The symptoms of Hartnup disease vary greatly from one person to another. The majority of affected individuals do not have any apparent symptoms (asymptomatic). When symptoms do develop, they most often occur between the ages of 3-9. In rare instances, symptoms first appear in adulthood.
The most common symptom are red, scaly light-sensitive (photosensitive) rashes on the face, arms, extremities, and other exposed areas of skin.
A wide variety of neurological abnormalities can occur including sudden episodes of impaired muscle coordination (ataxia), an unsteady walk (gait), impaired articulation of speech (dysarthria), occasional tremors of the hands and tongue, and spasticity, a condition marked by increased muscle tone and stiffness of the muscles, particularly those of the legs.
There have been reports of delayed cognitive development and, in rare instances, mild intellectual disability in some children. It is, however, unclear whether these symptoms are related to Hartnup disorder or incidentally occurred in the same individual and were therefore attributed to Hartnup disorder. Similarly, Seizures, fainting, trembling, lack of muscle tone (hypotonia), Headaches, Dizziness and/or vertigo, and delays in motor development have been observed but may be unrelated. Some affected individuals may experience psychiatric abnormalities including emotional instability such as rapid mood changes, depression, Confusion, anxiety, delusions, and/or hallucinations.
Some children experience growth delays and may be shorter than would be expected based upon age and gender (short stature). In some instances, the eyes may be affected and individuals may experience double vision (diplopia), involuntary rhythmic movements of the eyes (nystagmus), and droopy upper eyelids (ptosis).
Diarrhea may precede or follow an episode of this disorder. Some adults with Hartnup disease have been reported whose initial symptom was the onset of Seizures during adulthood. Heartburn has been reported in adults with the disorder.