Shares of Isis Pharmaceuticals advanced yesterday after the company reported positive results from a study of a drug designed to treat spinal muscular atrophy, a rare genetic disorder.

Isis said most of the children who received the largest doses of its drug ISIS-SMN continued to show improvements in muscle function up to 14 months after treatment. The study involved 24 children between the ages of 2 and 14 who had type 1, 2, or 3 of spinal muscular atrophy. Each child received a single injection of the drug.

Isis Pharmaceuticals Inc. shares picked up $4.25, or 13.4 percent, to $36.05 in afternoon trading. The stock has more than doubled in value over the last year, and earlier Thursday it reached a 13-year high of $36.38.

Spinal muscular atrophy is a disorder that causes muscle atrophy and weakness. Symptoms generally develop a few months after birth. Patients with the most severe type of the disease generally live less than two years. Patients with less severe forms may still have a severely shortened lifespan and may never be able to stand on their own.

Based on the results, Isis said it will test a larger dose of the drug in an active mid-stage study involving infants. The Carlsbad, Calif., company wants to start late-stage testing of ISIS-SMN in early 2014.

Isis is developing ISIS-SMN with Biogen Idec Inc.