Decoding early gait disturbance using the ‘ear of the lynx’ imaging sign in hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of genetically and clinically heterogeneous inherited neurological disorders characterised by progressive degeneration of the corticospinal tracts, leading to progressive spasticity and weakness of the lower limbs. ¹ Our case report highlights the diagnostic significance of the ‘ear of the lynx’ sign in early‐stage HSP, where subtle symptoms—limited to a mild spastic gait—posed an early diagnostic challenge.

A previously healthy 12‐year‐old male presented with a 6‐month history of progressive gait impairment, bradykinesia and bilateral lower extremity spasticity. Neurological examination revealed increased muscle tone and hyperreflexia in the lower extremities, accompanied by sustained bilateral ankle clonus (Video S1‐A). Gait assessment revealed a spastic, slow gait characterised by bilateral lower limb adduction and slight toe‐walking (Video S1‐B). Ophthalmological examination showed no retinitis pigmentosa. His 7‐year‐old sibling was neurologically asymptomatic.

Findings from brain magnetic resonance imaging (MRI) demonstrated bilateral symmetric T2/fluid‐attenuated inversion recovery (FLAIR) hyperintensities and T1 hypointensities in the forceps minor of the corpus callosum, consistent with the characteristic ear of the lynx sign. Associated corpus callosal atrophy was also noted (Fig. 1A–C). The proband's sibling showed no structural or signal abnormalities on brain MRI (Fig. 1D–F).

FIGURE 1.

Brain MRI of the proband (A–C) and asymptomatic sibling (D–F), DNA sequence chromatography for the proband and his family (G and H), and pedigree of the family with SPG11 variants (I). (A) Axial T2/FLAIR and (B) T1‐weighted sequences demonstrate bilateral symmetric signal abnormalities in the forceps minor of the corpus callosum (hyperintense on FLAIR, hypointense on T1), characteristic of the ear of the lynx sign. (C) Sagittal T1‐weighted sequence shows corpus callosal atrophy. Corresponding axial FLAIR (D), axial T1‐weighted (E) and sagittal T1‐weighted (F) sequences in the sibling reveal normal brain structure and signal intensity. (G) Heterozygous c.5925_5934dup (p.V1979*) variant in the proband and his father. (H) Heterozygous c.733_734del (p.M245Vfs*2) variant 1979 in the proband and his mother. The younger sibling was WT at both sites on Sanger sequencing. (I) Squares and circles indicate males and females respectively. Filled and unfilled symbols represent affected and unaffected individuals respectively. The arrow denotes the proband. SPG11 genotypes are shown below each symbol. FLAIR, fluid‐attenuated inversion recovery; MRI, magnetic resonance imaging; WT, wild‐type.

Whole‐exome sequencing revealed compound heterozygous variants in SPG11 (OMIM #604360), consistent with AR SPG11‐HSP. The identified variants included a maternally inherited 10‐base pair duplication (c.5925_5934dup; p.V1979*) at chr15:44574974‐44 and a paternally inherited two‐base pair deletion (c.733_734del; p.M245Vfs*2) at chr15:44657230‐44. According to American College of Medical Genetics and Genomics/Association for Molecular Pathology criteria, both variants were classified as pathogenic. Sanger sequencing confirmed the asymptomatic sibling was wild‐type at both loci (Fig. 1G,H).

Hereditary spastic paraparesis (HSP) with thin corpus callosum represents an autosomal recessive subtype characterised by progressive spastic paraplegia, cognitive deterioration, dysarthria, nystagmus and muscle weakness, with distinctive MRI features associated with SPG11 variants. ² Neuroimaging studies by Riverol and colleagues ³ demonstrated preferential involvement of periventricular fibres projecting to the prefrontal cortex in SPG11 patients, while anterior genu‐to‐cingulate projections remain relatively spared. The ear of the lynx sign can be detected in pre‐symptomatic stages. ³ Pascual and colleagues ⁴ validated this radiological marker, reporting sensitivity of 78.8% to 97.0% and specificity of 90.9% to 100% on FLAIR sequences for SPG11/SPG15‐related HSP; it has also been reported in SPG76. ⁵

This case illustrates the diagnostic utility of neuroimaging biomarkers in early‐stage HSP. The index patient presented with pure mild spastic gait, making the early clinical diagnosis challenging. However, the combination of juvenile‐onset, ear of the lynx sign and thin corpus callosum on neuroimaging effectively narrowed the differential diagnosis to SPG11 or SPG15. The absence of retinitis pigmentosa, a distinguishing feature of SPG15, directed the diagnosis toward SPG11, which was subsequently confirmed by whole‐exome sequencing demonstrating compound heterozygous SPG11 variants. The diagnostic specificity was further supported by the sibling's normal neuroimaging and absence of pathogenic variants.

This case demonstrates how the triad of early‐onset, characteristic radiological findings and preserved fundoscopic examination can guide precise genetic diagnosis in HSP, potentially reducing diagnostic delay and avoiding unnecessary testing. The ear of the lynx sign serves as a valuable radiological biomarker, particularly in cases where clinical manifestations are subtle or non‐specific.

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Supporting information

Video S1. Neurological examination findings. Segment A demonstrates pathologically brisk deep tendon reflexes in the patellar and Achilles tendons bilaterally, with sustained ankle clonus. Segment B shows the patient's gait pattern, characterised by spastic paraparesis with bilateral lower extremity adduction and mild foot drop, for comparison, it also includes the asymptomatic sibling demonstrating a normal gait.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.