Hereditary Spastic Paraplegia Genetic Test: Spasticity Genes and Results

A hereditary spastic paraplegia genetic test looks for DNA variants that cause progressive stiffness and weakness of the legs. Hereditary spastic paraplegia, or HSP, includes many distinct disorders rather than one condition, and more than 80 genetic types have been described. Some forms mainly affect the long motor pathways controlling the legs. Others also cause neuropathy, ataxia, seizures, intellectual disability, vision or hearing loss, movement disorders, or abnormalities on brain MRI. Testing may use a multigene panel, exome sequencing, genome sequencing, mitochondrial analysis, or targeted testing for a known familial variant. A molecular diagnosis can clarify inheritance and guide condition-specific monitoring, but it does not replace evaluation for spinal cord disease, vitamin deficiencies, infection, immune disorders, and other treatable causes of spasticity. Negative and uncertain results are common because HSP is genetically complex and because not every disease mechanism is captured by a routine sequencing panel.

• A pathogenic result can confirm a specific HSP type when the variant, inheritance pattern, and clinical features agree.
• A negative result does not exclude hereditary spastic paraplegia, especially after limited panels or sequencing without copy-number and mitochondrial analysis.
• SPAST, ATL1, REEP1, KIF5A, SPG7, SPG11, KIF1A, and AP-4 genes are frequent targets, but the appropriate list depends on age and associated findings.
• Children labeled with cerebral palsy may need genetic testing when spasticity progresses, family history is suggestive, or the birth history does not explain the pattern.
• Sudden or rapidly worsening leg weakness is not typical of HSP and needs prompt assessment for an acquired spinal cord or brain disorder.

Table of Contents

• The Clinical Pattern Behind HSP Testing
• Pure and Complex HSP Gene Groups
• Tests Used to Find an HSP Cause
• Conditions That Can Mimic HSP
• Interpreting the Laboratory Report
• Inheritance Risks for Family Members
• How a Diagnosis Changes Care
• Reanalysis After an Inconclusive Test

The Clinical Pattern Behind HSP Testing

HSP primarily damages the longest corticospinal axons, the nerve fibers that carry movement signals from the brain to the spinal cord. Their length helps explain why symptoms often begin in the legs. Typical findings include increased muscle tone, brisk knee and ankle reflexes, ankle clonus, extensor plantar responses, reduced ankle movement, toe walking, scissoring gait, and weakness that is often most noticeable at the hips or ankles.

Progression varies greatly. Some people develop mild stiffness in adulthood and continue walking independently for decades. Others begin in infancy or childhood and need mobility support early. Even relatives with the same pathogenic variant may differ in onset age and disability.

Clinicians often divide HSP into two broad clinical groups:

• Uncomplicated or pure HSP mainly causes bilateral leg spasticity and weakness. Urinary urgency and mildly reduced vibration sensation may occur without changing the classification.
• Complex HSP includes additional neurologic or systemic features such as developmental delay, cognitive change, seizures, ataxia, neuropathy, muscle wasting, dystonia, parkinsonism, optic atrophy, retinal disease, hearing loss, thin corpus callosum, white-matter changes, or short stature.

This division helps organize testing, but it is not absolute. A gene associated with pure HSP in one person may produce a complex phenotype in another. SPG7 can present as spastic paraplegia, spastic ataxia, cerebellar ataxia, or optic neuropathy. KIF1A-related disease ranges from relatively isolated spasticity to severe childhood neurodevelopmental disease. SPAST, the most common cause of autosomal dominant pure HSP, can occasionally produce additional findings.

The family history may show affected people in several generations, only affected siblings, affected males connected through women, or no obvious pattern at all. A negative family history does not rule out HSP. Late onset, reduced penetrance, small families, adoption, inaccurate diagnoses, and a new de novo variant can conceal inheritance.

A broad neurologic genetic panel may be preferable when spasticity overlaps ataxia, neuropathy, dystonia, or motor neuron disease rather than fitting one narrow syndrome.

