There are five clinical subtypes of MSUD. These include:

• The classic
• Intermediate
• Intermittent
• Thiamine responsive
•  E3 deficient subtypes

They are clinically distinguished based on their clinical presentation, age of onset, and residual BCKAD enzyme activity.[1] There is no good correlation between genotype and phenotype. The clinical presentation relies on the BCKAD residual function.[2] However, phenotypic classification depends on the response to metabolic decompensation and leucine tolerance. The specific characteristics and the differences between the different types are shown in table 1.[2][3] 

Evaluation of maple syrup urine disease requires prompt identification of clinical characteristics by a physician. In addition to the clinical features, biochemical testing and molecular testing play a significant role in disease phenotype evaluation.

Clinical Characteristics

Clinical characteristics depend on the underlying metabolic phenotype of MSUD. It is important to note the following manifestations:

• Maple syrup odor in cerumen is the first detectable clinical sign within the first 12 hours after birth.
• 2 to 3 days of age: Irritability, ketonuria, and poor feeding 
• A worsening course of encephalopathy presenting as apnea, opisthotonus, and lethargy. Furthermore, fencing and bicycling movements can also develop by the age of 4 to 5 days.
• Coma and respiratory failure by the age of 7-10 days.
• Milder forms of the disease can present later in infancy/childhood as anorexia, stunted growth, and delayed milestones. Such cases will only develop encephalopathy and ketonuria during metabolic decompensation during an acute illness.

Diagnosis

Children and newborns should be tested for MSUD if the following are present:

• Sporadic encephalopathy and ketoacidosis 
• Intermittent encephalopathy and ketoacidosis
• Encephalopathy and ketoacidosis during an underlying illness
• Encephalopathy and ketoacidosis following trauma or fasting
• Encephalopathy and ketoacidosis with a negative newborn screening result

Prenatal Diagnosis

Diagnosis requires the measurement of BCKAD enzyme activity in cultured chorion villus cells or amniocytes using mutational analysis. Branched-chain amino acid concentrations can also be measured in amniotic fluid. Pre-implantation diagnosis requires the identification of familial pathogenic variants. The preferred diagnostic method is molecular analysis. 

Newborn Screening and Biochemical Evaluation

Since 1964, routine newborn screening has been performed for MSUD. The screening is performed in all 50 United States and is best performed within 24-48 hours after birth. It is performed with tandem mass spectrometry amino acid profiling (MS/MS). The process examines the fisher ratio and concentrations of leucine-isoleucine as a standard measure.[2] Additional laboratory studies need to be performed if elevated branched-chain amino acids are detected.[2] Tandem mass spectrometry readily detects classic MSUD. However, it may not detect milder forms of MSUD due to normal leucine levels. MS/MS does not possess the capability to distinguish amino acids with the same mass, such as hydroxyproline, leucine, isoleucine, and allo-isoleucine.[2] In such cases, a second-tier test such as liquid chromatography must be conducted to analyze allo-isoleucine on dry blood spots. Second-tier testing can help distinguish MSUD from hydroxyprolinemia. Additional laboratory studies include dinitrophenylhydrazine test, gas chromatography, liquid chromatography, BCKAD enzyme activity, and molecular testing.[2] 

