Progressive hemifacial atrophy (PHA), also known as Parry-Romberg syndrome, is characterized by slowly progressive deterioration of the skin and soft tissues on one side of the face. It sometimes occurs on both sides of the face and occasionally involves the arm, trunk, and/or leg. The condition may worsen for 2 to 20 years and then stabilize. It usually begins around age 10 but can begin as early as infancy or as late as mid-adulthood. The severity varies greatly. While the cause is not well understood, it may differ among affected people. PHA is often associated with a type of linear scleroderma called "en coupe de sabre" (ECDS), and many researchers believe that PHA is also a form of linear scleroderma. Therefore, treatment for PHA often includes medications used to treat other forms of linear scleroderma. After progressive hemifacial atrophy has stopped progressing, reconstructive surgery may be used to restore the natural shape of the face and eye.

The cause of progressive hemifacial atrophy is unknown. Due to its similarity to localized, it has been considered as an autoimmune disorder. Other causes or triggers that have being suggested include:

• inflammation of the brain (encephalitis) or blood vessels (vasculitis)
• trauma
• scleroderma
•  migraine headaches
• infections (such as Lyme disease and herpes infections)
• abnormalities of the way the body processes fat (fat metabolism)
• blood vessel disorders in the brain (cranial vascular malformations)
• noncancerous (benign) tumors.

The first signs of progressive hemifacial atrophy usually are skin changes similar to those in localized scleroderma. It is more commonly seen in women and it usually affects the left side of the face more than the right side. Not everyone will have all of the possible symptoms, and the symptoms may range from mild to severe. In general, if the disease start later in life the atrophy is less severe because facial growth is almost complete in the second decade of life. Signs and symptoms may include:

• Characteristic thinning or shrinkage (atrophy) of the various tissues of one side of the face, mainly involving fat tissue, but that can include skin, connective tissues, muscle, and sometimes, bone, with can result in a mild unnoticed asymmetry, or that can be very severe and cause disfigurement of the face 
• Eye problems (in around 10-35% of cases) including changes to the eyelid and eye socket, where the eyeball may appear to sink into the eye socket (enophthalmos) as the supporting tissue atrophies. Other problems may include corneal and retinal changes, or may involve the optic nerve, or the muscle of the eyes 
• Neurological complications occur in 45% of cases, and may include seizures, migraine headaches, hair loss, facial nerve pain (trigeminal neuralgia) 
• Darkening of skin (hyperpigmentation)
• Teeth changes (in about 50% of the cases) which result in a deviation of nose and mouth to the affected side, and teeth exposure
• Tongue atrophy
• Very rarely there is also lost of fat tissue in the same side of the body where the face is affected, including arms, legs and thorax. 

According to the severity of the disease, there are three types of progressive hemifacial atrophy:

• Type I, is the most common and involves a localized area in the face affecting only the  superficial layer of the skin (epidermal layer) that follows the distribution of the fifth cranial nerve (trigeminal nerve)
• Type II, is more severe compared with type I
• Type III affects both sides of the face including all soft and underlying bone (the less frequent).

Currently there is no treatment to stop the progression of progressive hemifacial atrophy, but various treatments have been tried. Because it is very similar to  linear scleroderma treatment is also similar, and may include:

• Antimalarials
• Methotrexate
• Local or systemic steroids
• Tetracycline
• Cyclophosphamide
• Non-cosmetic skin softeners (emollients)
• Vitamin D3 analogues
• Treatment with drugs that become active when exposed to ultraviolet light (phototherapy), for example PUVA (Psoralen plus ultraviolet A)

Hemicranial pain syndrome (can be in form of migraine or continuous severe headache, but on one side of the head) has been treated successfully by repetitive local botulinum toxin A injections. Treatment of the eye and nerve involvement may include the use of some medications, such as steroids, and surgical procedures, depending on the specific problem. Progressive hemifacial atrophy stops progressing on its own within 2 to 20 years, which makes it hard to decide if a therapy is successful. After the condition does stabilize, reconstructive surgery can be done to restore the fat tissue that is lost with the disease and, therefore, recover the natural shape of the face, and treat the sunken eye (enophthalmos) and the eyelid retraction.  The following techniques are options for surgical reconstruction:

• Silicone implants
• Muscle flap grafts
• Fat grafts with or without stem cells
• Bone and cartilage grafts
• Injections to fill in hollows below the skin such as hyaluronic injection, which may be also improve the enophthalmos and eyelid retraction.

Since multiple systems of the body may be affected, a multidisciplinary team of physicians, surgeons, dentists, and psychologists may be needed to manage the different symptoms.

Progressive hemifacial atrophy is a slowly progressive, self-limiting condition. However, the type and severity of symptoms varies greatly. The length of time the disease progresses before it reaches stabilization also varies greatly. While most sources state the course of the disease will stabilize in 2-10 years, the most recent review suggests 2-20 years. The condition may be less severe when it begins in adulthood than in younger ages, probably due to the complete development of craniofacial and nervous structures. To our knowledge, recurrence in the same person after the condition has stabilized has not been reported in the medical literature. Progressive hemifacial atrophy can have a significant psychological and social impact on a person, depending on the severity, progression, age of onset, and success of cosmetic repair.

There is no specific test to diagnose progressive hemifacial atrophy, so the initial diagnosis can be difficult, often taking years. It is usually suspected because of the characteristic atrophy affecting tissues of the face. Diagnosis may include a medical examination, a full medical history, magnetic resonance imaging (MRI), removal of a tissue sample to study in the laboratory (biopsy), and/or and ruling out other causes.

Most cases of progressive hemifacial atrophy are sporadic, which means they occur in people with no family history of the condition. However, in rare cases, more than one family member is affected. No clear pattern of inheritance in these cases has been seen. Research into the cause of progressive hemifacial atrophy, including any possible genetic predisposition, is ongoing.