Pure and Complex HSP Gene Groups

The numbered “SPG” labels reflect the order in which genetic loci were described, not a severity ranking. Modern reports may use both an SPG number and a gene-based name.

Genes often associated with dominant HSP

SPAST (SPG4) is a leading cause of autosomal dominant HSP. Testing must include deletion and duplication analysis because exon-level copy-number variants contribute substantially. ATL1 (SPG3A) is an important cause of early-onset dominant HSP and may initially resemble nonprogressive spastic diplegia. REEP1 (SPG31), KIF5A (SPG10), KIAA0196/SPASTIZIN-related nomenclature distinctions, WASHC5 (SPG8), NIPA1 (SPG6), and RTN2 (SPG12) are additional dominant causes.

Some dominant genes have broader phenotypes. KIF5A variants can cause HSP, Charcot-Marie-Tooth neuropathy, or motor neuron disease depending on the variant location and mechanism. ATL1 can rarely cause a severe congenital motor phenotype. Gene-level interpretation must therefore include the exact variant rather than relying only on the gene name.

Genes often associated with recessive or complex HSP

SPG11 is a common cause of autosomal recessive complex HSP, particularly when there is cognitive decline, neuropathy, a thin corpus callosum, or juvenile-onset parkinsonism. ZFYVE26 (SPG15) can produce a similar pattern. SPG7 commonly causes adult-onset spastic ataxia and may involve optic neuropathy. KIAA0415/SPG48-related nomenclature, CAPN1 (SPG76), CYP7B1 (SPG5), FA2H (SPG35), GBA2 (SPG46), DDHD2 (SPG54), CYP2U1 (SPG56), and PNPLA6 illustrate the wide metabolic and cellular pathways involved.

The adaptor protein complex 4 genes AP4B1, AP4E1, AP4M1, and AP4S1 cause AP-4-associated HSP. Affected children often have early hypotonia, delayed milestones, absent or limited speech, progressive lower-limb spasticity, microcephaly, seizures, and characteristic facial or MRI findings. Many are initially diagnosed with cerebral palsy.

X-linked, mitochondrial, and other mechanisms

X-linked causes include L1CAM and PLP1, although both can produce phenotypes broader than classic HSP. Mitochondrial DNA variants and nuclear mitochondrial genes can cause spasticity with ataxia, neuropathy, hearing loss, epilepsy, or muscle disease. Structural variants, repeat expansions, deep intronic variants, and unusual inheritance mechanisms may escape a standard panel.

Additional finding	Examples of relevant genes or disorders	Useful accompanying evaluation
Thin corpus callosum and cognitive change	SPG11, ZFYVE26, AP-4 genes, KIDINS220	Brain MRI, neuropsychological or developmental assessment
Ataxia or optic neuropathy	SPG7, PNPLA6, CAPN1, mitochondrial disorders	Eye examination, cerebellar assessment, mitochondrial testing
Neuropathy or distal wasting	KIF5A, SPG11, BSCL2, GBA2, ATL1	Nerve conduction studies and electromyography
Early developmental delay and seizures	AP4B1, AP4E1, AP4M1, AP4S1, KIF1A, MAG	Trio exome or genome sequencing and brain MRI
Adrenal or metabolic abnormalities	ABCD1, CYP7B1, inborn errors of metabolism	Targeted biochemical and endocrine testing

Tests Used to Find an HSP Cause

No single assay is best for every patient. The testing plan should reflect the phenotype, family structure, and previous results.

Targeted familial-variant testing is the clearest choice when a pathogenic variant has already been confirmed in an affected relative. It is faster and avoids uncertain findings in unrelated genes.

Multigene panel testing provides deep coverage of selected HSP and overlapping genes. A high-quality panel should state whether it assesses exon-level deletions and duplications, difficult regions, mitochondrial DNA, and relevant repeat expansions. Panel size varies widely, so the name “HSP panel” does not guarantee the same content across laboratories.

