia or psychosis are not uncommon 36. In those cases, the diagnosis of X-ALD is often delayed; particularly when no family history of X-ALD is present and when clinical symptoms of Addison's disease are absent.

Approximately 10% of boys or adolescents with cerebral ALD may not develop the rapidly progressive neuroinflammatory stage of the disease. The same may occur in men with ACALD or in men with AMN who develop secondary cerebral demyelination (see below). This cerebral demyelinating form of X-ALD is often referred to as “chronic or arrested cerebral X-ALD”. The cerebral demyelinating process arrests spontaneously and the patient can remain stable for a decade or even longer. But even after a 10–15 year period of stability, sudden onset of rapid neurologic deterioration may occur, reflecting a full progression to the neuroinflammatory stage of the disease.

Virtually all patients with X-ALD who reach adulthood develop AMN, usually in the 3^rd and 4^th decade. Initial symptoms are limited to the spinal cord and peripheral nerves. Patients develop gradually progressive spastic paraparesis, sensory ataxia with impaired vibration sense, sphincter dysfunction (mostly urinary), pain in the legs and impotence 37. The clinical burden of peripheral nerve involvement is usually moderate and difficult to assess because of prominent spinal cord symptoms. If polyneuropathy is investigated electrophysiologically, an axonopathy is found in the majority of the patients 38. Rarely, the initial symptomatology may be that of a peripheral neuropathy, either demyelinating or axonal 3940.

Before the availability of MR imaging, AMN was often misdiagnosed as multiple sclerosis or hereditary spastic paraparesis (HSP). The pathological basis of AMN is a noninflammatory distal axonopathy that involves the long tracts of the spinal cord and, to a lesser extent, the peripheral nerves 41. This phenotype is in most cases slowly progressive, causing severe motor disability of the lower limbs over one or two decades but mild or no significant deficits in arms and hands. Brain MRI is normal or may show subtle abnormalities such as moderately increased signal intensities of the pyramidal tracts in brainstem, pons and internal capsules on FLAIR and T2 sequences that reflect likely Wallerian degeneration in patients with longstanding symptoms of AMN (Figure 2). These abnormalities are not considered manifestations of cerebral ALD. However, if the increased signal of the pyramidal tracts becomes more intense and extends beyond the internal capsules into the white matter of the centrum semiovale, this is considered a manifestations of cerebral ALD (Figure 3). MRI of the spinal cord eventually shows non-specific atrophy, but no demyelination or gadolinium enhancement as seen in multiple sclerosis. In contrast, magnetization transfer or diffusion tensor-based imaging sequences show significant abnormalities of the spinal cord tracts 424344.

A retrospective study revealed that over a period of 10 years, approximately 20% of AMN patients developed additional cerebral demyelination 45. After an initial progression demyelinating lesions can stabilize spontaneously leading to moderate cognitive deficits. However, once the cerebral demyelinating lesions have entered the active phase of neuroinflammation with gadolinium enhancement, the prognosis is as poor as in CCALD. The neurologic symptoms of AMN patients that develop cerebral ALD are identical to those observed in adults with adult cerebral ALD, with additional symptoms of the pre-existing myelopathy. Approximately 70% of AMN patients have adrenocortical insufficiency 46. An equal percentage of affected males have subclinical signs of testicular insufficiency 47. Clinical symptoms of testicular insufficiency are rare. Hair of patients with AMN is often thin and sparse; these patients often show balding at an early age (Figure 4). This typical scanty scalp hair was first described in 1955 23.