Plasma amino acid testing is the most important diagnostic test for MSUD. It is used to assess elevated levels of BCAAs and allo-isoleucine. Elevated allo-isoleucine levels (> 5 micromols/L) confer high specificity and sensitivity for diagnosis. Even with elevated leucine levels, allo-isoleucine levels are not detectable until the age of six days. The urinary organic acids can be detected using gas chromatography-mass spectrometry, dinitrophenylhydrazine tests, and urine test strips. Gas chromatography-mass spectrometry detects branched-chain ketoacids, which further support the diagnosis of MSUD. Branched-chain ketoacids usually follow after BCAA elevation and are detected 48-72 hours after birth.[2] The dinitrophenylhydrazine (DNPH) test can also be used to detect branched-chain ketoacids in the urine. DNPH reagent and urine are mixed in equal volumes. The sample is then observed for 10 minutes for any precipitation and color changes. Clear urine with no precipitate indicates a score of zero. Whereas, a yellow-white precipitate with opaque urine indicates a score of 4. The advantage of using the DNPH test stems from its ability to detect urinary branched-chain ketoacids in an outpatient setting. Standard urine test strips enable the detection of ketonuria. It offers an alternative source of testing in settings of restricted access to other diagnostic tests. Ketonuria is termed as a surrogate marker of underlying metabolic instability.[3] BCKAD enzyme activity can be measured in cells such as lymphoblasts, skin fibroblasts, and liver biopsy. In vivo measurements of BCKAD activity are not clinically useful because the activity levels do not correlate with leucine tolerance and oxidation.[3]

Molecular testing:

Molecular testing is available for three biallelic pathogenic gene variants. These include:

• BCKDHA gene: Encodes the E1-alpha subunit of the BCKAD enzyme complex (MSUD Type 1A).[3]
• BCKDHB gene: Encodes the E1-beta subunit of the BCKAD enzyme complex (MSUD Type 1B).[3]
• DBT gene: Encodes the E2 subunit of the BCKAD enzyme complex (MSUD Type 2).[3]

Genetic testing allows for a better understanding of the prognosis and genetic counseling of the family. Furthermore, it allows for accurate assessment of the deficient BCKAD subunit. This helps determine the individualized therapies. There are over 190 pathogenic variations in BCKAD enzyme subunits. All detected variants are homozygous or compound heterozygous

Diagnostic Strategies

Adult with symptoms of MSUD:

• DNPH testing can be used to alpha-ketoacids in the urine.[3]
• The most informative test is the identification of allo-isoleucine using plasma amino acid analysis.
• Branched-chain ketoacids and other organic acids can be detected using gas chromatography-mass spectrometry.

Newborn with signs and symptoms and a positive screening test for leucine, isoleucine, and valine or unexplained ketonuria:

• If infants are older than 48 to 72 hours, screening tests such as DNPH and urine ketone test strips can be used.
• Plasma amino acid analysis to detect elevated BCAA and allo-isoleucine.
• Gas chromatography-mass spectrometry is used to analyze the urine for ketoacids.
• Newborns and infants should not be challenged with higher than normal protein intake.[3]

Newborn with an affected sibling:

• If familial pathogenic variants are known, isolated blood from the umbilical cord can be used for a pathogenic variant detection by polymerase chain reaction (PCR), advanced sequencing, and melting analysis.
• If pathogenic variants are unknown, obtain blood from the umbilical cord to allow for pathogenic variant detection. The first diagnostic sign is the presence of maple syrup odor in the cerumen of a newborn. Therefore, inquire for a maple syrup smell 12 to 24 hours after birth. Allow protein intake to measure serum amino acid levels between 18 to 24 hours of life. If the results are vague, repeat the test between 24 to 36 hours of life. Amino acid profile indicative of MSUD should initiate dietary therapy. Furthermore, molecular testing and urine organic acid analysis should follow. It is important to remember that DNPH can't be used as a screening test until the age of 48 to 72 hours.

Effective treatment of maple syrup urine disease requires addressing the nutritional needs and optimally managing acute metabolic decompensations.[9] A pediatric nutritionist and metabolic disease specialist should be involved in the management of MSUD patients.

Medical Nutritional Therapy

The initiation of nutritional therapy requires clinical confirmation and a positive newborn screening result. The mainstay of treatment remains the dietary restriction of branched-chain amino acids. These dietary modifications need to be maintained throughout life. The goals of nutritional therapy are as follows [10]:

• Promote anabolism
• Prevent catabolism 
• Promote normal growth and weight gain
• Preserve intellectual function
• Enable restriction of branched-chain amino acid in the diet, which helps reduce toxic metabolites
• Maintain plasma BCAA levels within the required treatment ranges
• Evaluate thiamine responsiveness

