The infantile neuroaxonal dystrophy rating scale (INAD-RS)

Abstract

Background: INAD is an autosomal recessive neurogenetic disorder caused by biallelic pathogenic variants in PLA2G6. The downstream enzyme, iPLA₂, plays a critical role in cell membrane homeostasis by helping to regulate levels of phospholipids. The clinical presentation occurs between 6 months and 3 years with global developmental regression, hypotonia, and progressive spastic tetraparesis. Progression is often rapid, resulting in severe spasticity, visual impairment, and cognitive decline, with many children not surviving past the first decade of life. To date, no accepted tool for assessing the severity of INAD exists; other commonly used scales (e.g. CHOP-INTEND, Modified Ashworth, Hammersmith Functional Motor Scale) do not accurately gauge the current severity of INAD, nor are they sensitive/specific enough to monitor disease progression. Finally, these other scales are not appropriate, because they do not address the combination of CNS, peripheral nerve, and visual pathology that occurs in children with INAD.

Methods: We have developed and validated a structured neurological examination for INAD (scored out of 80). The examination includes six main categories of pediatric developmental evaluation: 1) gross motor-and-truncal-stability skills, 2) fine motor skills, 3) bulbar function, 4) ocular function, 5) temporo-frontal function, and, 6) Functional evaluation of the autonomic nervous system. A cohort of patients diagnosed with INAD were followed prospectively to validate the score against disease severity and disease progression.

Results: We show significant correlation between the total neurological assessment score and months since symptom onset with a statistically significant (p = 6.7 × 10^{- 07}) correlation between assessment score and disease onset. As hypothesized, the coefficient of months-since-symptom-onset is strongly negative, indicating a negative correlation between total score and months since symptom onset.

Conclusion: We have developed and validated a novel neurological assessment score in INAD that demonstrates strong correlation with disease severity and disease progression.

Keywords: Clinical outcome assessment (COA); Infantile neuroaxonal dystrophy (INAD); Infantile neuroaxonal dystrophy rating scale (INAD-RS); PLA2G6-associated neurodegeneration (PLAN).

Fig. 1

INAD-RS Progression. Figure showing months since symptom onset on X-axis and neurological score on Y-Axis. There is a negative correlation and an average loss of 0.49 points per month of symptoms. The best fit trend line is logarithmic (r2 = 0.54)