Exome sequencing evaluates most protein-coding regions and can be efficient for complex childhood presentations or genetically heterogeneous adult disease. Testing the affected person and both biological parents as a trio helps identify de novo variants, phase recessive variants, and reduce uncertain interpretations. Exome limitations include uneven coverage, incomplete structural-variant detection, and poor assessment of most noncoding variants and repeat expansions.

Genome sequencing can detect more structural, noncoding, and mitochondrial variants and may support repeat-expansion screening. It still may not resolve every repeat, complex rearrangement, methylation disorder, or low-level mosaic variant. Confirmatory methods remain necessary for some findings.

Biochemical or enzyme testing can be more direct than DNA for selected treatable disorders. Very-long-chain fatty acids for X-linked adrenoleukodystrophy, vitamin levels, metabolic profiles, and targeted enzyme assays may be ordered alongside genetics. DNA alone should not delay a time-sensitive biochemical diagnosis.

The usual specimen is blood or saliva, and fasting is not required for the genetic sample. Blood is often preferred when multiple tests are planned. Turnaround commonly ranges from several weeks for a panel to several months for exome, genome, or research analysis.

The clinician should include precise phenotype information on the requisition. “Spasticity” alone is less useful than “progressive lower-extremity spasticity beginning at age 8, distal weakness, urinary urgency, thin corpus callosum, normal birth history, and affected younger brother.” Laboratories use these details to prioritize variants.

Conditions That Can Mimic HSP

HSP is usually slowly progressive and bilateral, but many acquired or treatable disorders can produce the same upper motor neuron signs. MRI of the brain and spinal cord is often needed before or during genetic testing to look for compression, inflammation, tumor, vascular malformation, demyelination, or structural abnormalities.

Important mimics include:

• Cervical or thoracic spinal cord compression
• Multiple sclerosis, neuromyelitis optica spectrum disorder, and other inflammatory myelopathies
• Vitamin B12, vitamin E, copper, or folate deficiency
• Human T-cell lymphotropic virus type 1, HIV, syphilis, and selected infections
• Primary lateral sclerosis and amyotrophic lateral sclerosis
• Dopa-responsive dystonia, which can resemble childhood spasticity
• X-linked adrenoleukodystrophy or adrenomyeloneuropathy
• Cerebral folate, amino acid, lipid, and other metabolic disorders
• Toxic exposure, including nitrous oxide–related functional B12 deficiency
• Structural brain injury and true cerebral palsy

A child’s diagnosis deserves review when “cerebral palsy” was assigned without a clear prenatal or perinatal brain injury, when symptoms progress, when multiple siblings are affected, or when MRI does not match the motor pattern. Genetic cerebral palsy mimics are increasingly recognized, and a diagnosis can change surveillance or reveal a treatment.

Conversely, finding a genetic variant does not prove that all current disability comes from HSP. A person with HSP can also develop a slipped disc, stroke, medication toxicity, vitamin deficiency, or inflammatory disease. A sudden stepwise decline should not be attributed automatically to the inherited condition.

Clinical evaluation may include blood counts, electrolytes, liver and thyroid tests, B12 with methylmalonic acid, vitamin E, copper and ceruloplasmin, infectious testing based on exposure, very-long-chain fatty acids, cerebrospinal fluid studies, nerve conduction testing, and imaging. The exact workup depends on age, tempo, examination, and geography.

Interpreting the Laboratory Report

A report combines variant classification with inheritance, zygosity, and phenotype. All four must fit.

A pathogenic or likely pathogenic dominant variant may confirm HSP when one altered copy is sufficient and the presentation is compatible. A result inherited from an apparently unaffected parent may reflect late onset or reduced penetrance, but it also calls for a careful phenotype review.