As in many X-linked diseases, it was assumed that female carriers remain asymptomatic. However, many women develop AMN-like symptoms 48. Physical examination of a large group of female carriers attending family conferences in the United States revealed that more than 50% had some kind of abnormality on neurologic examination 30. An increasing number of symptomatic heterozygous women are identified as the first member of their family to be affected by X-ALD and the real incidence of AMN in heterozygous women is likely to be close to 65% by the age of 60 years (Engelen et al, manuscript in preparation). Onset of neurologic symptoms mainly occurs between the 4^th and 5^th decade and they are very similar to those observed in adult males with AMN. Sensory ataxia, fecal incontinence and pain in the legs are however often more prominent in symptomatic women with AMN. Cerebral involvement and adrenocortical insufficiency are rare, 2% and 1%, respectively 3049. So far, neither have been documented in the Netherlands and France. It is speculated that skewed X-inactivation in neuronal cells may contribute to the manifestation of neurologic symptoms in X-ALD carriers 5051. In a recent study the association of skewed X-inactivation and symptomatic status could not be confirmed 52. However, this may be attributable to differences in mean age between the symptomatic and asymptomatic group in this study.

A diagnosis of X-ALD must be followed by extended family screening together with a geneticist. This enables the detection of: 1) heterozygous women who can be offered prenatal or in some countries preimplantation diagnosis for future pregnancies, and 2) boys or adult males who are asymptomatic but at risk to develop cerebral demyelination or adrenocortical insufficiency 8. It is important to detect these complications as early as possible for treatment, as described in the section on ‘clinical management’.

X-ALD phenotypes are not static. Presymptomatic males are nearly all at risk to develop neurologic (cerebral ALD, AMN) or endocrinologic (Adisson’s disease) symptoms. Addison-only males can develop AMN or cerebral ALD and AMN males can develop cerebral demyelination. It is estimated that over a period of 10 years about 20% of patients with AMN will progress to a cerebral phenotype 45. Presymptomatic women with X-ALD are mostly at risk to develop AMN. Progression of X-ALD in a specific individual can not be predicted.

X-ALD is characterized by the absence of a genotype-phenotype correlation. In spite of identical ABCD1 gene mutations, patients can have markedly divergent neurological and neuropathologic characteristics 5354. There is ample evidence that other genetic and/or environmental factors may influence the clinical presentation of X-ALD. Segregation analysis suggests that the phenotypic variability is due to at least one autosomal modifier gene 5556. Head trauma may be an environmental factor triggering the onset of cerebral ALD 57. Other brain lesions, like a stroke, may also trigger cerebral demyelination in patients with X-ALD.

In boys and men presenting with Addison’s disease, including those who have only a glucocorticoid deficiency, and in whom there are no detectable steroid-21-hydroxylase or other organ-specific antibodies, X-ALD should be considered and determination of plasma VLCFA levels must be performed 7071. Young boys and adult males presenting with cognitive and neurological symptoms with (usually enhancing) white matter lesions on brain MRI should be tested for X-ALD.

In adult men, the most common presenting symptom of X-ALD is a chronic myelopathy. In the past, AMN was often misdiagnosed as primary progressive multiple sclerosis or hereditary spastic paraparesis. After ruling out a compressive myelopathy by MRI of the spinal cord and other common causes of chronic myelopathy (some possible diagnoses for chronic myelopathy with a (near) normal MRI are summarized) 72, X-ALD should be considered. A clinical clue to the diagnosis can be the presence of adrenocortical insufficiency and early baldness. However, even in the absence of clinical signs of adrenocortical insufficiency AMN should be considered.

Differential diagnosis of chronic myelopathy with normal MRI:

Deficiencies: Vitamin B12, folic acid, copper

Hereditary spastic paraparesis with amyotrophy

Infections (e.g., HTLV-1, HIV)

Primary lateral sclerosis

Radiation myelopathy

Cerebrotendinous xanthomatosis

Metachromatic leukodystrophy

Krabbe disease

A category that remains underdiagnosed relatively often are women with AMN. Physicians familiar with X-ALD are aware that it is not uncommon that women with X-ALD develop a myelopathy 46. However, most neurology textbooks do not list X-ALD in the differential diagnosis for chronic myelopathy in women. The diagnosis can only be excluded by ABCD1 mutation analysis. As mentioned before, there is an increasing number of X-ALD families in which the index case is a woman with clinical symptoms of AMN.

