Laboratory Studies
Whole blood, serum, and plasma studies
Findings reveal elevated plasma and serum cholestanol levels and low-to-normal cholesterol levels (usually 115-220 mg/dL). The cholestanol level is typically 3 to 15 times higher than mean levels in unaffected individuals and can range from 1.3 mg/dL to 15 mg/dL. [23] The cholesterol-to-cholestanol ratio is a better indicator of disease than cholestanol concentration alone. [51] A low cholesterol-to-cholestanol ratio is diagnostic in the appropriate clinical setting; however, additional confirmatory testing is recommended if feasible.
Low-density lipoprotein and triglyceride levels are also usually normal. High-density lipoprotein and very-low density lipoprotein levels vary. Although not typically obtained, bile acid intermediate and other sterol levels are also elevated, such as 7-dehydrocholesterol, [52] 7-alpha-hydroxycholesterol, and lathosterol levels. Some serum bile acid levels themselves are low. Measurement of 7-alpha-hydroxy-4-cholesten-3-one facilitates rapid, convenient diagnostic testing for CTX and may be useful in monitoring biochemical response to treatment. [53, 54] Measurement of the bile acid precursor (7-alpha, 12-alpha-dihydroxy-4-cholesten-3-one) enables sensitive dried bloodspot testing for CTX, showing that dried bloodspot newborn screening is technically feasible. [55] Additional studies are ongoing to determine the suitability of CTX for newborn screening programs. Recently, excellent diagnostic performance has been found for 5β-cholestane-3α,7α,12α,25-tetrol glucuronide in dried blood spots. [56]
As is true for many other genetic diseases, CTX is increasingly being identified by exome or genome sequencing. [57]
Urine studies
Urine testing of bile alcohols (typically pentols) using fast ion bombardment–mass spectroscopy for pattern recognition of bile alcohol glucuronides is typically abnormal in patients with CTX. [58, 59] Concurrent use of propofol may lead to false positive urine bile acid profile testing. [60]
Bile analysis
HIgh levels of cholestanol are present in the bile of almost all individuals affected by CTX and concentrations of cholestanol decrease in response to treatment. [61, 62] As expected given the enzymatic defect, a low concentration of CDCA) is observed along with high concentrations of bile alcohols (conjugated with glucuronic acid).
Cerebrospinal fluid (CSF) studies
Salen et al have reported an increased cholesterol-to-cholestanol ratio (1.5-20 times the reference range); however, this is not routinely analyzed for diagnostic purposes. [63]
Mutation studies
CYP27A1 sequencing and copy number analysis is increasingly the initial test indicating a diagnosis of CTX. Biallelic mutations are detectable in the vast majority of patients with clinical features and biochemistry that are consistent with CTX.
Electrophysiology studies
Decreased nerve conduction velocities as well as somatosensory, motor, brainstem, and visual evoked potentials all relate to peripheral neuropathy. [64, 65] These abnormalities may improve with treatment.
