What is the definition of a rare disease, and what are its main characteristics?

A rare disease is defined as a health condition that affects a relatively small number of people within a given population. In the United States, for example, a disease is classified as rare if it affects fewer than 200,000 people [1]. Although each rare disease affects a small group, there are more than 10,000 known rare diseases, which collectively affect millions of people [2].

Characteristics of Rare Diseases

Rare diseases are characterized by several features that make them unique and complex, significantly impacting the lives of patients and their families. Some of the most prominent characteristics include:

• Impact on Life: Rare diseases are often chronic, cause disability, and frequently lead to early death [3].
• Difficulty in Diagnosis: Diagnosing rare diseases can be extremely challenging, as they are often incorrectly diagnosed [3]. This challenge stems from a lack of awareness about the disease among physicians and the diversity of symptoms that may mimic more common conditions.
• Complexity: Rare diseases are often very complex in nature [3], requiring a deep understanding of the underlying biological mechanisms.
• Genetic Causes: Many rare diseases, about 80% of them, are caused by a defect in a single gene [1]. This makes gene therapy a promising approach for treating some of these conditions.
• Lack of Approved Treatments: There are no approved treatments for many rare diseases, or the available treatments may be ineffective [3]. This is partly due to a lack of financial incentives for pharmaceutical companies to develop treatments for diseases that affect only a small number of people [1].

Diagnostic and Treatment Challenges

Patients with rare diseases face significant challenges in their journey toward diagnosis and treatment. Finding a specialist knowledgeable in treating a specific rare disease can be difficult [3]. Researching and developing new treatments for rare diseases is time-consuming, complex, and costly [1]. For example, Spinal Muscular Atrophy (SMA) is a group of rare genetic disorders that cause the loss of nerve cells that control skeletal muscles, leading to weakness [1]. Although there is no cure for SMA, there are treatments to help prevent and manage symptoms, such as the drug nusinersen, which was among the first approved gene therapies for a rare disease [1].

The Role of Gene Therapy in Rare Diseases

Gene therapy represents a promising path for treating rare diseases, especially those caused by genetic mutations [1]. These therapies work to "fix" genetic mutations by replacing or altering a non-functioning gene with a functioning one [1]. For example, in the case of Spinal Muscular Atrophy, nusinersen targets a different gene called SMN2 to help produce more SMN protein, which is essential for muscle and nerve function [1]. There are also other gene therapies that use a safe virus to deliver a new copy of the SMN gene to nerve cells to improve muscle movement [1]. For more information on this topic, you can visit Rare Diseases: Diagnosis, Challenges, and the Future of Gene Therapy (Arabic).

Practical Steps for Patients and Their Families

If you suspect that you or a family member has a rare disease, it is important to take the following steps:

• Search for Specialists: Patient support groups, rare disease organizations, and genetics clinics can help you find a specialist with experience in rare diseases [3].
• Communicate with Physicians: Continue communicating with doctors to discuss symptoms and diagnostic possibilities. It may take several visits and consultations to obtain an accurate diagnosis.
• Follow Research: Stay informed about the latest research and developments in the field of gene therapies and rare diseases.

In cases where acute or rapidly worsening symptoms appear, such as severe muscle weakness that affects breathing or swallowing, emergency care must be sought immediately. These symptoms may indicate an underlying condition is worsening and requires urgent medical intervention to preserve vital bodily functions.

Medical references

• Gene therapies are a promising path to treating rare diseases
• Rare Diseases at FDA
• Rare Diseases