Increased plasma VLCFA are not pathognomonic for X-ALD or might even be false positive, because of hemolysis of the sample or dietary causes, for example a ketogenic diet 73. Alternatively, some dietary products like rapeseed oil or mustard seed oil that are rich in erucic acid (C22:1) can result in the lowering of C26:0 therefore causing a false negative result (Ann Moser, personal communication). If metabolic screening reveals increased VLCFA, the next step is to confirm the diagnosis by performing ABCD1 mutation analysis. If this is negative, it is important to consider other peroxisomal disorders, such as 1) peroxisomal biogenesis disorders with late onset of symptoms 7475 (Ebberink et al, in press), and 2) peroxisomal acyl-CoA oxidase 1 (ACOX1) or D-bifunctional enzyme (DBP) deficiency with late onset of symptoms. This requires further testing for bile acid intermediates, phytanic and pristanic acids in plasma, and plasmalogens in erythrocytes 76. A further metabolic screening of peroxisomal dysfunction must also be performed on skin fibroblasts, at least by performing catalase immunofluorescence after culturing cells at 37°C, but also at 40°C 77.

In males with confluent white matter changes, X-ALD should be considered, especially when there is increased signal intensity on T2-weighed and FLAIR sequences in the parieto-occipital region and the splenium of the corpus callosum 33. Rim enhancement of demyelinating lesions is usually observed when boys or adult males present with overt neurological symptoms from cerebral ALD. Therefore, intravenous administration of gadolinium is advised if X-ALD is considered. However, in about 20% of males with X-ALD, the white matter changes occur predominantly in the genu of corpus callosum and frontal lobes, or involve the pyramidal tracts with extension in the white matter of the centrum semiovale 33. The brain MRI pattern of cerebral X-ALD is usually readily recognized by neuroradiologists. A helpful algorithm for diagnostic work-up of patients with white matter changes on brain MRI is suggested by Schiffmann and Van der Knaap 78.

There are other peroxisomal disorders that present with MRI abnormalities resembling X-ALD, in particular peroxisomal biogenesis disorders and ACOX1 or DBP deficiency with late onset of symptom 79. Although the radiologic findings might suggest X-ALD, the clinical presentation is very different and therefore these disorders are easily differentiated.

Follow-up in boys and men with X-ALD is important for two reasons: 1) early detection of adrenocortical insufficiency and 2) early detection of cerebral ALD to propose allogeneic hematopoietic stem cell transplantation (HCT) if a HLA-matched donor or cord blood is available. Despite significant mortality risk, allogeneic HCT remains the only therapeutic intervention that can arrest the progression of cerebral demyelination in X-ALD, provided the procedure is performed very early, i.e., when affected boys or men have no or minor symptoms due to cerebral demyelinating disease 8182.

In the future, transplantation of autologous hematopoietic stem cells that have been genetically corrected with a lentiviral vector before re-infusion might become an alternative to autologous HCT, once the very encouraging results obtained in the first two treated patients will have been extended to a larger number of patients with cerebral X-ALD 83.

If boys or men do not have Addison’s disease it is recommended that they are evaluated yearly by an endocrinologist for adrenocortical dysfunction by measuring the plasma ACTH levels and performing an ACTH stimulation test 71. Steroid replacement therapy can then be initiated if necessary.

