The mutated gene responsible for CADASIL (Notch3) has been identified and characterized, and genetic testing for the disease has become commercially available. Prior to development of genetic testing for CADASIL, the disease could be diagnosed only through a combination of clinical history, physical examination, and nonmolecular laboratory testing. Despite the availability of genetic testing, CADASIL can still be reliably diagnosed using clinical and nonmolecular criteria.
Clinical diagnosis. An important procedure to establish the diagnosis of CADASIL is MRI of the brain that will reveal evidence of multiple subcortical lacunar infarcts and a diffuse leukoencephalopathy. In many cases, these pathological changes will be evident to some degree on MRI years before the clinical onset of strokes or dementia. Therefore, MRI can potentially be used to diagnose CADASIL in at-risk presymptomatic individuals.
The presence of white matter hyperintensity on T2-weighted MRI images in temporal lobe poles is highly specific for CADASIL, in comparison to patients with hypertensive white matter changes (O’Sullivan et al 2001). In patients with genetically determined CADASIL, white matter changes in the anterior temporal pole on MRI had a sensitivity of 89% and specificity of 86% for CADASIL. Involvement of the external capsule has a high sensitivity (93%) but is not as specific (45%) for CADASIL. In contrast, skin biopsy is only 45% sensitive but 100% specific for CADASIL (Markus et al 2002).
For individuals with an established family history of CADASIL, a characteristically abnormal MRI is necessary and usually sufficient evidence to establish the diagnosis.
Patients presenting with symptoms of CADASIL and a family history of strokes, but no firmly established family history of CADASIL require a much more extensive diagnostic evaluation in order to assess for other causes of stroke (familial and otherwise). This laboratory evaluation should begin with the "routine" labs obtained for stroke patients, including a complete blood count and white blood cell differential, coagulation profile, serum electrolytes, renal and liver function tests, blood glucose, platelet count, erythrocyte sedimentation rate, and lipid profile. All of these laboratory studies will typically be normal in patients with CADASIL. A duplex carotid ultrasound, ECG, and chest x-ray should be performed to assess for carotid and cardiac sources of embolism. The results of these procedures are all typically negative in patients with CADASIL.
Depending on the age, past medical history, and other items peculiar to each patient, a set of more specialized laboratory tests may be obtained, such as a serum amino acid analysis, VDRL, and antinuclear antibody to assess for homocystinuria, neurosyphilis, and lupus, respectively. Again, all of these laboratory tests would be expected to be negative or normal in CADASIL.
Many relatively young patients with stroke will undergo cerebral angiography. CADASIL induces a small vessel arteriopathy, leaving large vessels intact and patent; thus, the angiographic studies in CADASIL are typically normal. Furthermore, some evidence suggests that an uncommonly high rate of neurologic complications is induced by cerebral angiography in this disease, leading some authors to conclude that the cerebral angiogram is contraindicated in patients with possible CADASIL (Dichgans and Petersen 1997).
Biopsy of skin, muscle, or sural nerve has been used to diagnose CADASIL, as arteries in the periphery develop ultrastructural changes similar to cerebral arteries. These pathognomonic abnormalities on biopsy can often be seen before the onset of strokes or dementia (Ebke et al 1997). Finding granular osmiophilic material on biopsy is specific for CADASIL, but the sensitivity of the angiopathic changes in peripheral tissues may be as low as 45% (Markus et al 2002). The sensitivity of skin biopsy for CADASIL can be greatly increased by ensuring that a biopsy of sufficient depth and size is obtained. In particular, the skin biopsy should include the border zone between the deep dermis and the upper subcutis, where small arterial vessels of the correct size reside (Tikka et al 2009).
Some studies of families affected by CADASIL have found that skin and muscle biopsies are reliable in establishing the diagnosis (Mayer et al 1999) as both clinically affected subjects and clinically asymptomatic patients with minimal MRI abnormalities had characteristic abnormalities of granular electron-dense deposits. The discovery that CADASIL mutations induce an accumulation of Notch3 protein at the cytoplasmic membrane of vascular smooth muscle in the periphery as well as in the brain has led to the development of immunostaining techniques for the diagnosis of CADASIL (Joutel et al 2001). Skin biopsies are less invasive than muscle biopsies and less costly than a search for a mutation in the large Notch3 gene. Thus, electron microscopy and immunostaining of skin biopsy specimens has emerged as a powerful method in the diagnosis of CADASIL.
Molecular genetic testing for CADASIL has become available on a commercial basis. Two types of molecular genetic tests for CADASIL are available. The first type is linkage analysis for chromosome 19. This type of testing may be conducted appropriately for patients with multiple-affected family members. The second type is direct sequencing of the Notch3 gene to identify point mutations in individuals (Joutel et al 1997b; Oberstein et al 1999). An important advantage of sequence analysis over linkage analysis is that the former allows individual patient testing without having to recruit family members or to rely on a dubious family history.
For the molecular diagnosis of CADASIL, sequencing of the Notch3 gene is undertaken in a stepwise strategy. The first step is to sequence exon 3 and 4, which is the site of the mutation in two thirds of cases. If no mutations are found in exons 3 and 4, the remaining exons that code for EGF-repeat-like domains should be sequenced. Together, the first 2 steps will identify the Notch3 mutation in 90% of CADASIL cases. Sequencing of the entire Notch3 gene is undertaken only if no mutation is found in the first 2 steps.
Genetic testing for CADASIL in all patients with lacunar strokes who have no other typical features of CADASIL has a low yield (Dong et al 2003). Therefore, genetic testing should be initiated with patients who have the characteristic signs of lacunar stroke, white matter changes on MRI, family history (if available), and skin or nerve biopsy.
The nature of CADASIL and the inability of modern medicine to influence its course dictate that ethical considerations are addressed prior to genetic testing for the disease, especially in asymptomatic at risk individuals. CADASIL has no known specific treatment and ultimately leads to severe disability, dementia, and death, thus, presymptomatic testing requires protocols similar to those used for Huntington disease (International Huntington Association 1994).
A study of at-risk people seeking presymptomatic genetic testing for CADASIL has demonstrated that most applicants choose not to complete the testing or to obtain their genetic status once the disease and the implications of genetic testing are fully explained to them (Reyes et al 2012).