CARASIL
Introduction
CARASIL (Cerebral Autosomal Recessive Arteriopathy with Subcortical Infarcts and Leukoencephalopathy) is a rare hereditary cerebral small-vessel disease characterized by recurrent ischemic strokes, progressive subcortical dementia, alopecia, and early degenerative spinal disease. It belongs to the group of inherited cerebral arteriopathies that affect small penetrating arteries of the brain and lead to progressive white matter damage. The disorder typically presents in early adulthood and progresses over time due to chronic cerebral ischemia. CARASIL differs from the more common hereditary small-vessel disease CADASIL in that it follows an autosomal recessive inheritance pattern and lacks several features typical of CADASIL, such as migraine with aura.
Genetics
It is caused by biallelic pathogenic variants in the HTRA1 gene located on chromosome locus 10q26. The HTRA1 gene encodes a serine protease that plays an important role in regulating extracellular matrix proteins and modulating transforming growth factor-β (TGF-β) signaling pathways. The loss-of-function mutations (inactivating mutation) in HTRA1 reduce the activity of the encoded protease, leading to abnormal regulation of extracellular matrix components and increased TGF-β signaling. The disease is inherited in an autosomal recessive pattern, and affected individuals carry pathogenic variants in both alleles of the gene. Heterozygous carriers are usually asymptomatic, although some may show mild manifestations. More recently, heterozygous HTRA1 variants have also been recognized to cause a distinct form of autosomal dominant cerebral small-vessel disease that may clinically resemble CARASIL but generally has a milder phenotype.
Pathophysiology
The pathophysiological mechanism in CARASIL involves progressive degeneration of small arteries and arterioles in the brain. Impaired HTRA1 protease activity results in dysregulation of TGF-β signaling and abnormal deposition of extracellular matrix proteins within the vessel wall. These changes lead to arterial wall thickening and fibrosis, loss of vascular smooth muscle cells, and narrowing of the vascular lumen. As a result, cerebral blood flow becomes chronically impaired, particularly in deep brain structures supplied by small penetrating arteries. The resulting ischemia leads to widespread white matter damage and the formation of lacunar infarcts. Neuropathological examination typically demonstrates severe arteriolosclerosis, diffuse rarefaction of the cerebral white matter, and multiple lacunar infarcts. Similar degenerative processes may also affect connective tissues in the spine and hair follicles, which likely explains the presence of early-onset spondylosis and alopecia.
Epidemiology
CARASIL is an extremely rare disorder, with a small number of cases reported worldwide. This condition was first described in Japan. However, subsequent reports have shown that more patients are in other regions, including Europe and China.
The symptoms become clinically apparent in the second to fourth decades of life. Because of its autosomal recessive inheritance, the disease may occur more frequently in populations or families with consanguinity. The true prevalence remains unknown but is considered to be extremely low.
Clinical Manifestations
The clinical manifestations of CARASIL are primarily characterized by progressive cerebral small-vessel disease. Neurological symptoms usually begin in early adulthood and include gait disturbance, mild cognitive impairment, or focal neurological deficits related to the location of lacunar strokes. Patients experience recurrent ischemic strokes, usually affecting deep brain structures. Diffuse white matter damage leads to dementia (subcortical), which is characterized by cognitive slowing, executive dysfunction, and impaired attention.
Additional neurological symptoms include dysarthria, pseudobulbar palsy, spasticity, and emotional lability. Gait disturbances and motor impairment gradually worsen as the disease progresses. CARASIL is distinguished from many other hereditary small-vessel diseases by the presence of characteristic systemic manifestations. One of the earliest features is diffuse alopecia, which often develops in adolescence or early adulthood and can precede neurological symptoms. Another typical manifestation is early-onset degenerative disease of the spine, particularly lumbar spondylosis and intervertebral disc degeneration. Patients frequently experience chronic low back pain beginning at a relatively young age. Migraine, which is commonly observed in CADASIL, is generally absent in CARASIL.
Diagnosis
The diagnosis of CARASIL is based on the clinical manifestations, neuroradiological findings, and genetic analysis. This disorder should be suspected in individuals who present with early-onset cerebral small-vessel involvement in association with alopecia and degenerative spinal disease, particularly when there is a family history of autosomal recessive inheritance. Magnetic resonance imaging of the brain typically demonstrates diffuse white matter hyperintensities on T2-weighted and FLAIR sequences, and reflects extensive leukoencephalopathy.
Lacunar infarcts are frequently observed in the basal ganglia, thalamus, and brainstem. White matter abnormalities may appear early and progress over time. Compared with CADASIL, involvement of the anterior temporal lobes is usually less prominent.
Definitive diagnosis is established by molecular genetic testing demonstrating biallelic pathogenic variants in the HTRA1 gene. Genetic confirmation is important for differentiating CARASIL from other hereditary or sporadic forms of cerebral small-vessel disease.
Treatment
There is currently no curative treatment for CARASIL, and management is supportive. Treatment focuses on the prevention of stroke complications, symptomatic management, and rehabilitation. Although vascular risk factors are not the primary cause of the disease, standard measures for stroke prevention may still be considered, including antiplatelet therapy in appropriate patients. Supportive treatment includes physiotherapy, speech therapy, and cognitive rehabilitation. Pain management may be required for chronic back pain related to spinal degeneration.
A multidisciplinary team involving neurologists, rehabilitation specialists, and other healthcare professionals is often necessary to address the progressive neurological disability associated with the disease. Genetic counseling should also be offered to affected families because of the autosomal recessive inheritance pattern.
Prognosis
CARASIL is a progressive neurovascular disorder that leads to advanced neurological disability over time. Recurrent lacunar strokes and subsequent white matter degeneration lead to a gradual deterioration of motor, sensory, and cognitive functions. Patients develop severe gait impairment, spasticity, and advanced subcortical dementia as the disease progresses. Functional independence is often lost in mid-adulthood, and patients may eventually require full-time care. Although the rate of progression varies, the overall prognosis is poor, with significant disability typically developing by the fourth or fifth decade of life.
