HPP is an extremely variable disorder. Six major clinical forms have been identified based primarily upon the age of onset of symptoms and diagnosis. These are known as perinatal, infantile, childhood (severe or mild), adult, and odontohypophosphatasia. Generally, the severity of these different forms of HPP correlates to the residual alkaline phosphatase activity in the body, with less enzyme activity causing more severe disease. Because HPP has broad-ranging serenity, it is important to note that affected individuals may not have all of the symptoms discussed below and that every individual case is actually unique. Some children will develop severe complications early in life; others have mild disease that may improve during young adult life. Parents should talk to their child’s physician and medical team about the specific symptoms and overall prognosis.

Perinatal HPP is associated with profound inactivity of alkaline phosphatase and markedly impaired mineralization. Consequently, the skeleton fails to form properly in the womb. Specific skeletal malformations may vary, but short, bowed arms and legs and underdeveloped ribs often occur. Some pregnancies end in stillbirth. In other cases, affected newborns survive for several days, but pass away from respiratory failure due to deformities of the chest and underdeveloped lungs.

Prenatal benign HPP is associated with bowed limbs at birth. The skeletal deformities can be apparent by ultrasound studies before birth. The skeletal malformations associated with this form improve after birth, eventually resembling the signs and symptoms of individuals who range from infantile HPP to odontohypophosphatasia.

Infantile HPP may have no noticeable abnormalities at birth, but symptoms may become apparent at any time within the first six months. The initial symptom may be the failure to gain weight and grow at the expected rate for age and gender referred to as “failure to thrive.” Some affected babies later exhibit early fusion of the bones of the skull (craniosynostosis), which can result in the head appearing disproportionately wide (brachycephaly). Craniosynostosis may be associated with increased pressure of the fluid that surrounds the brain (cerebrospinal fluid), a condition known as “intracranial hypertension.” This complication can cause headaches, swelling of the optic disk (papilledema), and bulging of the eyes (proptosis). Affected infants have softened, weakened bones that lead to the skeletal malformations of rickets. Rickets is a general term from the bone disease that occurs during growth with softening of bone and characteristic bowing deformities of the legs from growth plate abnormalities. Enlarged wrist and ankle joints may occur. Affected infants may also have deformities in the chest and ribs and rib fractures, predisposing them to pneumonia. Varying degrees of pulmonary insufficiency and breathing difficulties may develop, potentially progressing to life-threatening respiratory failure. Episodes of fever and bone pain, or tender bones may occur. Some infants have diminished muscle tone (hypotonia), so that a baby appears “floppy”, attributable to elevated levels of calcium in the blood (hypercalcemia). Hypercalcemia can cause vomiting, constipation, weakness, and poor feeding. Increased excretion of calcium may lead to kidney (renal) damage. In rare cases, vitamin B6-dependent seizures may occur. Sometimes there is spontaneous improvement in mineralization during early childhood. Short stature and skeletal deformities may persist lifelong.

Childhood HPP is highly variable, and severe and mild forms should be considered, these are less severe than the infantile form. Affected children may sometimes have craniosynostosis and exhibit signs of intracranial hypertension. Skeletal malformations that resemble rickets may become apparent at 2 to 3 years of age. Bone and joint pain and fractures may occur. Typically, one or more baby teeth fall out earlier than normal. Some children are weak and experience delays in walking, and when they do learn to walk may have a distinct, waddling gait. Spontaneous remission of bone symptoms has been reported in young adult life, but such symptoms can recur during middle-age or late adulthood.

Adult HPP is characterized by a wide variety of symptoms. Affected individuals have osteomalacia, a softening of the bones in adults. Some individuals have a history of rickets during childhood, or have experienced premature loss of their baby teeth. Individuals with adult HPP can suffer fractures, especially stress fractures in the feet or pseudofractures of the thigh. Repeated fractures can result in chronic pain and debility. Fractures of the spine seem less common, but also occur. Bone pain is a common complication. Some affected adults develop joint inflammation and pain near or around certain joints due to the accumulation of calcium crystals (calcific periarthritis) or a condition called chondrocalcinosis, characterized by the accumulation of calcium crystals within the cartilage of joints, sometimes damaging the joint. Others have sudden, severe pain in a joint (pseudogout). Affected adults may experience loss of adult teeth.

Odontohypophosphatasia is characterized by the premature loss of deciduous teeth in childhood, or loss of teeth in adulthood. The dental problems are an isolated finding that does not occur along with the characteristic bone symptoms of other forms of HPP.