The allowed amounts of dietary BCAA are titrated into the diet using biochemical lab values and growth measurements during respective life periods. Long-term treatment warrants accurate assessment of caloric needs, BCAA restriction, BCAA-free amino acid supplementation, and valine and isoleucine supplementation.[2] BCAA-free medical foods can provide up to 80%-90% of protein needs. Valine and isoleucine help promote anabolism. They are supplemented because they are low in content in medical nutrition. Leucine supplementation is usually not required because they are found in ample amounts in breast milk or formula. Favorable intellectual outcomes can be achieved if leucine concentrations are maintained between 75 to 300 micromol/L.[11] It is also recommended that plasma valine and isoleucine levels are maintained between 200 to 400 micromol/L.[10] Thiamine should be supplemented at a dose of 50 to 200 mg/day in all patients with MSUD for four weeks. In thiamine responsive patients, thiamine supplementation should be continued. Thiamine should not be supplemented in patients diagnosed with the homozygous 1312T>A mutation or with mutations leading to less than 3% of BCKAD activity. Administration of sodium chloride may help maintain serum sodium concentrations and plasma osmolarity. It can help prevent the development of cerebral edema or fatal herniation. 

Treatment of Acute Metabolic Decompensation

Metabolic decompensations (plasma leucine >380 micromol/L) usually occur due to dietary non-compliance and infections. Dietary non-compliance raises the BCAA levels and rarely progresses to decompensation and encephalopathy. However, trauma and infections can trigger large protein catabolism leading to a metabolic crisis. Decompensations arise more commonly in the first year of life and after the age of 15.[12] Strauss et al. indicated that vomiting and viral gastroenteritis are the most common cause of hospitalization during acute decompensation.[13] Other common causes of hospitalization include viral bronchiolitis, sinusitis, neonatal encephalopathy, and urinary tract infections.[13] Residual BCKAD activity and liberation of leucine from catabolism determine the risk of metabolic crisis.[3] Patients with a higher residual BCKAD activity have a better leucine tolerance when well. Furthermore, during an illness, these patients face less severe elevations in leucine. The main aim of therapy is to suppress protein catabolism and promote protein synthesis.[2] Management strategies in more severe cases include:

• Effectively treating the underlying stressor causing the metabolic crisis 
• Restrict protein intake for 24 to 72 hours
• Provide ample caloric support
• Provide adequate hydration to maintain metabolic homeostasis
• Provide supplementation with cofactors
• Eliminate toxic metabolites
• Treat associated clinical sequelae
• Correct metabolic abnormalities

Home Therapy 

Providers can be instructed to use a dinitrophenylhydrazine reagent to detect high urine branched-chain ketoacids. This allows for the timely detection and home management of mild to moderate cases of acute metabolic decompensation. In such cases, experienced providers can help manage with restriction of dietary leucine, sick day formulas, and outpatient monitoring.[3] More specifically, sick day protocols require a 120% increase in the intake of BCAA-free amino acid formula, fluid administration of 150 mL/kg, 50% to 100% decrease in leucine intake, and frequent small feedings.[2] It is important to ensure that nutritional treatment is aggressive and provides sufficient energy. 

In-Hospital Therapy[3]

Goals and treatment strategies of in-hospital therapy include:

• Effectively treating the underlying stressor (e.g., fever, dehydration, infection, and inflammation).
• To control nausea and vomiting, antiemetics such as ondansetron should be administered.
• Reduction in leucine concentration at 750 micromol/L or more per 24 hrs. This reduction in leucine can be achieved via insulin and glucose infusions. Ideally, leucine levels should be maintained from 200 to 300 micromol/L. Upon clinical improvement, total parenteral nutrition can be used to reintroduce protein back into the diet (25%-50% of normal intake). This intake can be increased depending on the clinical situation over the next few days.
• EER must be provided at least 1.25 times the weight or surface area. Lipid should constitute 40% to 50% of total calories. Nutritional goals can be achieved by combined parenteral and enteral feeding. 
• Isoleucine and valine should be supplemented at 20-120 mg/kg/day each. The intake of supplements is adjusted to maintain a steady plasma concentration of 400-600 micromol/L.
• Enteral supplementation of tyrosine (100-400 mg/kg/day) to treat focal or generalized dystonias.
• Glutamine and alanine supplementation at 150-400 mg/kg/day each.
• Supplement BCAA-free amino acids.
• Maintain sodium levels within the physiological range.
• Correct underlying acid-base disturbances.
• Avoid osmolarity fluctuations > 5 mosm/L per day and maintain urine output.
• Prevent and treat hypokalemia and hypophosphatemia associated with IV glucose and insulin therapy.