Two pathogenic or likely pathogenic variants in a recessive gene generally need to be in trans, one on each gene copy. Parental testing can establish phase. Two variants on the same chromosome may represent one allele and leave the case unresolved.

One pathogenic variant in a recessive gene usually indicates carrier status. It may also mean a second variant was missed by the method, especially when the phenotype strongly matches. Copy-number analysis, genome sequencing, RNA studies, or another targeted assay may be considered.

A variant of uncertain significance is not a confirmed explanation. A VUS should not be used by itself to label relatives, predict an unaffected child’s future, or support prenatal diagnosis. Segregation, functional evidence, phenotype specificity, and reanalysis may change its classification.

A negative result means no reportable cause was found within the assay’s scope. Review whether SPAST copy-number variants, mitochondrial DNA, structural variants, repeat expansions, and newly described genes were covered. A negative result on a small older panel has a different residual risk from a negative, well-analyzed genome.

A dual diagnosis is possible. Severe or unusual findings may result from two genetic conditions, or from one genetic disorder plus an acquired problem. Stopping after the first plausible variant can miss an explanation for features that do not fit.

A result can name the disorder without accurately predicting severity. Even SPAST-HSP shows substantial variability within families. The report should guide surveillance and family testing, not serve as a fixed forecast of wheelchair use or life expectancy.

Inheritance Risks for Family Members

Autosomal dominant HSP gives each child of a carrier a 50% chance of inheriting the familial variant. Inheritance does not ensure the same onset age or severity. Testing an unaffected adult relative is most informative after the familial variant is known and should include counseling about uncertain timing and available interventions.

Autosomal recessive HSP usually affects siblings born to healthy carrier parents. When both parents carry a pathogenic variant in the same gene, each pregnancy has a 25% chance of an affected child, a 50% chance of a carrier child, and a 25% chance of a child who inherited neither variant. The probabilities are the same for every pregnancy.

X-linked conditions may mainly affect males, although female carriers can sometimes have symptoms. Mitochondrial inheritance generally follows the maternal line, but the amount of altered mitochondrial DNA and tissue distribution can create substantial variability.

A new variant in a child lowers but does not always eliminate recurrence risk because a parent may have low-level germline mosaicism. Parental testing and condition-specific counseling refine that estimate.

Predictive testing of children is appropriate when results will change childhood care, as in many early-onset HSPs. Testing a healthy child solely for a late-onset adult condition is usually deferred so the person can decide as an adult. The distinction is based on medical benefit, not the emotional importance of the information.

Once the familial variants are confirmed, reproductive options can include prenatal diagnosis or preimplantation genetic testing. A discussion of autosomal dominant inheritance or recessive risk should also address variable expression; prenatal detection may identify the variant without predicting the future level of disability.

How a Diagnosis Changes Care

Most HSP care remains symptom focused, but the genetic subtype can determine which complications need surveillance. A person with SPG7 may need attention to ataxia, swallowing, and optic nerve function. SPG11 can involve cognition, neuropathy, parkinsonism, and swallowing. AP-4-HSP requires developmental, seizure, nutritional, orthopedic, communication, and caregiving support. Metabolic diagnoses may lead to targeted therapy or avoidance of specific exposures.

Core management often includes:

• Physical therapy for stretching, strength, gait efficiency, balance, and fall prevention
• Occupational therapy for transfers, self-care, work, school, and adaptive equipment
• Orthotics, canes, walkers, or wheelchairs selected before falls become frequent
• Oral antispasticity medication, focal botulinum toxin, or intrathecal baclofen when appropriate
• Bladder assessment for urgency, retention, or recurrent infection
• Speech and swallowing evaluation for complex forms
• Orthopedic monitoring for contractures, hip problems, foot deformity, and scoliosis
• Treatment of pain, fatigue, mood symptoms, and sleep problems

Reducing tone is not always the only objective. Some people use spasticity to stand or transfer when weakness is substantial. Aggressive treatment can therefore worsen function. Goals should be tested against walking, comfort, sleep, hygiene, caregiving, and participation rather than muscle tone alone.

