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Como a dor funciona

Illustration: Como a dor funciona

Esta página foi traduzida automaticamente e ainda não foi verificada por um médico. A versão em inglês é a versão oficial.

A dor é o sistema de alarme do corpo. Ela nos protege ao nos fazer parar, afastar-nos do estímulo e descansar enquanto algo cicatriza; a curto prazo, é uma das funções mais úteis do sistema nervoso. Contudo, a dor é mais complexa do que um simples “medidor de dano”. Compreendê-la realmente ajuda, especialmente quando a dor persiste: o próprio sistema de alarme pode tornar-se hipersensível, fazendo com que a dor continue ou seja sentida de forma desproporcional em relação ao que se observa em exames de imagem, mesmo após a cicatrização dos tecidos. Uma mensagem central, baseada em evidências, permeia esta página: sentir dor nem sempre significa haver lesão. Esta página explica, em linguagem simples, o que é a dor e como funciona; depois, para os mais curiosos, aprofunda-se na biologia por trás da persistência da dor.

O que é a dor e para que serve

A dor não é medida por um sensor e transmitida ao cérebro tal qual é, como a leitura de um termômetro. Na verdade, é uma resposta gerada pelo cérebro após avaliar diversos estímulos: sinais provenientes do corpo, sim, mas também o contexto, experiências passadas, o estado de humor e o grau de ameaça que o cérebro percebe na situação. Sua função é protetora: chamar a atenção da pessoa e modificar seu comportamento para que uma lesão possa cicatrizar.

É por isso que a mesma lesão pode causar dor de intensidades muito diferentes em dias distintos ou em pessoas diferentes; e é por isso que a dor é completamente real, mesmo quando não corresponde ao que se observa em um raio-X.

Dor aguda versus dor crônica

  • A dor aguda é o mecanismo de alarme normal: surge após uma lesão, é mais ou menos proporcional ao dano causado e desaparece à medida que a cicatrização ocorre. Essa dor está cumprindo sua função.
  • A dor crônica é aquela que persiste além do período esperado de cicatrização (geralmente definida como mais de três meses). Nesse caso, o problema frequentemente não é um dano tecidual contínuo, mas sim um sistema de alarme que se tornou excessivamente sensível e não consegue “desligar”. A dor é real e pode ser intensa, porém já não serve como sinal confiável de que algum tecido está sendo lesado.

Essa distinção é extremamente importante, pois os dois tipos de dor exigem abordagens distintas: a dor aguda é tratada cuidando da lesão e proporcionando alívio a curto prazo; já a dor crônica requer a gradual calma e reeducação do sistema nervoso, e não o uso contínuo de analgésicos cada vez mais potentes.

O que ajuda a aliviar a dor

  • Entendê-la. Compreender que “sentir dor não significa haver dano” realmente reduz a dor crônica e a incapacidade; o medo e a catastrofização intensificam a dor.
  • Movimento e ritmo adequado. Atividades suaves e progressivas ajudam a dessensibilizar um sistema hiper-sensível; o repouso prolongado e a evitação de movimentos geralmente pioram a dor crônica.
  • Sono, humor e estresse. O sono ruim, o humor negativo e o estresse aumentam a intensidade da dor; cuidar desses fatores ajuda a diminuí-la.
  • Os medicamentos certos para cada tipo de dor. Tratamentos curtos com analgésicos simples são úteis para a dor aguda; a dor crônica responde melhor a medicamentos que atuam nos nervos, à prática de exercícios e a estratégias psicológicas do que aos opioides.

Em maior profundidade

Advanced reading: the deeper science (optional)

Esta seção oferece uma explicação mais detalhada, voltada para estudantes. Ela não é necessária para o controle da sua dor; porém, se você tem curiosidade sobre como a dor é gerada e por que pode persistir, continue lendo.

Como um sinal de dor se propaga

Terminações nervosas especializadas chamadas nociceptores detectam estímulos potencialmente prejudiciais: pressão extrema, calor ou substâncias químicas liberadas por tecidos lesionados. Elas enviam sinais por meio de dois tipos de fibras. As fibras A-delta, rápidas e isoladas, transmitem a dor aguda e imediata que faz com que você retire a mão rapidamente; as fibras C, mais lentas e sem isolamento, transmitem a dor surda, contínua e duradoura que se segue. Esses sinais chegam à medula espinhal, onde são retransmitidos (e já modulados) antes de seguirem para o cérebro. Não existe um único “centro da dor”; diversas regiões cerebrais decidem em conjunto se a dor será percebida e qual sua intensidade. Portanto, a dor é “construída”, e não meramente recebida.

