What is the prevalence of primary lateral sclerosis (PLS) in the US?

Updated: Sep 06, 2019
  • Author: Carmel Armon, MD, MSc, MHS; Chief Editor: Nicholas Lorenzo, MD, MHCM, CPE, FAAPL  more...
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Answer

Data on the incidence of primary lateral sclerosis (PLS) are uncertain. In contrast, data on ALS are well documented; ALS affects 2-3 individuals per 100,000 population each year. The 8 patients with PLS reported by Pringle et al in 1992 were identified over a period of 10 years among a population of 500 patients with ALS. [2] Inferring a population base of approximately 4 million people from the ALS patient data (assuming these are mixed prevalence and incidence data) would result in a prevalence of 2 per million for PLS, assuming all cases were identified.

Further assuming an average disease duration of 20 years (close to the reported median of 19 y), this prevalence would translate into an annual PLS incidence rate of 1 per 10 million (0.01 case per 100,000 population per year), which is approximately 0.5% of that for ALS. The tentative nature of these estimates should be emphasized. They are consistent with a conservative estimate that not more than 500 people with PLS currently are living in the United States. Independent validation of this estimate would be difficult. In addition, review of the cases in the Pringle study suggests that half may not have had PLS; this would reduce the above estimates accordingly.

Repeating this calculation, using the more recent numbers 43 patients with PLS and 661 patients with ALS seen over a period of 17 years, [4] results in a presumptive population base of 13,220,000. Factoring an average PLS duration of 20 years, of the 43 PLS patients, approximately one half would be alive at any point in time, giving a prevalence of 1.6 per million, which translates into an incidence rate of 0.8 per 10 million per year and an estimated 400 people with PLS currently living in the United States. These estimates are lower than the previous estimates, in which the author did not take into account loss of PLS patients over the time they were accrued.


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