Physical therapy is well positioned to have a profound impact on advancements in treatment of people with complex and rare diseases. This vantage is due, in part, to the amount of time physical therapists spend with their patients. Time allows a more thorough evaluation of both the clinical presentation and the treatment efficacy, which illuminates the complexity of presentation and allows a deep understanding of its impact on the individual.

There are more than 6800 known rare diseases1 defined as a disease that affects fewer than 200 000 people in the United States at any given time. These diseases are often referred to as "orphaned diseases" because their small numbers mean that they are rarely the target of research and development for pharmacological or biological interventions. While relatively small in comparison to the general population, it is estimated that more than 25 million people in the United States have a rare disease, making it a highly relevant issue in health care. In addition, if we consider those individuals with complex diagnosis, for which a precise definition is currently being discussed,2 the number of challenging medical conditions markedly increases.

People with rare or complex diseases are often poorly served by the health care community, many spend years visiting different providers, and are often misdiagnosed or treated with ineffective or unsuitable therapies. Physical therapists are often among the clinicians who encounter these patients, and physical activity or other rehabilitation therapies can be a key part of improving their quality of life. Understanding how to treat people with conditions that fall in this category is challenging, and when we look to the literature for guidance, we often find nothing of relevance for the patient sitting in front of us. The "gold standard" randomized controlled clinical trial is not suited for research related to rare and complex diseases due to high variability of clinical presentation and small numbers of cases. This results in a dilemma: how do we conduct effective research and then develop and disseminate treatment guidelines that are useful for the treatment of patients with rare and complex medical conditions?

The contents of this special issue include manuscripts that may not provide simple answers to these difficult, but do use high-quality approaches to address treatment questions in a variety of difficult diagnoses. A systematic review by Koter and colleagues considers appropriate outcome measures for use in evaluation of impairment and functional limitations associated with the complex condition of contraversive lateropulsion (previously referred to as Pusher syndrome) in persons with stroke. Fritz and colleagues employ a case series to illustrate a treatment-based classification scheme useful in the development and examination of the physical therapy management for Huntington disease. De Patre and colleagues describe a case study of cortical blindness, a novel approach of therapeutic exercises integrated with cognitive tasks, and remarkable recovery of visual and sensorimotor function more than 2 years after ischemic cortical damage. Another case study by McCain and colleagues details an innovative approach to improve walking in a person with chronic traumatic brain injury using high intensity task practice. Schubert and colleagues describe a unique type of positional nystagmus that does not follow the currently proposed criteria for the atypical light cupula phenomena. Finally, the special interest article by Lobo and colleagues provides an excellent summary of appropriate "small N" designs and analysis techniques as well as guidelines for how to appraise the quality of literature using these techniques.

As neurologic physical therapy research reveals the mechanisms underlying beneficial effects of our task-specific training, exercise, and neuromodulation interventions, the foundation of effective clinical care in the profession will continue to grow. This foundation will benefit from case studies and small N research reports disseminated by the physical therapists who share their clinical experiences treating patients with rare and complex neurologic disorders. Together with larger-scale trials and mechanistic studies, physical therapy practice will continue to bring meaningful change to the patients who come to us for answers.

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