The severity of the symptoms does depend on 3 things...
A) When you were diagnosed
B) If you follow your diet and formula consumption
C) The type of MSUD you have and how it has affected you.
Every person is different. While the disease remains the same, no two cases are alike. The earlier you are diagnosed, the better to avoid coma and brain damage. However, following your diet and making sure to take your formula is also extremely key in keeping your amino acid levels down and staying healthy. Lastly, the type of MSUD is also important because some tend to be more severe than others in regard to protein/leucine tolerance. However, when a person with MSUD is sick, it makes very little difference whether they're a classic or variant. Sick is sick.
Some of the worst symptoms are things like being ataxic and finding you're unable to walk, being unable to read. Not knowing where you are and even being unable to recognize people. Extreme stomach pains, and intense muscle weakness, and various other debilitating symptoms.
The symptoms may vary depending on the person.