A ideia do controle do “portão”

Diagrama simplificado do controle de portão: um neurônio de projeção que recebe estímulos de uma fibra A-beta rápida e de uma fibra C lenta (responsável pela dor).
O “portão”, de forma simplificada: o estímulo Aβ não doloroso pode fechar o “portão” para os sinais de dor provenientes das fibras C nos neurônios da medula espinhal. Wikimedia Commons, CC BY-SA 3.0

A medula espinhal funciona como um portão capaz de aumentar ou diminuir os sinais de dor que chegam. Estímulos não dolorosos podem “fechar esse portão” para a dor; é por isso que massagear um local lesionado ou usar um aparelho TENS alivia a dor. O cérebro também envia sinais descendentes à medula espinhal, regulando a intensidade da dor conforme a atenção, o humor e o contexto (modulação descendente). Esse é o mecanismo real por trás de observações cotidianas: uma lesão esportiva quase imperceptível durante uma partida, dor que piora quando estamos ansiosos ou exaustos, alívio proporcionado por distração ou palavras de conforto. Esse “portão” não é uma metáfora; é um fenômeno fisiológico.

Sensibilização periférica e central

Após uma lesão, o sistema aumenta deliberadamente sua sensibilidade para proteger a área afetada:

  • Sensibilização periférica: as substâncias químicas liberadas no local da lesão tornam os nociceptores locais mais excitáveis, fazendo com que a área dolorida doa mais (é por isso que a pele queimada pelo sol reage intensamente a um banho morno).
  • Sensibilização central: a medula espinhal e o cérebro também se tornam mais excitáveis, amplificando todo sinal que chega.

Os principais sinais são a hiperalgesia (coisas que normalmente causam dor passam a doer mais do que deveriam) e a alodinia (estímulos que não deveriam causar dor, como um toque leve ou o contato com roupas, tornam-se dolorosos). Em casos de lesão aguda, esse mecanismo é útil e desaparece com o tempo. Na dor crônica, porém, ele pode persistir e se auto-manter.

Quando a dor persiste após a lesão: dor crônica e nociplástica

Em algumas pessoas, o sistema nervoso permanece sensibilizado muito tempo depois de os tecidos terem cicatrizado; o “alarme” fica “ligado” continuamente. A dor que surge desse processamento alterado, e não de algum dano contínuo ou lesão nervosa, é chamada de dor nociplástica (a sensibilização central é seu principal mecanismo). Isso explica um quadro clínico comum e, antigamente, difícil de compreender: dor intensa, real e incapacitante, sem nada de anormal ser detectado por exames de imagem. Reconhecer esse fenômeno não equivale a minimizar a dor como sendo “só na sua cabeça”; as alterações são reais e mensuráveis no sistema nervoso. Isso também redefine o objetivo do tratamento: não se trata mais de procurar por mais danos estruturais a serem corrigidos, mas sim de reduzir a sensibilidade excessiva do sistema nervoso. Condições como a fibromialgia e muitas formas de dor musculoesquelética persistente se enquadram nessa categoria.

Por que analgésicos mais fortes não são a solução para a dor crônica

Os opioides são úteis para a dor aguda grave e de curta duração, mas não são indicados para a dor crônica. Com o uso contínuo, o corpo desenvolve tolerância (precisando de doses maiores para obter o mesmo efeito) e, paradoxalmente, hiperalgesia induzida por opioides: esses medicamentos podem tornar o sistema nervoso mais sensível à dor ao longo do tempo. Como a dor crônica é, em grande parte, resultado de um sistema nervoso sensibilizado, as ferramentas eficazes são aquelas que reeducam esse sistema: exercícios progressivos, sono adequado, estratégias psicológicas e medicamentos que atuam diretamente sobre os nervos (em vez de opioides).

O que ajuda e o que prejudica

  • Atividade gradual e controle do ritmo desensibilizam o sistema; a evitação motivada pelo medo e o repouso prolongado o sensibilizam ainda mais.
  • Sono, controle do humor e gestão do estresse diminuem a sensibilidade; sono ruim e sofrimento emocional a aumentam.
  • Educação e redução da catastrofização diminuem significativamente a dor e a incapacidade.
  • Medicamento adequado para cada tipo de dor: analgésicos simples de uso breve para dor aguda; exercícios, terapia psicológica e medicamentos direcionados aos nervos (sem aumentar o uso de opioides) para dor crônica.

Ver também


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