Mitochondrial disorders may affect almost all organs of the human body. However, various tissues need different amounts of energy. For example, brain, heart and skeletal muscles require a lot of energy in the form of ATP; thus, these organs are often the first affected in children and adults with mitochondriopathies.
Children with developmental delay (including gross-motor and cognitive delays) or a history of development regression could be referred for an evaluation. If the developmental delay of a patient can be explained based on a history of birth trauma with typical findings on brain MRI, the patient does not have to undergo evaluation to rule out mitochondrial disorder. If, however, no explanation for cause of a delay is evident, mitochondrial disorder may be considered. As well, gross motor delay in patients with a mitochondrial disorder is frequently accompanied by hypotonia and weakness. Children with mitochondrial disorders may have episodes of deterioration of their motor function and language with subsequent full or incomplete recovery. Regression may sometimes be provoked by viral illness. Recovery may be quick but, in occasional instances, may take months or even years.
Children with mitochondrial disorders may have significant feeding difficulties - frequently right after birth - along with failure to gain weight and, sometimes, constipation. In some patients, symptoms related to gastrointestinal tract dysmotility are the first presentation of mitochondrial disease.
Most, but not all, patients with mitochondrial diseases have elevated blood and cerebrospinal fluid lactate levels. Measurement of blood lactate has become one of the screening tests in patients with developmental delay. Children with elevated blood or spinal fluid lactate levels need to be seen by a mitochondrial specialist. However, lack of lactate elevation does not rule out mitochondrial disorder.
If a primary care physician (PCP) suspects a mitochondrial disorder, he or she should make a referral to a mitochondrial specialist. Most physicians who work in this area are either neurologists or specialists in metabolic disorders. Several hospitals in the country have established Mitochondrial Programs. A PCP can initiate the workup for potential mitochondrial disorder. This includes a liver function test, checking blood lactate and electrolytes, and checking blood and urine amino and organic acids. Data collected by a primary care provider can be helpful to a mitochondrial specialist in making a decision about how aggressively to pursue the potential diagnosis of a mitochondrial disease.
Selected sections from Irina Anselm's "Diagnosis of Mitochondrial Disorders, Deciding When to Refer" in the Mitochondrial and Metabolic Disorders, Primary Care Physician's Guide, second edition, pages 14 & 15.