Role of Sodium Phenylbutyrate (NaPBA)

Sodium phenylbutyrate is a nitrogen scavenger. It is primarily used in the treatment of urea cycle disorders. NaPBA can also reduce branched-chain amino acid levels. It can be used in patients with intermediate MSUD.[14] Hemodialysis and peritoneal dialysis can also be used to rapidly correct BCAA and ketoacids during an acute decompensation.

Orthotopic Liver Transplantation

Indications:

• Psychomotor disabilities
• Poor metabolic control
• Frequent metabolic decompensations

The liver is responsible for expressing 10% of BCKAD activity. Therefore, liver transplantation is recommended for classic MSUD patients who are difficult to manage. More commonly, the liver from an undeceased and unrelated individual is used.[2] Post-transplantation, the residual activity of BCKAD, can rise to that of mild MSUD levels. Therefore, transplantation tends to reduce the requirement of dietary restrictions and episodes of metabolic decompensation. It also does not reverse previously inflicted brain damage, cognitive dysfunction, and psychiatric illnesses.[15] However, transplantation doest restrict further progression of neurological impairment and prevent the development of cerebral edema.[16][17]

Management in Pregnancy

It is indeed possible for women with MSUD to deliver a healthy child. The mother must be educated about the potential teratogenic risks of elevated maternal leucine concentration. In such patients, tight metabolic control before and throughout gestation is critical. As the placenta and fetus develop, the maternal need for protein and BCAA exponentially increases. Therefore, measurements of plasma amino acid concentrations and fetal growth are imperative to avoid possible nutritional deficiencies. Maintaining BCAA levels between 100 and 300 micromol/L is said to be compatible with normal infant delivery. At the time of delivery, referral to a metabolic center must be made. This is because the post-partum period is deemed dangerous for the mother. Events such as labor stress, internal blood sequestration, and uterine involution can act as a source of metabolic decompensation. Therefore, extra measures must be taken to counteract catabolism during the post-partum period.

Management of other MSUD Complications

Cerebral Edema

• Administer furosemide (0.5 to 1.0 mg/kg)
• Infusion of mannitol (0.5 to 1.0 g/kg) followed by hypertonic saline (3%-5%) 

The risk of developing cerebral edema can be reduced by elevating the head of the bed.[3] Furthermore, it is recommended to use a PICC line in encephalopathic patients. The use of a PICC line allows the delivery of nutrition without the need for excess fluid volume.[3]

Cerebral Herniation

• Elevate the head
• Induce hyperventilation with the help of a face mask or endotracheal tube.
• Infuse mannitol and hypertonic saline.
• Transfer the patient immediately to a pediatric/neonatal intensive care unit.

Infection

Patients with MSUD are predisposed to candida infections and catheter-based bacterial or fungal infections. It is vital to monitor patients for hospital-acquired infections continuously. Failure to treat them can result in an episode of acute metabolic decompensation.

Acute Pancreatitis

During an acute metabolic decompensation episode, a patient may develop symptoms such as epigastric pain, mid-back pain, anorexia, and vomiting. This should raise suspicion of acute pancreatitis and immediately order serum levels for lipase and amylase. Also, the physician must suspend enteral feeding and keep the patient NPO. Treatment is usually supportive, and the patient's nutritional needs can be managed with the use of special parenteral solutions.