Boys without neurological deficits should be monitored closely for radiological signs of cerebral ALD. CCALD has not been reported before the age of 2.5 years 8. We recommend an MRI of the brain every 6 months in boys aged 3 to 12 years old to screen for early signs of CCALD. If symptoms occur suggestive of cerebral ALD (for instance declining school performance) the MRI should be performed at the earliest available opportunity, but it is our experience that the detection of brain MRI abnormalities precedes any detectable cognitive dysfunction by at least 6 months to 1 year. After the age of 12 years, the incidence of CCALD decreases, but an MRI scan must be performed yearly or earlier if new symptoms occur. It is important to detect cerebral ALD as early as possible, preferably in the asymptomatic stage with only moderate radiological abnormalities to discuss the possibility to perform allogeneic HCT. Accordingly, if a brain MRI shows abnormalities, even very limited such as an increased signal intensity on T2 or FLAIR sequences in the splenium or genu of the corpus callosum, brain MRI must be repeated within 3 months to evaluate disease progression and in particular to identify the presence of gadolinium rim enhancement of lesions. Because the disease can be very rapidly progressive, it is strongly advised to discuss the possibility of allogeneic HCT as soon as brain MRI abnormalities typical of cerebral ALD are detected. After a successful transplant, the lesions on MRI stabilize and even regress. Treatment results are better the earlier treatment is started 82.

For adult men with or without signs of AMN, we advise evaluation by a neurologist yearly or bi-annually to screen for symptoms of AMN and to administer symptomatic treatment if necessary (for instance, medication against spasticity). Referral to a rehabilitation physician and urologist will often become necessary.

Adult men can develop cerebral ALD and in our centers, we offer an MRI of the brain every single year 3045. There is no proven treatment for cerebral ALD in adults. It seems likely that allogeneic HCT is also effective in adults with early stage cerebral ALD, but there are no published studies or cases describing this treatment. We tend to consider allogeneic HCT in an adult patient with early stage cerebral ALD, after carefully counseling the patient about the lack of evidence for the treatment and the risk of the procedure which is significantly higher than in boys. Whereas the onset of demyelinating lesions involving the corpus callosum and adjacent parieto-occipital or frontal white matter leaves no doubt about the onset of cerebral ALD, the situation is different when there are only slightly increased signal abnormalities in the pyramidal tracts of AMN patients that gradually become more intense and involve the white matter of the centrum semiovale. This can herald the onset of cerebral ALD, but can also reflect Wallerian degeneration in severe AMN.

For AMN there is no effective disease modifying therapy available yet. Although Lorenzo’s oil (LO) had great promise, several open-label trials have shown that the disease progresses even when plasma VLCFA are normalized by LO treatment 8485. A large randomized placebo-controlled clinical trial was designed to provide a definitive answer, but was unfortunately aborted before completion by the safety monitoring board because of presumed side effects of the placebo treatment. There is also a retrospective study suggesting that if presymptomatic boys are started on LO, it may delay the onset of neurological symptoms 86. We consider the scientific evidence to support the efficacy of LO weak, and do not offer this treatment to our patients. Regular follow-up in AMN remains important, however, mainly to provide symptomatic treatment.

Lovastatin also lowered plasma VLCFA 87, but a placebo-controlled trial revealed that lovastatin did not have an effect on the C26:0 levels in peripheral blood lymphocytes and erythrocytes nor on the VLCFA content of the low-density lipoprotein fraction 88.

More research and new treatments strategies are desperately needed, especially for those affected by AMN, which is relentlessly progressive and causes severe disability. Antioxidants reduce markers for oxidative stress and axonal degeneration in the spinal cord of Abcd1 knockout mice 89. Based on this observation a clinical trial with anti-oxidants in X-ALD is ongoing in Spain.

Women with X-ALD should be evaluated for the development of neurologic symptoms. Since women with X-ALD very rarely develop adrenocortical insufficiency or cerebral involvement, periodic evaluation of adrenocortical function and brain MRI is not mandatory 37. Greater awareness among physicians that women can develop neurologic symptoms is important for counseling but also to prevent unnecessary diagnostic tests and erroneous diagnosis. We know of cases of women with X-ALD who underwent cervical laminectomy for presumed cervical spondylogenic myelopathy. For symptomatic women with X-ALD, we advise (as for men with AMN) a yearly evaluation by a neurologist to discuss the indication of rehabilitation, the referral to an urologist and treatment of spasticity and neuropathic pain.