HTRA1 disorder is a phenotypic spectrum in which some individuals have few to no symptoms and others manifest with the more severe CARASIL (cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy) phenotype. Those who have a heterozygous HTRA1 pathogenic variant may have mild neurologic findings (sometimes identified only on neuroimaging) or mild-to-moderate neurologic signs and symptoms of CARASIL. In this chapter, the term "classic CARASIL" refers to the more severe phenotype associated with biallelic pathogenic variants, and "HTRA1 cerebral small vessel disease" (HTRA1-CSVD) refers to the milder phenotype associated with a heterozygous HTRA1 pathogenic variant. Classic CARASIL is characterized by early-onset changes in the deep white matter of the brain observed on MRI, and associated neurologic findings. The most frequent initial symptom is gait disturbance from spasticity beginning between ages 20 and 40 years. Forty-four percent of affected individuals have stroke-like episodes before age 40 years. Mood changes (apathy and irritability), pseudobulbar palsy, and cognitive dysfunction begin between ages 20 and 50 years. The disease progresses slowly following the onset of neurologic symptoms. Scalp alopecia and acute mid- to lower-back pain (lumbago) before age 30 years are characteristic. The most frequent initial symptom in individuals with HTRA1-CSVD is slowly progressive gait disturbance after age 40 years, which may be followed by the development of mood changes and cognitive dysfunction. A majority of affected individuals have a stroke-like episode after age 40 years. Spondylosis and alopecia are seen in a minority of individuals with HTRA1-CSVD.

AP-4-associated hereditary spastic paraplegia (AP-4-HSP) is a childhood-onset and complex form of hereditary spastic paraplegia. Spastic paraparesis is a universal feature in affected individuals. Manifestations typically begin before age one year, with infants presenting with hypotonia, mild postnatal microcephaly, and delayed developmental milestones. Seizures are common in early childhood, often starting as prolonged febrile seizures. As the disease progresses, older children have intellectual disability that is usually moderate to severe; most affected individuals communicate nonverbally. Neurobehavioral/psychiatric manifestations (e.g., impulsivity, hyperactivity, and inattention) are common. Hypotonia transitions to progressive lower-extremity weakness and spasticity, accompanied by pyramidal signs such as plantar extension, ankle clonus, and hyperreflexia. Although some children achieve independent ambulation, most eventually lose this ability and rely on mobility aids or wheelchairs. In adolescence or early adulthood, spasticity may affect the upper extremities in some individuals but is generally less severe and not significantly disabling. Complications in some individuals include contractures, foot deformities, and bladder and bowel dysfunction. Dysphagia may emerge in advanced stages of the disease.