A study by Fermaglich and Miller (2023) evaluated trends in orphan drug designations and approvals after the Orphan Drug Act of 1983 was passed in the US. The authors use FDA data to evaluate between 1983 and 2022 and find that:

Over the 40 years of the ODA, 6,340 orphan drug designations were granted, representing drug development for 1,079 rare diseases. Additionally, 882 of those designations resulted in at least one FDA approval for use in 392 rare diseases. Much of this development has been concentrated in oncology as seven of the top ten most designated and approved diseases were rare cancers.

Of the orphan drug designations (approvals), the top 5 top diseases areas were:

• Oncology: 38% of designations (38% of approvals)
• Neurology: 14% (10%)
• Infectious Disease: 7% (10%)
• Metabolism 6% (7%)
• Hematology: 5% (8%)

You can find more detail on orphan drug designations and approvals in the full paper here.