A molecular diagnosis may support eligibility for natural-history studies and gene-specific trials. It does not guarantee an experimental treatment or prove that a therapy is effective. Standard rehabilitation and prevention of complications remain important while research advances.

Reanalysis After an Inconclusive Test

An inconclusive test should lead to a defined review plan rather than indefinite uncertainty. Ask the laboratory whether it offers periodic reanalysis, whether raw data are retained, and whether new genes or improved structural-variant methods can be applied without another sample.

Reanalysis is especially reasonable when:

1. The original panel was ordered several years ago.
2. The phenotype has expanded to include ataxia, neuropathy, seizures, cognitive change, or optic disease.
3. Only sequencing was performed without deletion and duplication analysis.
4. One pathogenic variant was found in a recessive gene.
5. Trio testing was not available during the first analysis.
6. The clinical diagnosis remains strong despite a negative result.

Updated family information can be as useful as new technology. Examining a mildly affected parent, obtaining records from a relative, or testing another affected family member can clarify segregation. DNA banking may preserve options when an affected relative is seriously ill or access to testing is limited.

Do not wait for a genetic answer before treating spasticity, preventing falls, evaluating bladder dysfunction, or checking reversible causes. Genetics and clinical care should proceed together.

The clinical record should preserve the full molecular report, not only the final diagnosis entered into a problem list. Variant nomenclature, transcript, zygosity, phase, copy-number findings, laboratory methods, and report date may be needed when a relative seeks testing years later. A scanned report also helps distinguish a true negative comprehensive analysis from a limited test that examined only a handful of genes.

When a VUS is present, collect evidence systematically rather than repeatedly ordering unrelated tests. A genetics team may compare the person’s detailed phenotype with published cases, test informative relatives, review population databases, or ask whether RNA or functional studies are available. Testing an unaffected relative simply to see whether the VUS is present is not always helpful; the relative must be selected because the result can meaningfully change the segregation evidence.

A multidisciplinary review can be valuable for difficult cases. Neurology, genetics, neuroradiology, rehabilitation, metabolic medicine, and laboratory specialists may recognize a pattern that no single clinic sees. Reclassification should be communicated in a way that updates both the patient and relatives who made decisions based on the earlier report.

Long-term follow-up should include emotional and social effects as well as motor change. Progressive walking difficulty can affect driving, employment, parenting, sexuality, and caregiver needs. Early social-work and mental-health support can reduce crisis-driven decisions, while school or workplace accommodations may preserve participation even when the molecular diagnosis remains uncertain. Regular reassessment also helps match equipment and home support to changing daily function before preventable injuries or hospitalization occur.

Seek urgent care for sudden weakness, new numbness with a sensory level, loss of bladder or bowel control, severe back pain, fever, acute confusion, or rapidly changing gait. These symptoms can indicate spinal cord compression, inflammation, infection, or stroke and should not be assumed to represent ordinary HSP progression.

References

• Practical recommendations for the clinical evaluation of patients with hereditary ataxia and hereditary spastic paraplegia 2024 (Review)
• Uncomplicated (Pure) Hereditary Spastic Paraplegia Overview 2022 (Review)
• Childhood-onset hereditary spastic paraplegia and its treatable mimics 2022 (Review)
• AP-4-Associated Hereditary Spastic Paraplegia 2025 (Review)
• Biomarkers in Hereditary Spastic Paraplegias 2025 (Review)
• Natural history in hereditary spastic paraplegias: a multicenter prospective cohort study 2026 (Study)

Disclaimer

This article is educational and cannot diagnose hereditary spastic paraplegia or identify the correct test for an individual. Results require interpretation by a neurologist or genetics professional with the complete report, examination, imaging, family history, and evaluation for acquired or treatable causes. Sudden weakness or loss of bladder or bowel control requires urgent medical care.