Neuropsychiatric Illnesses

Adult and adolescent patients are at an increased risk of developing anxiety, depression, and ADHD. These can be treated by prescribing standard anti-depressants and psychostimulant drugs.

Secondary Complications

Surgical procedures and trauma care should be planned in coordination with a metabolic specialist.

General Prevention

Patients must be advised never to exceed their daily allowed dose of branched-chain amino acids.

It is essential to exclude other clinically distinct entities that can also manifest as neonatal encephalopathy. These include:

• Hypoglycemia
• Status epilepticus
• Meningitis
• Encephalitis
• Kernicterus
• Birth asphyxia
• Urea cycle defects[3]
• Organic acidopathies such as propionic acidemia and methylmalonic acidemia[3]
• HMG-CoA lyase deficiency
• Beta-ketothiolase deficiency[3]
• Non-ketotic hyperglycinemia[3]

Sotolone, which is found in fenugreek and lovage, is responsible for the characteristic maple syrup odor in cerumen and bodily secretions.[3] Excess ingestion of fenugreek during pregnancy can result in a false diagnosis of MSUD.[3] In the setting of NICUs, the use of topical benzoin can also give off a sweet odor.[3]

Good prognosis can be expected in patients who begin therapy before or immediately after developing symptoms. Plasma leucine concentrations are known to affect neurocognitive outcomes. Classic MSUD in school-aged patients has shown high performance than IQ.[18] 61% of adult MSUD patients live independently and integrate well into society.[12] However, 56% of patients still require psychological and psychiatric care.[12] Patients with late-onset MSUD can suffer from slight developmental delays depending on the residual BCKAD activity.[3] A delayed diagnosis of more than 7 to 14 days of classic MSUD can result in irreversible learning disability and cerebral palsy.[3]

Failure to diagnose and treat MSUD in a time-sensitive manner can result in serious consequences. Patients undergoing a treatment plan can develop an acute illness resulting in a sudden increase in levels of branched-chain amino acids. This metabolic crisis is usually indicated by the development of clinical symptoms such as extreme fatigue, irritability, vomiting, and loss of alertness. If the patient remains undiagnosed or untreated, the following complications can arise:

• Seizures
• Metabolic acidosis
• Cerebral edema
• Cerebrovascular ischemia
• Intellectual disabilities
• Blindness
• Muscle spasticity
• Irreversible neurological damage
• Osteoporosis
• Acute pancreatitis
• Recurrent esophageal candidiasis due to T-cell suppression from elevated plasma leucine.
• Essential amino acid deficiency presenting as anemia, hair loss, growth failure, and acrodermatitis

A thorough understanding of the patient and the public's clinical condition will ensure adherence to optimal patient care strategies. Furthermore, it allows physicians and other healthcare team professionals to provide the best patient outcomes based on evidence-based medicine. Educational resources should include:

• Educational handouts obtained online from the New England Consortium of metabolic programs
• The American College of Pediatrics website
• MSUD family support group
• National Library of medicine genetics home reference
• NCBI genes and diseases
• Metabolic Support UK
• European Registry and Network for Intoxication Type Metabolic Diseases (E-IMD)
• Organic Acidemia Association

Optimal management of maple syrup urine disease requires an interprofessional team consisting of an internist, pediatrician, geneticist, metabolic specialist, pediatric nutritionist, social worker, and pediatric nurse. The mainstay of therapy is medical, nutritional therapy. The healthcare team must collaborate to assess caloric needs and execute dietary treatment strategies such as BCAA restriction, BCAA-free amino acid supplementation, and valine and isoleucine supplementation. The pediatric nutritionist must educate the patient never to exceed the daily intake of amino acids. They should also educate the patient to adhere to a lifelong restricted diet. The geneticist must counsel the patient or their parents about their inherited disorder's implications to help them make informed medical and personal decisions. Outcomes of the patient depend upon biochemical findings, clinical severity, and molecular testing. A combination of early diagnosis, aggressive treatment, and controlling plasma leucine levels help improve neurological outcomes.[19]