Imaging Studies
The brains of individuals with CTX often show both supratentorial and infratentorial abnormalities on MRI. The findings on MRI and CT scanning include cortical and cerebellar atrophy of the brain, as well as focal lesions (including demyelinating lesions and, rarely, xanthomata) in the cerebellum, basal ganglia, and cerebrum. T2/FLAIR hyperintensity of the subcortical, periventricular, cerebellar white matter, brainstem, and dentate nuclei are characteristic of CTX. [66, 67] T2 abnormalities are also found in the globus pallidus, substantia nigra, and inferior olives with extension into the surrounding white matter in later years of the disease. Cerebrotendinous xanthomatosis should be considered in the differential diagnosis of leukodystrophies. [68]
A 2017 study showed T1/FLAIR hypointensity consistent with cerebellar vacuolation and T1/FLAIR/SW hypointense alterations compatible with calcification in a subgroup of patients with CTX. Long-term follow-up showed that clinical and neuroradiological stability or progression were almost invariably associated. In patients with cerebellar vacuolation at baseline, worsening over time was observed, while patients lacking vacuoles were clinically and neuroradiologically stable at follow-up. Infratentorial abnormalities on MRI are related to clinical disability. The presence of cerebellar vacuolation may be regarded as a useful biomarker of disease progression and unsatisfactory response to therapy. Conversely, the absence of dentate nuclei signal alteration should be considered an indicator of better prognosis. [67]
Brain MRI fluid attenuation inversion recovery (FLAIR) sequences in one patient revealed cortical and subcortical hyperintensities in the temporal lobes) and globus pallidus. T2-weighted MRI revealed cerebellar hyperintensities within the dentate nucleus. Hypointensities were seen on T1-weighted and susceptibility MRI scans within the cerebellum at the level of the midbrain. [66]
Magnetic resonance spectroscopy reveals diffuse mitochondrial dysfunction and axonal damage, with large amounts of lactate and decreased N -acetylaspartate in the periventricular white matter and cerebellar hemispheres. [66]
Diffusion tensor imaging (DTI) may show abnormalities despite normal conventional brain MRI findings. DTI showed reduced fractional anisotropy (FA) and tract-density in the cerebellum and widespread cerebral reductions of FA. DTI after therapy initiation showed progressive increases in cerebellar tract density and cerebral FA. [69]
Isolated spinal cord white matter disease has been described. [70, 71] MRI may reveal increased intensity in the lateral and dorsal columns. [66] Magnetization transfer imaging has been found to be a reliable quantitative indicator of the extent of damage in the brain parenchyma. [71] MRI can also be used to evaluate for tendon xanthomata.
Other Tests
Other studies include electrophysiologic testing, prenatal testing (only performed if a family member is affected), and population-wide newborn screening (not yet implemented). [72, 73] Electrophysiological testing may reveal motor or sensorimotor peripheral neuropathy (demyelinating, axonal, or mixed). Somatosensory evoked potentials (SSEPs) are commonly affected. Delayed brainstem auditory evoked potentials are common, and visual evoked potentials are often abnormal. [74]
Histologic Findings
CNS findings
Xanthomas and granulomas are found in multiple areas of the brain, including the cerebellar hemispheres, cerebellar peduncles, and globus pallidus. Further studies have reported more involvement of gray matter and have also described pathognomonic lesions of spindle-shaped lipid crystal clefts, fibrosis, and hemosiderin deposition in the dentate nucleus. [75] Neuropathologic findings described include lipid crystal clefts, neuronal loss, demyelination, reactive astrocytosis, and perivascular macrophages, suggesting the limited reversibility of the disease and poor prognosis if treatment is not started early.
CNS findings
Xanthomas and granulomas are found in multiple areas of the brain, including the cerebellar hemispheres, cerebellar peduncles, and globus pallidus. Further studies have reported more involvement of gray matter and have also described pathognomonic lesions of spindle-shaped lipid crystal clefts, fibrosis, and hemosiderin deposition in the dentate nucleus. [72] Neuropathologic findings described include lipid crystal clefts, neuronal loss, demyelination, reactive astrocytosis, and perivascular macrophages, suggesting the limited reversibility of the disease and poor prognosis if treatment is not started early.
Peripheral nerve findings
“Onion bulbs” have been described; this suggests that the process is chronic and demyelinating. Demyelination, remyelination, and primary axonal degeneration have been reported. Whether or not the process is a primary neuroaxonal process is unclear.
Muscular findings
Atrophy and pyknotic nuclei have been reported. Mitochondrial dysfunction has also been noted, as revealed by decreased respiratory chain activity and increased levels of pyruvate and lactate in the serum and cerebrospinal fluid. [39]
Liver findings
Typical findings include amorphous pigment and crystalloid forms associated with smooth endoplasmic reticulum, with some free-floating in the cytoplasm. [61]
Xanthomas
Birefringent crystals are surrounded by giant cells with foamy cytoplasm.
-
Sixteen-year-old male with cerebrotendinous xanthomatosis. Note the xanthomas on his knuckles.
-
Sixteen-year-old male with cerebrotendinous xanthomatosis.
-
Xanthomas of the Achilles tendon. Photo courtesy of William Connor, MD, Oregon Health and Science University.
-
Xanthomas on the knees in a patient with cerebrotendinous xanthomatosis. Photo courtesy of William Connor, MD, Oregon Health and Science University.
